chapter 4 Mechanics Flashcards
1
Q
What is a vector?
A
A physical quantity that has a direction as well as a magnitude
2
Q
Examples of vectors?
A
- field strength
- force
- momentum
- weight
- velocity
- acceleration
- displacement
3
Q
What is a scalar?
A
A physical quantity that is not directional
4
Q
Examples of scalars?
A
- density
- charge
- resistance
- work done
- energy
5
Q
How can a vector be represented?
A
By an arrow
6
Q
What does the length of an arrow representing a vector determine?
A
The magnitude of the vector quantity
7
Q
How to resolve vectors into their horizontal and vertical components?
A
- use angle of vector (Θ)
- for sinΘ
- then for cosΘ
8
Q
If two forces act on an object, when is equilibrium achieved?
A
When the two forces are equal and opposite
9
Q
If three forces act on an object, when is equilibrium achieved?
A
When the resultant of any two of the forces is equal and opposite to the third force
10
Q
What are the ways to work out forces on an object when it is in equilibrium?
A
- vector triangle
* resolve the components
11
Q
What is the centre of gravity of an object?
A
The point through which the entire weight of the object may be considered to act
12
Q
Word equation to calculate the moment of a force?
A
Force x Perpendicular distance from the line of action of the force to the pivot
13
Q
What is the principle of moments?
A
For any object that is in equilibrium, the sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments about that same point
14
Q
What is a couple?
A
A pair of equal and opposite forces, not acting in the same straight line
15
Q
What is the turning effect that couples create called?
A
A torque
16
Q
How to calculate the torque of a couple?
A
torque = one of the forces x perpendicular distance between them
17
Q
What are the two conditions for equilibrium?
A
- there is no net (resultant force)
2. there is no turning effect (moment) about any point
18
Q
What is the centre of mass of a body?
A
The point on an object where the mass may thought to be concentrated
19
Q
How to find C.O.M of symmetrical objects?
A
Along the line of symmetry, or where multiple lines of symmetry intersect
20
Q
How to find Center Of Mass of a non-symmetrical objects?
A
If an object swings freely, when it stops the COM is on a vertical line passing through the pivot
21
Q
When an object is in equilibrium, what is true of the vertical forces?
A
They are equal in magnitude and opposite in direction
22
Q
When an object is in equilibrium, what is true of the horizontal forces?
A
They are equal in magnitude and opposite in direction
23
Q
When an object in equilibrium is supported at one point, what is the support force equal to?
A
It is equal and opposite to the total downward force acting on the object
24
Q
the equation for velocity is on the data sheet. what do teh symbols stand for?
v=s/t
A
v=s/t
velocity= distance / time
25
The acceleration equation is given in the formula booklet. what do the symbols stand for?
a=v/t
a=v/t
| acceleration=change in velocity / time
26
How to calculate average velocity?
displacement / time
27
What does velocity measure?
The rate of change of displacement
28
What does acceleration measure?
The rate of change of velocity
29
What is the gradient in a distance-time graph?
Speed
30
What is the gradient in a displacement-time graph?
Velocity
31
What is the gradient in a velocity-time graph?
Acceleration
32
What is the gradient in an acceleration-time graph?
Rate of change of acceleration
33
What is the area underneath a velocity-time graph?
Displacement
34
What is the area underneath an acceleration-time graph?
Velocity
35
What do negative values of displacement or velocity on a graph indicate?
Motion in the opposite direction
36
On a velocity-time graph, what does a straight upwards line represent?
Constant acceleration
37
On a velocity-time graph, what does a straight horizontal line represent?
Constant velocity
38
On a velocity-time graph, what does a straight downwards line represent?
Constant deceleration
39
On an acceleration-time graph, what does a horizontal line represent?
An object travelling at constant acceleration - or no acceleration so constant velocity
40
On an acceleration-time graph, what does a straight line downwards represent?
Acceleration is speeding the object up, but at a slowing rate because acceleration is decreasing
41
On an acceleration-time graph, what does a straight downwards line, below the zero line, represent?
Negative acceleration - the deceleration is slowing down, and area below zero line so negative change in velocity
42
What is the equation for an object's speed, moving at constant speed on a circle?
v = 2πr / T
where T = time to move round once
43
What are suvat equations?
Equations describing motion
44
When can suvat equations be used?
When acceleration is constant
45
What do the letters in suvat stand for?
* s - displacement
* u - initial velocity
* v - final velocity
* a - acceleration
* t - time
46
How is the equation a=(v-u)/t derived using a graph?
* velocity-time graph
* gradient = acceleration
* increase in y = v-u
* over increase in x = t
47
How is the equation s=(u+v)t÷2 derived using a graph?
* velocity-time graph
* area underneath = displacement
* area of trapezium = 1/2(a+b)h
* s = 1/2(u+v)t = (u+v)t÷2
48
What is the suvat equation that doesn't include u?
s=vt-1/2at²
49
What is the suvat equation that doesn't include v?
s=ut+1/2at²
50
What is the suvat equation that doesn't include a?
s=[(u+v)/2]t
51
What is the suvat equation that doesn't include t?
v²=u²+2as
52
What experiment can be used to measure g?
* drop an object from a height - measure using meter rule
* use light gates to measure time taken for object to fall
* s=1/2at² ∴ s=1/2gt²
53
What is a projectile?
An object that is projected or thrown through the air at an angle
54
What can be done with a projectile's initial velocity?
Can be separated into components
55
When calculating projectiles, what force is ignored?
Air resistance
56
Ignoring air resistance, what is the only force acting on a projectile?
Gravity
57
What does gravity acting objects mean for the components of projectiles?
* gives a downward acceleration - so affects vertical component of velocity
* horizontal component of velocity remains constant
58
Which component of acceleration of a projectile is affected by gravity?
The vertical component
59
Which value is the same for both components when calculating projectiles?
Time
60
What is a resultant force?
A single force that has the same effect as all the forces combined
61
What is Newton's 1st law?
An object will remain at rest or continue to move with a constant velocity as long as the forces acting on it are balanced
62
When is an object's momentum not constant?
When there is a resultant unbalanced force acting on it
63
What is inertia?
The resistance to change velocity
64
What does the inertia of an object depend on?
Its mass
65
How does a bigger mass affect inertia?
Bigger mass = bigger force to overcome its inertia and change its motion
66
When travelling in a car and decelerating, what keeps you moving?
Your inertia - until something stops you, hopefully your seatbelt
67
Word equation for momentum?
Mass x velocity
68
Symbol equation for momentum?
ρ = mv
69
What is the unit for momentum?
kgms⁻¹ or Ns
70
What do objects have when stationary, in terms of momentum and inertia?
They have no momentum, but still have inertia
71
Is inertia a scalar or a vector?
Scalar
72
Is momentum a scalar or a vector?
momentum is a Vector
73
What is Newton's 2nd law?
The rate of change of momentum of an object ∝ to resultant force acting on it. Change in momentum takes place in the direction of the force
74
If mass is constant, how can change in momentum be calculated?
Δρ = mv - mu
75
What can F ∝ Δρ/Δt be simplfied to? And under what condition?
F = m [(v-u)/Δt]
| if the mass of the object doesn't change
76
What is tension equal to (in terms of mass and force)?
T = ma + mg
77
What is force equal to (in terms of tension and weight)?
F = T - mg
78
What happens in terms of forces when two objects interact?
They exert equal and opposite forces on eachother
79
What is true about pairs of forces acting on different objects?
They are always the same type of force
80
What is Newton's 3rd law?
If object A exerts a force on object B, then object B exerts an equal and opposite force of A
81
Do forces act in isolation?
No - they act in pairs
82
Why do pairs of forces not cancel each other out?
They act on different objects
83
How can Newton's laws show conservation of momentum?
* 3rd says F₁=-F₂
* 2nd says F=ma so ΔPB/ΔTB = ΔPA/ΔTA
* ΔTB=ΔTA ∴ ΔPB=-ΔPA
* ∴ ΔPB+ΔPA = 0
84
What is the principle of conservation of momentum?
When bodies in a system interact, the total momentum remains constant - provided no external force acts on the system
85
What is the principle of conservation of momentum - in simple terms?
Total momentum before = total momentum after
| provided no external forces act
86
What happens to momentum when objects collide?
Momentum is conserved
87
What is assumed when two objects collide and momentum is conserved?
There are no external forces acting
88
During a collision, will energy and momentum be conserved if there are no external forces acting?
* momentum will be conserved
| * energy might be conserved
89
How is the kinetic energy before and after an elastic collision?
Ek before = Ek after
90
What are the types of collision?
* elastic
| * inelastic
91
Are collisions between snooker balls elastic or inelastic?
Very nearly elastic
92
What is the difference between elastic and inelastic collisions?
* elastic - Ek before = Ek after
| * inelastic - Ek before > Ek after
93
Are collisions between molecules in a gas elastic or inelastic?
Elastic
94
What would it mean if collisions between molecules in a gas were inelastic instead of elastic?
Repeated collisions would slow gas molecules down, and would eventually settle at the bottom of a container
95
Are most collisions elastic or inelastic?
Inelastic
96
What happens to the kinetic energy that is 'lost' during an inelastic collision?
It is transferred to other forms
97
What is the 'lost' energy usually converted to during inelastic collisions?
Internal (heat) energy
98
What type of collision is undergone in crash barriers and crumple zones of cars?
Inelastic
99
Why are crash barriers and crumple zones designed to collide inelastically?
To absorb the kinetic energy in a crash
100
Which principle can be applied to explosions?
The principle of conservation of momentum
101
What can the principle of conservation of momentum be used for?
* collisions
| * explosions
102
In exam questions, what is the movement of all objects involved in explosions?
Initially, they are all stationary
103
What is the initial momentum of explosions when the objects are both stationary?
Total initial momentum is zero
104
What is impulse equal to?
The change in momentum
105
the equation for impulse is found on the data sheet. what do the symbols stand for?
f △t = △ (mv)
f △t = △ (mv)
change in momentum = impulse
f=force(N)
△t= change in time (seconds)
m= mass (kg
v=velocity
106
What is the advantage of 'following through' in sports?
* keeps force acting on ball for longer
| * refer to F = Δp/Δt
107
What is the advantage of drawing your hands back when catching a ball?
* reduces sting
* as Δp happens over a longer time
* reducing force on hands
108
What happens to Δp when there is a greater force on an object, acting for longer?
Greater change in the object's momentum
109
What is the area underneath a force-time graph?
Impulse
110
What is the area underneath a force-time graph NOT referred to as?
Change in momentum
111
What is the area underneath a force-time graph?
Impulse (kgms⁻¹)
112
the equation for work done that includes displacement is found on the data sheet. what do the symbols stand for?
w= F cos(Θ)
w= F cos(Θ)
w=work done (j)
F=force (N)
cos(Θ) = horizontal displacement (m)
113
For equation W=Fd, what must be done if force and displacement are not in the same direction?
The force needs to be resolved to find the component acting in the direction of the displacement
114
What can work done also be referred to as?
Energy transferred
115
What is the definition of 1J
The work done when a force of 1N moves through a distance of 1m (in the direction of the positive force)
116
What is power?
The rate at which work is done
117
the symbol equation for power is found on the data sheet. what do the symbols stand for?
P = ΔW / Δt
P = ΔW / Δt
p=power ( watts)
W= work done (joules)
t= time ( seconds)
118
What is 1W in derived units?
1 Js-1
119
the equation for power is found on the data sheet. what do the symbols stand for?
P = Fv
P = Fv
p=power (watts)
F=force(N)
v= velocity (ms)
120
What is true for the velocity in P=Fv?
It is constant
121
What are the types of energy?
* heat
* light
* chemical
* sound
* electrical
* nuclear
* elastic potential
* kinetic
* gravitational potential
122
How is the equation for gravitational potential energy derived?
* W = Fx
* = Weight x Δh
* Work done = Ep gained
* so Ep = weight x change in height
* Ep = mgh
123
How is the equation for elastic potential energy derived?
* F = kx
* when a spring is stretched, work is done
* (area under f-x graph) = 1/2kx
* W = Fx
* W = 1/2 kx x x
* W = 1/2ke²
124
How is the equation for kinetic energy derived?
* an object gains Ek if a force does work on it
* W = Fx
* & F=ma → so W=mas
* suvat to find F → s=[(u+v)/2]/t & a=(v-u)/t
* subs a and s into W=mas to get W=1/2m(v²-u²)
* so Ek = 1/2mv²-1/2mu²
125
What is the principle of conservation of energy?
Energy can be transferred from one form to another, but it cannot be created or destroyed
126
What is true of the total energy when using the law of conservation of energy?
The total amount of energy always stays the same
127
What is the unit for efficiency?
No unit
128
What does it mean when energy is 'wasted'?
It is transferred to internal (heat) energy
129
the equation for force is found on the data sheet. what do the symbols stand for?
F=ma
force (N) = mass(kg) x acceleration ( ms-2)
130
The equation for force is found on the data sheet. what do the symbols stand for?
F= △(mv)/△t
force= change in momentum (mass X velocity) / time
131
What is a scalar quantity?
A quantity that only has a magnitude, without direction.
132
What is a vector quantity?
A quantity that has both magnitude and direction.
133
```
Sort these into scalar and vector:
• Mass
• Displacement
• Velocity
• Time
• Force
• Acceleration
• Distance
• Speed
• Energy
• Momentum
```
```
SCALAR
• Mass
• Temperature
• Time
• Distance
• Speed
• Energy
VECTOR
• Displacement
• Velocity
• Force
• Acceleration
• Momentum
```
134
Is mass vector or scalar?
Scalar
135
Is temperature vector or scalar?
Scalar
136
Is displacement vector or scalar?
Vector
137
Is velocity vector or scalar?
Vector
138
Is time vector or scalar?
Scalar
139
Is distance vector or scalar?
Scalar
140
Is force vector or scalar?
Vector
141
Is speed vector or scalar?
Scalar
142
Is weight vector or scalar?
Vector
143
Is energy vector or scalar?
Scalar
144
Is acceleration vector or scalar?
Vector
145
Is momentum vector or scalar?
Vector
146
What is the term for adding two forces together?
Finding the resultant of them.
147
What are the two methods for finding the resultant of two forces?
1 - Scale drawings
| 2 - Pythagoras + Trigonometry
148
How can the resultant of two forces be found using scale diagrams?
* Draw the two forces tip to tail
* Draw the resultant force
* Measure the length of the resultant force and the angle from the horizontal
149
150
How can the resultant of two forces be found using Pythagoras and trigonometry?
* Only works if the forces are at right angles
* Draw a right angled triangle out of the forces
* Use Pythagoras to find the magnitude of the resultant
* Use trig to find the direction of the resultant
151
When finding the resultant of two forces, when can Pythagoras + trigonometry be used?
When the forces are at right angles to each other.
152
What is splitting a force into horizontal and vertical components called?
Resolving the force.
153
When resolving a force (F) to the top right, what is the horizontal component equal to?
F x cosθ
Where θ is the angle from the horizontal.
154
When resolving a force (F) to the top right, what is the vertical component equal to?
F x sinθ
Where θ is the angle from the horizontal.
155
What makes vertical and horizontal components easy to find when resolving forces?
The two components don’t affect each other, so the two directions can be dealt with separately.
156
On a free-body diagram, do the sizes of the arrows matter?
Yes, because they represent the size of the force.
157
What is a free-body diagram?
A diagram that shows all the forces acting on a single body (and NOT the forces that the body exerts).
158
What are coplanar forces?
* Forces that are all in the same plane
| * You’ll only have to deal with these types of forces
159
What are the conditions for an object to be in equilibrium?
* Forces acting on the object in each direction must be balanced.
* No resultant moment acting on the object.
160
Describe the motion of a body in equilibrium.
Either:
• At rest
• Moving at constant velocity
161
Why might you resolve a force for calculations?
If a force is in an awkward direction, resolving it into vertical and horizontal components can make calculations easier.
162
How can you demonstrate that three coplanar forces give no resultant force?
When you draw them out as a triangle, they form a closed loop.
163
When there are three forces acting on a body in equilibrium and one is unknown, how can it be calculated?
* Resolve each force horizontally and vertically
* The horizontal forces must add up to 0.
* The vertical forces must add up to 0.
* Find the missing force.
164
When resolving a force acting in an unusual direction, what is it important to do?
Choose axis that are sensible for the problem.
165
What axis should you choose when resolving a force acting on an object on a slope?
• Choose axis at right angles to the slope.
• Turning the page to be parallel with the slope will help.
(See diagram pg 41)
166
What is a moment?
* The turning effect of a force around a point.
| * Equal to the force multiplied by the perpendicular distance from the line of action to the pivot.
167
What is the equation for the moment of a force?
Moment (Nm) = Force (N) x Perpendicular distance from the point to the line of action of the force (m)
M = F x d
168
What is d in the moments equation?
Perpendicular distance from the turning point to the line of action of the force (m).
169
What is the unit for moments?
Newtonmeter (Nm)
170
State the principle of moments.
For an object in equilibrium, the sum of the clockwise moments about any point equals the sum of the anticlockwise moments.
171
In a lever, what forces act against each other?
The effort force acts against the load force.
172
What is a couple?
* A pair of coplanar forces of equal size that act parallel to each other but in opposite directions.
* Produce a turning effect.
173
What is the equation for the moment of a couple?
Moment (Nm) = Size of one of the forces (N) x Perpendicular distance between the lines of action of the forces (m)
M = F x d
174
What is mass?
The amount of matter in an object.
175
What is the unit for mass?
Kilograms (kg)
176
What is inertia?
An object’s resistance to change in velocity.
177
What affects an object’s inertia?
* Its mass.
| * The greater the mass, the greater the inertia.
178
Is an object’s mass affected by the gravitational field strength?
No
179
What is weight?
The force exerted on an object due to the Earth’s gravitational field.
180
What is the unit for weight?
Newton (N)
181
What is the equation for weight?
Weight = Mass x Gravitational Field Strength
W = m x g
182
What is the value of g?
9.81 N/kg
183
What is an object’s centre of mass?
* The point through which the whole weight can be said to act through.
* Object will balance around this point.
184
Is the centre of mass always on an object?
No, sometimes it is outside the object.
185
Where is the centre of mass of a uniform, regular solid (e.g. a sphere)?
At its centre.
186
How can the centre of mass of a regular object be found?
* Look at the lines of symmetry
| * The centre of mass is where the lines cross
187
How can the centre of mass of an irregular object be found?
* Hang object from a point
* Draw vertical line downwards from point (using a plumb bob as guidance)
* Repeat with different point
* Centre of mass is where the lines cross
188
When will an object topple and why?
* When the vertical line from its centre of mass falls outside of the base area.
* Because the centre of mass causes a resultant moment around the pivot.
189
What makes an object stable?
• Low centre of mass
• Wide base
E.g racing cars won't topple over on fast corners
190
For supports:
| The closer the object's centre of mass .....
....the stronger the force on the support ...
191
What is speed?
How fast an object is moving, regardless of direction.
192
What is displacement?
How far an object has travelled from its starting point in a given direction.
193
What is the symbol for displacement?
s
194
What is velocity (in terms of displacement)?
The rate of change of an object’s displacement.
195
What is the symbol for velocity?
v
196
What is acceleration?
The rate of change of an object’s velocity.
197
What is the symbol for acceleration?
a
198
What is the equation for velocity?
v = Δs/Δt
199
What is the equation for acceleration?
a = Δv/Δt
200
What is instantaneous speed?
The speed at a given moment (as oppose to average speed).
201
Describe the displacement-time graph for an accelerating object.
* Curved graph
| * If acceleration is constant, the rate of change of the gradient is constant.
202
Remember to practice predicting different displacement-time graphs.
Pg 46 of revision guide.
203
What does the gradient and area under the curve represent in a displacement-time graph?
* Gradient = Velocity
| * Area = Nothing
204
How do you find the velocity from a displacement-time graph?
* Find the gradient.
| * You may need to draw a tangent if the graph is a curve.
205
On a curved displacement-time graph, how do you find the AVERAGE velocity?
* Divide the overall displacement by the overall time.
| * No need for tangents.
206
What is the difference between a speed-time and velocity-time graph?
A velocity-time graph can have a negative part to show motion in the opposite direction.
207
How is uniform acceleration shown on a velocity-time graph?
Straight line
208
How is a changing acceleration shown on a velocity-time graph?
Curved line
209
What does the gradient and area under the graph represent on a velocity-time graph?
* Gradient = Acceleration
| * Area = Displacement
210
How do you find the acceleration from a velocity-time graph?
Gradient of the line
211
How did you find the displacement from velocity-time graph?
Area under the graph
212
On an acceleration-time graph, what does the area under the line represent?
The change in velocity.
213
What piece of equipment can be used in motion experiments?
Ultrasound position detector
214
How does an ultrasound position detector work?
* Records distance of object from the sensor several times a second
* Connected to computer with graphing software to get graph
215
What are some advantages of data-loggers over traditional methods of recording data?
1) Data is more accurate - don't have to allow for human reaction times.
2) Higher sampling rate than humans (for example, ultrasound position detectors can take a reading ten times every second)
3) Data displayed in real time
216
What are the suvat equations used for?
4 equations that can be used to solve constant acceleration motion problems.
217
What are the 4 suvat equations?
* v = u + at
* s = (u+v)/2 x t
* s = ut + 1/2 at²
* v² = u² + 2as
218
In suvat problems for a falling object, what is the value of a?
g (9.81m/s²)
219
Remember to practice suvat equations and problems.
Pg 50/51 of revision guide.
220
What is free fall?
The motion of an object undergoing an acceleration of ‘g’ (i.e. under gravity and nothing else).
221
In free fall, what is the only force acting on an object?
Its weight.
222
Can objects with an original velocity undergo free fall?
Yes, as long as the force providing the initial velocity is no longer acting.
223
Describe the rate at which objects fall to the Earth.
All objects fall to the Earth at the same rate due to gravity (9.81m/s²).
224
Describe an experiment to calculate g.
1) Set up a circuit with a switch that controls two parallel circuits: one with an electromagnet and ball, the other with a timer and trapdoor
2) Measure the height from the bottom of the bearing to the trapdoor.
3) Flick the switch to start the timer and release the bearing.
4) Bearing falls, hits trapdoor and stops the timer. Record the time.
5) Repeat 3 times at this height and average the time.
6) Repeat at various heights.
7) Plot a graph of height (m) against time taken squares (s²).
8) a = 2 x Gradient
(See diagram pg 52)
225
In the experiment to calculate g, how is error reduced?
* Bearing is small and heavy -> Means air resistance is negligible
* Computer releasing and timing fall -> Reduces uncertainty
226
In the experiment to calculate g, what is the biggest source of error?
RANDOM error: The measurement of h (using a ruler = uncertainty of + or - 1mm).
227
In the experiment to determine g, describe what graph should be plotted and why?
```
Height (m) against time squared (s²) because:
• s = ut + 1/2 at²
• Since u = 0, s = 1/2 at²
• 1/2 a = s/t² = Gradient
• a = g = 2 x Gradient
```
228
Describe two experiments to find g.
1) Ball bearing drop with timer.
| 2) Card drop through a light gate.
229
How do you find the acceleration from a displacement-time graph?
* Find the gradient (velocity) at two separate points
| * Find the difference between these two points and divide it by the time (a = Δv/Δt)
230
Remember to revise experiments to determine g.
Pg 52/53 of revision guide.
231
When considering vertical motion, what can the acceleration be taken as?
g (-9.81m/s²)
If you use a negative for upward stay consistent
232
How do you approach projectile motion questions when an object is projected horizontally?
* Consider vertical motion separately.
* Use suvat to determine the time in the air.
* Now consider horizontal motion.
* Use distance = speed x time to find the horizontal distance travelled.
233
```
In suvat equations, what signs can these values be (can they be positive/negative)?
• g
• t
• u and v
• s
```
g = Usually negative (depends)
t = Positive
u and v = Positive or negative
s = Positive or negative
234
How do you approach projectile motion questions when an object is projected at an angle?
* Resolve the velocity into horizontal and vertical components.
* Use vertical component with suvat to work out the time in the air.
* Use the horizontal component with distance = speed x time to work out the horizontal distance travelled.
235
What is Newton’s 1st Law?
The velocity of an object will not change unless a resultant force acts on it.
236
How are the forces on an apple on a table balanced?
Reaction force of table = Weight of apple
237
What is Newton’s 2nd law?
| Written in two ways
• The force required to accelerate an object is equal to its mass times the acceleration
• F = m x a
OR
• The rate of change of momentum of an object is directly proportional to the resultant force which acts in the object.
• F = Δ(mv)/Δt
238
What is the equation for Newton’s 2nd law?
Resultant force (N) = Mass (kg) x Acceleration (m/s²)
F = m x a
239
Explain why all objects fall at the same rate.
```
Consider Newton’s 2nd Law:
• F = ma
When an object falls:
• mg = ma
• a = g
```
240
What is Newton’s 3rd Law?
* If an object A exerts a force in object B, then object B exerts an equal and opposite force on object A.
* i.e. Every action has an equal and opposite reaction
241
What must you be careful of with Newton’s 3rd Law?
* The opposite reaction is must be the EXACT reverse of the original action.
* The forces cannot both be acting on the same object!
242
What is the Newton’s 3rd Law opposite reaction of a person pushing on a wall?
The wall pushing back on the person.
243
What is the Newton’s 3rd Law opposite reaction of a person pulling a cart?
The cart pulling back on the person.
244
What is the Newton’s 3rd Law opposite reaction of a person pushing back on the water when swimming?
The water pushing the person forward.
245
What can be said about the types of forces in Newton’s 3rd Law?
The equal and opposite pairs are always of the SAME TYPE (e.g. both electrical).
246
What is a friction?
A force that opposes motion.
247
What are the two types of friction?
* Dry friction
| * Fluid friction
248
What is dry friction?
Friction between two solid surfaces.
249
What is fluid friction?
Drag or air resistance, due to a liquid or gas.
250
What is a fluid?
A liquid or a gas.
251
What factors affect the size of fluid friction (force)?
* Viscosity of liquid
* Speed of object (force increases = speed increases)
* Shape of object (larger area pushing against fluid= greater resistive force)
252
In projectile motion calculations, what is the significance of air resistance?
* Air resistance is usually ignored
| * This gives an answer which is too large
253
Can friction speed an object up or start it moving?
No
254
When energy transfers, what does friction result in?
Kinetic energy to heat and sound.
255
What is lift?
* An upwards force on an object moving through a fluid.
| * Perpendicular to the direction of fluid flow.
256
The resistive force from the fluid isn't necessarily in the same direction as...
...the direction of the movement of the fluid
| check the explanation but it should match this picture
257
How does lift happen?
When an object in a fluid causes the fluid flowing over it to change direction.
258
Give an example of when lift might happen.
* A plane wing moving through the air.
* Wing pushes down on the air, changing its direction.
* The air pushes back with an equal and opposite reaction force (Newton's 3rd)
259
When is terminal speed achieved?
When the friction force equals the driving force or weight.
260
Describe how a moving object reaches terminal speed.
1) Constant driving force causes acceleration.
2) As speed increases, so does the frictional force. This reduces the resultant force and therefore the acceleration.
3) Eventually, the frictional force equals the driving force and terminal speed is achieved.
261
What factors will cause a terminal speed in a moving object?
1) Driving force that stays constant
| 2) Frictional force that increases with speed
262
What things can be done to increase a moving object’s terminal speed?
1) Increase the driving force (e.g. bigger engine)
| 2) Reduce the frictional force (e.g. more streamlined object)
263
How does air resistance change with speed?
It increases with speed.
264
What does a v-t graph look like for terminal velocity?
265
What does an a-t graph look like for terminal velocity?
266
How can you find the terminal speed of a ball bearing in a viscous material?
Put elastic bands around the tube of viscous liquid at fixed distances using a ruler.
Drop the ball bearing into the tube.
Use a stopwatch and record the time at which it reaches each band.
…
(You could alter other variables instead of fluid velocity e.g the shape of the object)
267
Describe how and why the speed of a skydiver changes as he falls.
* Skydiver accelerates until air resistance equals weight.
* Travelling at terminal speed until parachute opens.
* Air resistance is now bigger than his weight.
* This slows him down until his speed has dropped so that the air resistance is equal to the weight again.
* This is the new terminal speed.
268
Describe the velocity-time graph of a falling skydiver.
* Velocity increases at a decreasing rate
* Until terminal speed (flat part of graph)
* Sharp drop in velocity when parachute opens
* New terminal speed (lower flat part of graph)
269
What is momentum?
The tendency of an object to keep moving in the same direction.
270
What two factors affect the momentum of an object?
* Mass
| * Velocity
271
What is the equation for momentum?
Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
p = m x v
272
What is the unit for momentum?
Kilogram metres per second (kg m/s)
273
The total momentum before a collision is...
...equal to the total momentum after the collision.
274
Is momentum conserved in a system?
* Yes, assuming no external forces act.
| * Even if the collision is inelastic.
275
The total momentum before an explosion is...
...equal to the total momentum after the explosion.
276
What can be said in terms of momentum after an air rifle is fired?
The forward momentum gained by the bullet is equal to the backward momentum of the rifle.
277
What are the two types of collision?
* Elastic
| * Inelastic
278
What is an elastic collision?
* One where kinetic energy is conserved (as well as momentum).
* No energy is dissipated as heat, sound, etc.
279
What is an inelastic collision?
* One where kinetic energy is not conserved (but momentum is conserved).
* Some energy is dissipated as heat, sound, etc.
280
What two equations represent Newton’s 2nd Law?
* F = ma
| * F = Δ(mv)/Δt
281
Give the momentum definition of Newton’s 2nd Law.
The rate of change of momentum of an object is directly proportional to the resultant force which acts on the object.
282
Express force in terms of momentum and time.
Force = Change in Momentum / Time
283
What is impulse?
* Force x Time
| * It is the change in momentum
284
What is the unit for impulse?
Newton seconds (Ns)
285
What is the equation for impulse?
Impulse = Change in Momentum
Ft = mv - mu
(Note: This is derived from Newton’s 2nd Law)
286
How can impulse be found from a force-time graph?
Area under the graph (because it is equal to force x time).
287
How can the force of an impact be reduced?
Increasing the impact time.
| For example packaging takes longer to deform on impact, so absorbs shock and protects packaged object.
288
Give some ways in which vehicles have safety features to reduce the force of an impact.
* Crumple zones -> Crumple on impact and increase time of crash
* Seat belts -> Stretch slightly to increase stopping time
* Air bags -> Slow passengers down more slowly and prevent them from hitting hard surfaces
289
When work is done...
...energy is transferred.
290
What is work?
The amount of energy transferred from one form to another.
291
Why is a force needed to move an object?
You have to overcome another force to move the object.
292
What is work done against when lifting a box and what is the final energy form?
* Work done against: Gravity
| * Final energy form: GPE
293
What is work done against when pushing a chair across a level floor and what is the final energy form?
* Work done against: Friction
| * Final energy form: Heat
294
What is work done against when pushing two magnetic north poles together and what is the final energy form?
* Work done against: Magnetic force
| * Final energy form: Magnetic energy
295
What is work done against when stretching a spring and what is the final energy form?
* Work done against: Stiffness of spring
| * Final energy form: Elastic potential energy
296
What is the equation that relates work done, force and distance?
Work done (J) = Force (N) x Distance (m)
W = F x d
297
What is the unit for work done?
Joules (J)
298
When work is done on an object, is the object’s energy equal to the work done?
No, it is the CHANGE in the object’s energy that is equal to the work done.
299
When using “W = F x d” with a changing force, what value of F is used?
The average force.
300
In “W = F x d”, what is F?
The force CAUSING MOTION.
301
What is an assumption of “W = F x d”?
The direction of the force is the same as the direction of movement (otherwise you must resolve forces).
302
Define a joule.
The work done when a force of 1N moves an object through a distance of 1 metre.
303
When using the equation “W = F x d” with a force in a direction different to the direction of motion, what must you do?
* Resolve the force so that you have the component that is equal to the direction of motion.
* Now use “W = F x d”
(See page 62 of revision guide)
304
Remember, the sledge is only moving horizontally
...
305
For a force F acting to the top right at an angle θ above the horizontal causing a motion horizontally to the right, give an equation for the work done.
W = (Fcosθ) x d
306
For a force F acting to the top right at an angle θ above the horizontal causing a motion horizontally to the right, why does the vertical component not have an effect on work done?
* It is not causing horizontal motion.
| * It is only balancing out some of the weight of the object, so there is a smaller reaction force (in a sledge example).
307
On a force-displacement graph, what does the area under the graph give you and why?
* The work done
| * Because of “W = F x d”
308
What is power?
* The rate of doing work
| * P = ΔW / Δt
309
What is the equation for power?
Power (W) = Change in energy or work (J) / Change in time (s)
P = ΔW / Δt
310
What is the unit for power?
Watt (W)
311
Define a watt.
The rate of energy transfer equal to 1 joule per second.
312
What equation relates velocity, power and force?
Power (W) = Force (N) x Velocity (m/s)
P = F x v
313
Derive the equation that relates power, force and velocity.
* P = W / t
* W = Fd
* Therefore, P = Fd / t = F x (d/t)
* v = d / t
* Therefore, P = F x v
314
For a force acting at angle θ to the direction of motion, what is the equation for the work done?
W = (Fcosθ) x d
315
For a force acting at angle θ to the direction of motion, what is the equation for the power?
P = (Fcosθ) x v
316
State the principle of conservation of energy.
* Energy cannot be created or destroyed.
| * Energy can be transferred from one form or another, but the total amount of energy in a closed system will not change.
317
In a closed system, total energy in...
...equals total energy out.
318
What is the equation for efficiency in terms of power?
Efficiency = Useful output power / Input power
319
When will the principle of conservation of energy come up?
When doing questions about changes between kinetic and potential energy.
320
What is kinetic energy?
The energy of an object due to movement.
321
What is the formula for kinetic energy?
Energy (J) = 1/2 x Mass (kg) x (Velocity (m/s²))²
E = 1/2 x m x v²
322
What are two types of potential energy?
* Gravitational
| * Elastic
323
What is gravitational potential energy?
The energy something gains if you lift it up.
324
What is the equation for GPE?
ΔGPE (J) = Mass (kg) x G.F.S. (N/kg) x ΔHeight (m)
ΔE = m x g x Δh
325
What is elastic strain energy (aka elastic potential energy)?
The energy stored in a stretched rubber band or spring, for example.
326
What is the equation for elastic strain energy?
Energy (J) = 1/2 x Spring Constant x (Extension (m))²
E = 1/2 x k x (Δl)²
327
Where does the energy for human interactions come from and what energy transfers occur?
* Food
* Chemical energy is converted to other forms, such as kinetic energy
* Some is wasted in, for example, heat
328
Describe the energy transfers when a ball is thrown vertically up.
* When the ball goes up, the kinetic energy is converted into GPE
* When the ball comes down, the GPE is converted back into kinetic energy
329
Describe the energy transfers when a person slides down a slide.
The gravitational potential energy is converted into kinetic energy.
330
Describe the energy transfers when a person jumps on a trampoline.
* When bouncing up, Elastic energy -> Kinetic energy -> GPE
| * When coming down, GPE -> Kinetic energy -> Elastic energy
331
What forces are involved when jumping on a trampoline?
Weight force.
| Tension in springs and trampoline material.
332
In reality there are other energy transfers in jumping on a trampoline, what are these?
To keep jumping at the same height each time, you would have to use some force from your muscles.
Each time the trampoline stretches, some heat is generated in the trampoline material.
333
In energy conservation problems, what must you usually assume?
That frictional forces are negligible.
334
Remember to revise conservation of energy problems.
Pg 65 of revision guide.
335
Question:
Remember: the height from the ground is different from the length of string.
You need to find change in height
336
In energy transfer problems where kinetic energy and GPE are exchanged, what must you do?
* Put the kinetic energy change equal to the GPE change.
| * mgΔh = 1/2mv²
