Mechanics Flashcards
1
Q
What is a scalar quantity?
A
A quantity that has no direction (just an amount)
2
Q
What is a vector quantity?
A
A quantity that has magnitude and direction
3
Q
Give 4 examples of scalar quantities
A
Mass
Temperature
Energy
Distance
4
Q
Give 4 examples of vector quantities
A
Acceleration
Velocity
Momentum
Displacement
5
Q
What is a bearing?
A
A 3-digit angle that is measured clockwise from north in degrees
6
Q
How do you find the resultant vector?
A
By adding 2 or more vectors together
7
Q
How do you find the resultant vector from a scale diagram?
A
Drawing a scale diagram with all the vectors joining in one line (tip-to-tail), then measuring the length and angle of the resultant vector from the diagram
8
Q
What is displacement?
A
The position relative to the start position
9
Q
What does cos60 and sin30 equal?
A
0.5
10
Q
What is an inclined plane?
A
A slope
11
Q
Which direction does weight always act?
A
Downwards
12
Q
What is happening when is a body is in equilibrium?
A
All the forces are balanced (no resultant force)
13
Q
What 2 states can an object in equilibrium be at?
A
Rest or moving with constant velocity
14
Q
When only 2 forces act on an object, what do they have to be for the object to be in equilibrium?
A
Equal and opposite (opposite direction)
15
Q
When forces act on an object in equilibrium, what do they form if you draw them tip-to-tail in a scale diagram?
A
Form a closed loop (a vector triangle)
16
Q
What is the magnitude of a vector?
A
The size of it (without direction)
17
Q
What is a moment of a force?
A
The turning point of a force around a point
18
Q
What are 2 moments of equal size acting parallel to each other called?
A
A couple
19
Q
What is the mass of an object defined as?
A
The amount of matter in it
20
Q
What is inertia?
A
A resistance to a change in motion
21
Q
What happens to the inertia if an objects mass is increased?
A
The greater the resistance to a change in motion will be (larger inertia)
22
Q
What is weight and what is it measured in?
A
A force measured in newtons
23
Q
Define weight
A
The force experienced by a mass due to gravity
24
Q
What equation links weight, gravity field strength and mass
A
W=mg
25
Is weight a scalar or vector quantity?
Vector as it has size and direction
26
What 2 things does the moment of a force depend on?
Size of the force and how far the force is applied from the turning point
27
What equation links moment, force and perpendicular distance from the turning point?
M = fd
28
What does the principle of moment state for a body to be at equilibrium?
The sum of the clockwise moments about any point equals the sum of the anti-clockwise moments at the same point
29
What will happen to an object if the moments aren't balanced?
The object will turn
30
How do levers work?
An effort force acts against a load force by means of a rigid object rotating around a pivot
31
What situations are levers useful in?
If you need a larger turning effect
32
Give 3 examples of levers
Spanners
Wheelbarrows
Scissors
33
How does a spanner (or any other lever) work?
They increase the distance from the pivot a force is applied, so you need less force to get the same moment
34
What does 'Σ' mean?
The sum of
35
What is a couple?
A pair of forces of equal size which act parallel to each other, but in opposite directions
36
What does a couple cause?
A turning effect (moment), doesn't cause any resultant linear force
37
What is an object's centre of mass?
The single point that it's whole weight acts through (whatever the orientation)
38
What will an object at centre of mass do?
Balance
39
For a uniform regular solid, where is the centre of mass?
In the centre
40
What are the 4 steps to find an objects centre of mass?
- Hang the object freely from a point
- Draw a vertical line downwards from the point of suspension
- Hang the object from a different point and draw a line downwards
- Centre of mass is where the 2 lines cross
41
Why will an object topple over?
If the line of action of its weight falls outside it's base area
42
Why does it cause an object to topple over if the line of action of its weight falls outside it's base area?
A resultant moment occurs, which provides a turning force
43
What cause an object to be less stable?
If the centre of mass is higher, and there is a small base area
44
What makes an object very stable?
If it has a low centre of mass and a wide base area
45
If an object is being held up on supports (e.g. a chair), why is the force acting on each support not the same?
the closer the centre of mass is to a support, the stronger the force on that support
46
What is speed?
How fast something is moving, regardless of direction
47
What is displacement?
How far an object has travelled from its starting point in a given direction
48
What is velocity?
The rate of change of an object's displacement (it's speed in a given direction)
49
What is acceleration?
The rate of change of an object's velocity
50
What does the U mean in the SUVAT equations?
Initial velocity (ms^-1)
51
In displacement-time graphs, what is plotted on the Y-axis?
Displacement
52
What does the gradient of a displacement-time graph show?
Velocity
53
What does a steeper gradient in a displacement-time graph mean?
A bigger acceleration
54
What is the equation linking velocity, change in displacement and time taken?
Velocity = Change in displacement / Time taken
55
What is an object's instantaneous velocity?
It's velocity at a particular moment in time
56
How did you find the instantaneous velocity of an object from a displacement-time graph?
At a given point, draw a tangent to the graph and find the gradient of it
57
What is the average velocity?
The total change in displacement of the object divided by the total time taken
58
What does the gradient of a velocity-time graph?
Acceleration
59
What does a steeper gradient on a velocity-time graph mean?
A greater acceleration
60
What is the difference between velocity-time graphs and speed-time graphs?
Velocity-time graphs can have negative regions to show something travelling in the opposite direction
61
What is the area under a speed-time graph equal to?
Distance travelled
62
What does a straight line on a velocity-time/speed-time graph represent?
A uniform acceleration or deceleration
63
When is a situation where you can't use the SUVAT equations?
If the acceleration isn't constant
64
How do you find the acceleration at a point on a velocity-time graph with changing acceleration?
Draw a tangent at that point and find the gradient of the tangent
65
What are 3 main advantages of using dataloggers over traditional methods?
The data is more accurate as you don't have to allow for human reaction times
Automatic systems have a much higher sampling rate than humans
You can see the data displayed in real time
66
What does the height at any point on an acceleration-time graph show?
The acceleration at that time
67
What does it mean if a=0?
Constant velocity
68
What does a negative gradient (above the x-axis) mean on an acceleration-time graph?
It means the object is still accelerating, but the rate of acceleration is decreasing
69
What is the area under an acceleration-time graph equal to?
The total change in velocity
70
What does Newton's 1st law of motion state?
The velocity of an object will not change unless a resultant forces acts on it
71
What does Newton's 1st law of motion mean?
A body will stay still or move in a straight line at a constant speed, unless there's a resultant forces acting on it
72
Why won't an apple sitting on a table move?
Because the forces are balanced. Reaction (force of table pushing up on apple) = Weight (force of gravity pulling apple down)
73
What is Newton's 2nd law of motion?
The acceleration of an object is proportional to the resultant force acting on it
74
What equation defines Newton's 2nd law?
F = ma
75
What 2 things can we see from the equation F = ma?
The more force you have acting on a certain mass, the more acceleration you get
For a given force, the more mass you have, the less acceleration
76
What is the resultant force?
The vector sum of all the forces
77
What is the resultant force always measured in?
Newtons
78
What is Newton's 3rd law of motion?
If an object A exerts a force on object B, then object B exerts an equal but opposite force on object A
79
What situations can Newton's 3rd law be used?
All situations, but both forces have to be the same (e.g. both gravitational)
80
What is freefall?
When there's gravity acting on an object and nothing else
81
Freefall is defined as...?
The motion of an object undergoing an acceleration, g
82
What type of quantity is acceleration?
Vector
83
What is the only force acting on an object in freefall?
It's weight
84
What did Galileo prove in his experiments?
That all objects fall at the same rate, and he showed mathematically that all objects are attracted towards the Earth due to a force called gravity
85
Why do all objects fall at the same rate (not accounting for air resistance)?
The acceleration is independent of the mass (mass makes no difference)
86
What is the sign convention for upwards and downwards when calculating motion?
Upwards is +ve and downwards is -ve
87
What direction does g act?
Downwards so it's normally a negative number in calculations
88
How do you set up the experiment for determining g using freefall method?
Put a trapdoor on the floor and measure a set distance up. At this distance, position an electromagnet with a ball bearing. Attach a timer and a switch in series with the electromagnet, and connect the trapdoor to the circuit
89
What are the 5 steps for the experiment to determine the value of g by freefall method?
- Measure the height, h, from the bottom of the ball bearing to the trapdoor
- Flick the switch to simultaneously start the timer and release the ball bearing
- When the ball bearing falls, it hits the trapdoor, thus breaking the circuit, so the timer will stop. Record the time, t, shown on the timer
- Repeat experiment 3 times and then average the time taken for the ball bearing to drop
- Use these results to find the value of g from a graph
90
What is the most significant source of random error for the experiment to determine g from freefall method?
Measurement of h, as the ruler will have an uncertainty of +- 1mm
91
In the experiment to determine g from freefall method, why would you use a small, heavy ball bearing?
Because we can then assume that air resistance is negligible
92
What is projectile motion?
Motion with a constant horizontal velocity and a vertical velocity affected by acceleration due to gravity
93
In projectile motion, the horizontal and the vertical velocity are...?
Independent of each other
94
Why do projectiles follow a curved path?
Because their horizontal velocity is constant, but their vertical velocity increases due to gravity
95
For projectiles, describe the horizontal acceleration and velocity
```
Acceleration = 0
Velocity = Constant
```
96
For projectiles, describe the vertical acceleration and velocity
```
Acceleration = -9.81ms^-1
Velocity = Changes with time according to equation V=U+AT
```
97
What are the 3 steps for solving projectile motion problems with an angle?
- Resolve the initial velocity into horizontal and vertical components
- Use the vertical component to work out how long it's in the air/how high it goes
- Use the horizontal component to work out how far it goes in the horizontal direction
98
When resolving vectors, how do you calculate the vertical component?
Vy = Vsinθ
99
When resolving vectors, how do you calculate the horizontal component?
Vx = Vcosθ
100
Air resistance causes a....that acts in the.....direction to motion and affects the trajectory of a projectile
Air resistance causes a drag force that acts in the opposite direction to motion and affects the trajectory of a projectile
101
What is the effect of the horizontal component of drag?
Reduces the horizontal speed of the projectile and reduces the horizontal distance it travels
102
What is the effect of the vertical component of drag?
Reduces the maximum height the projectile will reach and steepens the angle of descent
103
What is friction?
A force that opposes motion
104
What are the 2 main types of friction?
Contact friction
| Fluid friction
105
What is contact friction?
Friction that occurs between solid surfaces
106
What is fluid friction?
Friction when it's in gas or liquid, such as air resistance. Also known as drag
107
What 3 things affect fluid friction?
- The force depends on the viscosity of the fluid
- The force increases as speed increases
- The forces depends on the shape of the object moving through it
108
What are the 3 main things about frictional forces?
- They act in the opposite direction to the motion of the object
- They can never speed up or start something moving
- They convert kinetic energy into heat
109
What is lift?
An upwards force on an object moving through a fluid
110
What causes lift?
When the shape of an object causes the fluid flowing over it to change direction (such as an aircraft wing)
111
What direction does lift force work at?
Perpendicular to the direction in which the fluid is flowing
112
How does a plane wing cause lift?
As the wing moves through the air, it pushes down on the air (and changes it's direction). This causes an equal and opposite reaction force on the wing
113
What is terminal velocity?
The speed at which the driving force(s) match the frictional force(s) (and so the resultant force becomes 0)
114
When will an object reach terminal velocity?
When the driving forces stays constant, but the frictional force increases with speed
115
What happens to the air resistance on an object when speed increases?
Air resistance on an object increases with speed
116
What are 2 ways of increasing a vehicle's maximum speed?
Increasing the driving force (e.g. larger engine)
| Reducing the frictional forces (e.g. more aerodynamic)
117
On a velocity-time graph, what feature of the graph shows that the object has reached terminal velocity?
A gradient that equals 0 from a steady increasing gradient
118
Describe what happens when an object falling through the air reaching terminal velocity
The weight of the object is a constant force of acceleration downwards. Air resistance is a frictional force opposing this motion, which increases with speed. The object reaches a point where the resistance forces are equal to the driving force, so it reaches terminal velocity
119
What is the equation for momentum?
Momentum = Mass x Velocity
120
What type of quantity is momentum, scalar or vector?
Vector
121
What is momentum?
It's the product of an object's mass and velocity
122
What is the SI unit of momentum?
kg ms^-1
123
What is the equation for density?
Density = Mass/Volume
124
Assuming no external forces act, linear momentum is always...?
Conserved
125
What is the total linear momentum of 2 objects before they collide equal to?
The total linear momentum after the collision
126
Explain what happens when you fire a rifle in terms of the principle of linear momentum
The forward momentum gained by the bullet is equal in magnitude to the backward momentum of the rifle (the recoil)
127
What is an elastic collision?
A collision where momentum is conserved and kinetic energy is conserved
128
What is kinetic energy?
The energy an object has due to its motion
129
What is the equation for kinetic energy?
Ek = 0.5 mv^2
130
What does it mean if a collision is inelastic?
Some of the kinetic energy is converted into other forms during the collision
131
What is the SI units for rate of change of momentum?
kg ms^-2
132
What is force equal to in terms of momentum?
Force = Change in momentum = Δ(mv)/ Δt
133
What equation links change in momentum, mass, initial velocity and final velocity?
Δ(mv) = mv - mu
134
What is impulse?
The impulse acting on an object is equal to the change in momentum of the object
135
What is the SI unit of impulse?
Ns
136
What is impulse defined as?
The product of force and time
137
What represents impulse on a force-time graph?
The area under the graph
138
How can you reduce the force of an impact?
By increasing the time of the impact, as force = change in momentum
139
How can the force of an impact be increased?
By reducing the time of the impact
140
What are 3 safety features that cars have to decrease the force of a crash?
Crumple zones
Seat belts
Airbags
141
How do crumple zones on a car work?
The parts of the car at the front and back crumple up and deform plastically on impact. This causes the car to take longer to stop, increasing the impact time and decreasing the force on the passengers
142
How do seat belts work?
They stretch slightly, increasing the time taken for the wearer to stop, thus decreasing the force
143
How do airbags work?
They slow down the passengers more gradually, and prevent them from hitting any hard objects
144
Work is done whenever...?
… energy is transferred
145
What does work mean in physics terms?
The amount of energy transferred from one form to another when a force causes a movement of some sort
146
What is the equation for work done?
Work done = force x distance
| W=Fs
147
What does 1 joule mean?
The work done when a force of 1 newton moves an object through a distance of 1 metre
148
What is the equation for work done when the force is at an angle?
W = Fs cosθ
θ = angle the line of action of the force makes with direction of motion
149
What is power?
The amount of energy transferred from one form to another per second
150
What is the equation linking power, work done and time?
P = (ΔW)/(Δt)
151
What is a watt defined as?
Rate of energy transfer, equal to 1 joule per second
152
What is the area under a force-displacement graph equal to?
Work done
153
Why can't you use the equation W=Fs for a variable force?
Because the force isn't constant
154
What is the equation linking power, force, speed and angle at which the force acts to the direction of motion?
P = Fv cosθ
155
What does the principle of the conservation of energy state?
Energy cannot be created or destroyed, only transferred from one form to another (total amount of energy stays the same in a closed system)
156
What is efficiency?
The ratio of useful energy given out by a machine to the amount of energy put into a machine
157
What is the equation for efficiency of energy?
Efficiency = Useful energy output / Energy input
158
What is the equation for efficiency of power?
Efficiency = Useful power output / Power input
159
What is kinetic energy?
The energy of anything moving
160
What is the equation for kinetic energy?
Ek = 0.5mv^2
161
What is a kJ equal to?
1000J
162
When an object is falling, what type of energy does it lose?
Gravitational potential
163
What is gravitational potential energy?
The energy something gains when you lift it up in a gravitational field
164
What is the equation for gravitational potential energy?
Ep = mass x gravity field strength x height
165
What is elastic potential energy?
The energy stored in a stretched material
166
What is the equation for elastic potential energy?
E = 0.5 x spring constant x extension^2
167
Describe the energy transfers that occur when you throw a ball up
Kinetic energy is transferred to gravitational potential energy
168
Describe the energy transfers that occur when a ball is falling towards the ground
Gravitational potential energy is transferred to kinetic energy
169
If you're ignoring frictional forces, what is the equation to use for solving problems with energy conservation?
Change in potential energy = Change in kinetic energy
mgh = 0.5mv^2
