Forces Flashcards
1
Q
What do scalar quantities have?
A
Magnitude
2
Q
What do vector quantities have?
A
Magnitude and direction
3
Q
How are vectors represented?
A
To scale with an arrow:
- specific length representing magnitude
- head representing the direction
4
Q
Give examples of scalar quantities:
A
distance, speed, mass, time, temperature, energy
5
Q
Give examples of vector quantities:
A
displacement, velocity, acceleration, weight, momentum
6
Q
What do combined vectors give?
A
A single resultant
7
Q
What are the steps to find the resultant vector of two vectors at right angles?
A
1) choose scale for drawing
2) draw the two given vectors carefully, measuring the angle between them
3) complete the rectangle/parallelogram
4) draw the diagonal
5) measure the length and angle of the diagonal
8
Q
What can a resultant vector be turned into?
A
Its vertical and horizontal components
9
Q
How do planes keep on their desired course despite the wind?
A
The pilot has to adjust the velocity vector of the plane to compensate -they would have to determine a velocity relative to the wind speed and direction to ensure travel on the desired course
10
Q
What is a contact force?
A
A force which requires physical contact for interaction (ie. to do work)
11
Q
What is a non-contact force?
A
A force which requires no physical contact for interaction/ to do work
12
Q
What are examples of contact forces?
A
friction, tension, air resistance, normal contact force
13
Q
What are examples of non-contact forces?
A
- gravitational attraction or weight
- magnetism
- electrostatic force
14
Q
What is the principle of a gravitational force?
A
A massive body will cause another massive body to accelerate ie. a body’s speed and/or direction will change when the body is in a gravitational field
15
Q
What is weight?
A
The result of the acceleration due to gravity
16
Q
How do we calculate weight?
A
weight = mass x acceleration due to gravity W = mg
17
Q
What is the unit for weight?
A
Newtons
18
Q
What value has the units N/Kg?
A
Gravitational field strength
19
Q
What value has m/s^2?
A
Acceleration due to gravity
20
Q
What is the relationship between weight and mass?
A
they are directly proportional
21
Q
What type of quantity is mass? what does this mean?
A
- an invariant quantity which means it is not dependent on the location of the body
22
Q
What type of quantity is weight? what does this mean?
A
- a variable quantity which means it is dependent on the acceleration due to gravity at the location of the body
23
Q
What do we use to measure weight?
A
a calibrated spring balance (Newtonmeter)
24
Q
What is the centre of mass/gravity?
A
the point associated with a body through which its weight appears to act
25
Where is the centre of mass for a regular lamina?
At the geometric centre of it
26
How would you find the centre of mass for an irregular lamina?
- freely suspend the lamina and a plumb line from three different points on the lamina
- the plumb line will hang along a vertical line that contains the lamina's centre of mass
- where the three lines cross, is the location of the lamin'a centre of mass
27
What is work done?
the energy transferred by a force changing the displacement of a body
28
What is the equation for work?
W = Fs
29
What is the equivalent of 1J of work?
1 Nm
30
What is the work-energy theorem?
When work is done that causes a change in a body's velocirt, there is a corresponding change in the body's kinetic energy
31
What is the equation for the work-energy theorem?
Fs = Δ(0.5 x m x v^2)
32
What does the work energy theorem assume?
No energy is dissipated as thermal energy to the surroundings increasing their internal energy
33
What is Hooke's law?
The extension of a material is directly proportional to the force applied to it
34
What is the equation linked to Hooke's law?
F = ke
35
What types of objects above Hooke's law?
Those which behave elastically
36
What does behaving elastically mean?
That the material returns to its original, undeformed dimensions when the force is removed
37
What is the term used to describe something which behaves oppositely to something elastically?
Plastic
38
What is a machine?
A system that uses force to do work
39
What is a moment?
The result of a turning force acting at a perpendicular distance from a pivot point
40
How do you calculate a moment?
force x perpendicular distance
41
What is the unit for a moment?
Newtonmetre / Nm
42
What is required for an equilibrium for moments?
The sum of the clockwise moments must be equal to the sum of the anticlockwise movements
43
Why is it easier to hold a heavy object closer to your body than at an arms length for example?
- distance is greater between the object and the pivot, the shouder, when the object is held at arms length
- this causes a greater turning effect at the pivot since the turning effect (moment) is equal to distance x force
- this means the body has a greater turning effect to balance with a force from the muscles to hold the object
44
Why is a strong wind needed to topple a lorry?
- to topple the lorry, the weight has to act on the outside of the pivot
- therefore, the centre of mass has to not be inline with the base area/pivot of the lorry and outside of it
- so a wind will need to move the centre of mass a big distance
- so the wind needs to be strong
45
When will a suspended object stop swinging?
when the centre of mass is below the point of suspension
46
What must be true of a see-saw to move anticlockwise?
- the sum of the anticockwise moments must be greater than the sum of the clockwise moments
47
What do levers do?
they provide a large moment by increasing the distance at which the force is applied from the pivot
48
What is a gear?
A machine which transmits a turning/rotational force
49
What is the relationship between the rotation direction of two adjacent gears?
They will rotate in opposite directions
50
What does the rate of rotation of a child gear depend on?
- its radius
- the number of teeth
compared to the parent gear
51
What does the pressure result from in an object immersed in a fluid?
all forces that are normal to the surfaces of the body
52
How due you calculate the pressure due to a depth of fluid?
pressure = density x acceleration due to gravity x depth below surface
53
Why does atmospheric pressure decrease with altitude?
- Pressure results from collisions between a surface and air molecules
- The number of molecules in the air decreases as the altitude decreases as the atmosphere becomes less dense
- This reduces the number of collisions between molecules and a surface
- This reduces the rate of change of momentum which thus reduces the force and consequently the pressure decreases since pressure= force/area
54
What is a fluid?
A substance that can flow because the particles are able to move around
55
What does the pressure in a liquid depend on?
Depth and Density
56
What effect does depth have on pressure in a liquid and why?
As the depth of the liquid increases, the number of particles above that point increases. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth
57
What effect does density have on pressure in a liquid and why?
The more dense a given liquid, the more particles it has in a certain space. This means there are more particles that are able to collide so the pressure is higher.
58
Explain how upthrust works:
- Pressure increases with depth, so the force exerted on the bottom of the object is larger than the force acting on the top of the object
- this causes a resultant force upwards
59
What is the atmosphere?
It is a layer of air that surrounds the Earth
60
Aside from fewer molecule collisions, what else decreases the pressure as altitude increases?
- Fewer air molecules are above a surface as the height increases
- Meaning that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude
61
How does an object float?
When the upthrust on an object is equal to the objects weight, meaning the forces balance
62
How does an object float?
- an object that is less dense than the fluid it is placed in, has a weighs less than the equivalent volume of liquid
- this means it displaces a volume of fluid that is equal to its weight before it is completely submerged
- the objects weight is equal to the upthrust at this point
63
Why does an object sink?
- an object that is denser than the fluid it is placed in, is unable to displace enough fluid to equal its weight.
- thism eans that its weight is always larger than the upthrust
64
How do submarines use upthrust?
- to sink the submarines, large tanks are filled with water to increase the weight of the submarine so that it is more than the upthrust
- to rise to the surface, the submarine tanks are filled with compressed air to reduce the weight so it is less than the upthrust
65
What is speed?
distance/time
66
What is velocity?
the rate of change of displacement - Δs/Δt
67
What is acceleration in words?
the rate of change of velocity
68
What equations can be used to calculate acceleration?
a = Δv/Δt or a = (v-u)/t or a = (v^2 - u^2)/2s
69
SUVAT equation for velocity:
V^2=U^2+2AS
70
What do different lines on a displacement time graph represent?
- flat line means stationary
- flattening curve means constant deceleration
- steepening curve means constant acceleration
- straight line means constant velocity
71
What do different lines on a velocity time graph represent?
- flat line on x axis means stationary
- flat line not on x axis means constant velocity
- straight line with positive gradient means constant acceleration
- straight line with negative gradient means constant deceleration
72
What does the negative y quadrant in a displacement-time graph represent?
When the object moves in the opposite direction from the starting point
73
What does the negative y quadrant in a velocity time graph represent?
when the object is moving backwards
74
Why is the gradient for the velocity of a bouncing ball always constant?
Since the acceleration results from the acceleration due to gracity each time which is constant
75
What is Newton's first law?
A body will remain at rest, or continue moving at a constant velocity unless a non zero resultant force acts on it
76
What is Newton's second law?
The rate of change of a body's momentum is proportional to the non zero resultant force acting on it
77
What is Newton's third law?
If body A exerts a force on body B, then body B exerts a force on Body A which is equal in magnitude but opposite in direction
78
What is inertia?
A massive body's resistance to changes in motion
79
How do you calculate momentum?
mass x velocity
80
What does the rate of change of momentum result in?
A force
81
How do you calculate the force from a mometum?
force = momentum x acceleration
82
What is 1 kgm/s^2 equivalent to?
1N
83
What must the pairs of forces in Newton's third law be?
- equal in magnitude
- opposite in direction
- acting on different bodies
- the same type of force (both pushes or both pulls)
84
What is the third law pair for the weight of a ball?
pull of the Earth on the ball with the pull of the ball on the Earth
85
What are the rules for free body force diagrams?
- only show forces acting on a single body
| - so therefore do not include Newton's third law pairs
86
What is stopping distance?
thinking distance + braking distance
87
What is thinking distance dependent on?
The driver
88
What is the braking distance dependent on?
The car and the environment
89
What equation is used to calculate the magnitude of a braking force?
F = ma
90
Describe how brakes in a vehicle work in relation to energy transfers:
- Brakes exert a force on the wheel of the vehicle
- Friction between the brake and the wheel result in the transfer of kinetic energy to thermal energy
- The thermal energy of the brakes and the wheels is dissipated into the surroundings, increasing their internal energy
91
What is meant by the term thinking distance?
The distance travelled while the driver is reacting
92
What is meant by the term braking distance?
The distance travelled while the driver is braking
93
What is momentum?
A quantity possessed by all massive,moving bodies
94
How is mometum calculated?
mass x velocity
95
How is the change in mometum calculated?
Forces x change in time
96
When will mometum be conserved?
In a closed system ie. a system which has no external forces acting on it
97
What is the Law of the Conservation of Momentum?
In any interaction between two or more bodies, the total momentum before the interaction is equal to the toal mometum after the interaction as long as no external forces act on the system
98
Why is mometum not always conserved?
An external force (to that of the system) may act
99
Why does moving skateboard slow down and lose kinetic energy?
Work is done against friction betwen the skateboard's wheels and the ground, transferring energy from the skateboard's kinetic energy stores to the internal energy stores of the surroundings as thermal energy
