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GCSE AQA Physics > Topic 5 - Forces > Flashcards

Topic 5 - Forces Flashcards

1
Q

What type of quantity is

force?

(scalar or vector)

A

vector

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2
Q

Define

vector quantity.

A

a quantity which has a magnitude and a direction

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3
Q

What are some examples of

vector quantities?

(5)

A
  • force
  • velocity
  • displacement
  • acceleration
  • momentum
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4
Q

Define

scalar quantity.

A

a quantity which only has magnitude and no direction

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5
Q

What are some examples of

scalar quantities?

(5)

A
  • speed
  • distance
  • mass
  • temperature
  • time
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6
Q

Define

force.

A

a push or a pull on an object that is caused by it interacting with something

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7
Q

Define

contact force.

A

when two objects have to be touching for a force to act

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8
Q

What are some examples of

contact forces?

(3)

A
  • friction
  • air resistance
  • tension in ropes
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9
Q

Define

non-contact force.

A

when the objects do not need to be touching for the force to act

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10
Q

What are some examples of

non-contact forces?

(3)

A
  • magnetic force
  • gravitational force
  • electrostatic force
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11
Q

Define

interaction pair.

A

a pair of forces that are equal and opposite and act on two interacting objects

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12
Q

What are the

two important effects of gravity

around a planet?

A
  • on the surface of a planet, it makes all things
  • it gives everything a weight
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13
Q

What is

mass?

A

the amount of ‘stuff’ in an object

for any given object this will have the same value anywhere in the universe

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14
Q

Define

gravity.

A

the pull of the gravitational forces on the object

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15
Q

Define

weight.

A

the force acting on an object due to gravity

close to earth, this force is caused by the gravitational field around the earth

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16
Q

How does

gravitational field strength vary?

A

it varies with location

it’s stronger the closer you are to the mass causing the field
it’s stronger for larger masses

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17
Q

What is

weight

dependent on?

A

the strength of the gravitational field at the location of the object

this means that the weight of an object changes with its location

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18
Q

What is an object’s

centre of mass?

A

a point at which you assume the whole mass is concentrated

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19
Q

How is

weight

measured?

A

a calibrated spring balance

(or a newtonmeter)

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20
Q

How is

mass

measured?

A

a mass balance (kg)

mass is not a force

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21
Q

What equation links

mass, weight and gravitational field strength?

A

weight = mass x gravitational field strength

W = mg

W: N
m: kg
g: N/kg

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22
Q

What is the relationship between

weight and mass?

A

they are directly proportional

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23
Q

What equation links

distance, force and work done?

A

work done = force x distance

W = Fd

W: joules, J
F: newtons, N
d: metres, m

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24
Q

How do you convert between

joules and newton metres?

A

1J = 1Nm

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25
Q

How would you use a

scale drawing to find the resultant force?

A
  1. Choose a sensible scale.
  2. Draw all of the forces acting on an object, to scale, ‘tip to tail’.
  3. Draw a straight line from the start of the first force to the end of the last force (this is the resultant force).
  4. Measure the length of the resultant force on the diagram to find the magnitude.
  5. Measure the angle of the resultant force on the diagram to find the direction of the force.
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26
Q

Define

equilibrium.

A

when all of the forces on an object combine to give a resultant force of zero

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27
Q

When has an object been

elastically deformed?

A

when the object can go back to its original shape and length after the force has been removed

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28
Q

When has an object been

inelastically deformed?

A

when the object doesn’t return to its original shape and length after the force has been removed

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29
Q

What equation links

force, spring constant and extension?

A

force = spring constant x extension

F = ke

F: newtons, N
k: newton metres, N/m
e: metres, m

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30
Q

Define

limit of proportionality.

A

the maximum force above which a force against extension graph curves, showing that extension is no longer proportional to force

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31
Q

Describe an experiment that investigates the

link between force and extension.

(5 steps)

A
  1. Measure the natural length of the spring (when no load is applied) with a millimetre ruler clamped to the stand.
  2. Add a mass to the spring and allow it to come to rest.
  3. Record the mass and measure the new length of the spring. The extension is the change in length.
  4. Repeat this process until you have enough measurements (at least 6).
  5. Plot a force-extension graph of your results.

it will only start to curve if you exceed the limit of proportionality

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32
Q

How would you interpret a

force-extension graph?

A
  • when the line of best fit is a straight line it means that there is a linear relationship between force and extension (they’re directly proportional), so the gradient of the straight line is equal to the spring constant
  • when the line begins to bend, the relationship is now non-linear between force and extension
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33
Q

What is the equation for the

work done in stretching a spring?

(as long as the spring is not stretched past its limit of proportionality)

A

EPE = (ke^2) /2

(this is equal to the area under a force-extension graph)

EPE: joules, J
k: newton metres, N/m
e: metres, m

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34
Q

Define

moment.

A

the turning effect of a force

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35
Q

What equation gives the

size of the moment of the force?

A

moment = force x perpendicular distance from pivot to the line of action of the force

M = Fd

M: newton metres, Nm
F: newtons, N
d: metres, m

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36
Q

How do

levers make it easier for us to do work?

A

they increase the distance from the pivot at which the force is applied

this means that less force is needed to get the same moment

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37
Q

What is a

gear?

A

a circular disc with teeth around its edge

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38
Q

What do

gears do?

and how?

A

they transmit the rotational effect of a force from one place to another

this is because their teeth interlock so that turning one causes another to turn, in the opposite direction

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39
Q

Define

fluid.

A

a substance that can ‘flow’ because its particles are able to move around

(liquids or gases)

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40
Q

Define

pressure.

A

force per unit area

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41
Q

What equation allows you to calculate the

pressure at the surface of a fluid?

A

pressure = force normal to surface / area of that surface

p = F/A

p: pascals, Pa
F: newtons, N
A: metres squared, m^2

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42
Q

How does

density affect pressure?

(in a liquid)

A

the more dense a given liquid is, the more particles it has in a certain space, this means that there are more particles that are able to collide so the pressure is higher

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43
Q

How does

density vary in a liquid?

A

it doesn’t
for a given liquid, the density is uniform and it doesn’t vary with shape or size

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44
Q

How does

depth affect pressure

in a liquid?

A

as the depth of the liquids 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

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45
Q

What equation allows you to calculate the

pressure at a certain depth due to the column of liquid above?

A

pressure = density of the liquid x gravitational field strength xheight of column of liquid (the depth)

p = ρgh

p: pascals, Pa
ρ: kg/m^3
g: N/kg
h: metres, m

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46
Q

What is

upthrust?

A

the resultant force of the force exerted on the bottom of a submerged object and the force acting on the top of the object

(the force on the bottom is larger)

this is equal to the weight of fluid that has been displaced by the object

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47
Q

When does an object

float?

A

when the upthrust on an object is equal to the object’s weight

(the forces balance)

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48
Q

When does an object

sink?

A

when the object’s weight is more than the upthrust

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49
Q

What does

an object’s ability to float depend on?

A

its density

An object that is less dense than the fluid it is placed in weighs less than the equivalent volume of the fluid. This means that it displaces a volume of fluid that is equal to its weight before it is completely submerged.
At this point, the object’s weight is equal to the upthrust, so the object floats.

An object that is denser than the fluid it is placed in is unable to displace enough fluid to equal its weight. This means that its weight is always larger than the upthrust, so it sinks.

50
Q

How do

submarines make use of upthrust to sink and rise?

A

Submarines hold large tanks.
To sink, these tanks are filled with water to increase the weight of the submarine (but not the volume) so that it is more than the upthrust.
To rise to the surface, the tanks are filled with compressed air to reduce the weight so that it’s less than the upthrust.

51
Q

What is the relationship between

atmospheric pressure and altitude?

and why?

A

atmospheric pressure decreases with altitude

As the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface.
There are also fewer air molecules above a surface as the height increases. This meanse that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.

52
Q

What is the

atmosphere?

A

a layer of air that surrounds the Earth

this is thin compared to the size of the Earth

53
Q

What is

atmospheric pressure?

A

the pressure that is created on a surface by air molecules colliding with the surface

54
Q

Define

distance.

what type of quantity is this?

A

how far an object has moved

this is a scalar quantity (so it doesn’t involve direction)

55
Q

What is

displacement?

and what type of quantity is this?

A

the distance and direction in a straight line from an object’s starting point to its finishing point

this is a vector quantity

the direction could be relative to a point, or a bearing

56
Q

What is a

bearing?

A

a three-digit angle from north

e.g. 035°

57
Q

What is

speed?

and what type of quantity is this?

A

how fast you’re going with no regard to the direction

this is a scalar quantity

58
Q

What is

velocity?

and what type of quantity is this?

A

a speed in a given direction

this is a vector quantity

59
Q

What equation links

distance, time and speed?

A

distance = speed x time

60
Q

What is the typical speed for

a person walking?

(in m/s)

A

1.5 m/s

61
Q

What is the typical speed for

a person running?

(in m/s)

A

3 m/s

62
Q

What is the typical speed for

a person cycling?

(in m/s)

A

6 m/s

63
Q

What is the typical speed for

a car?

(in m/s)

A

25 m/s

64
Q

What is the typical speed for

a train?

(in m/s)

A

30 m/s

65
Q

What is the typical speed for

a plane?

(in m/s)

A

250 m/s

66
Q

What does

the speed at which a person can travel depend on?

(4 things)

A
  • their fitness
  • their age
  • the distance travelled
  • the terrain
67
Q

What are some examples of things that affect

wind speed?

(3 things)

A
  • temperature
  • atmospheric pressure
  • if there are any large buildings or structures nearby
68
Q

What is

uniform acceleration?

A

speeding up (or slowing down) at a constant rate

69
Q

Define

acceleration.

A

the change in velocity in a certain amount of time

70
Q

What equation allows you to calculate

the average acceleration of an object?

A

acceleration = change in velocity/time

a = Δv / t

a: m/s^2
v: m/s
t: s

71
Q

What is

deceleration?

A

negative acceleration

(if something slows down, the change in velocity is negative)

72
Q

What is

acceleration due to gravity near the Earth’s surface?

A

roughly 9.8 m/s^2

(the same value as gravitational field strength)

this is uniform for objects in free fall

73
Q

What equation links

final velocity, initial velocity, distance and acceleration?

(only for uniform acceleration)

A

(final velocity)^2 - (inital velocity)^2 = 2 x acceleration x distance

v^2 - u^2 = 2ad

74
Q

What does the

gradient of the line represent on a distance-time graph?

A

gradient = speed

(the steeper the graph, the faster it’s going)

75
Q

What does a

flat section represent on a distance-time graph?

A

the object is stationary

76
Q

What does a

straight line represent on a distance-time graph?

A

the object is travelling at a steady speed

77
Q

What does a

curve represent on a distance-time graph?

A

an acceleration or a deceleration

a steepening curve means it’s speeding up
a levelling off curve means it’s slowing down

78
Q

What does the

gradient of the line represent on a velocity-time graph?

A

gradient = acceleration

the steeper the graph, the greater the acceleration or deceleration

uphill sections are acceleration
downhill sections are deceleration

79
Q

What does a

flat section represent on a velocity-time graph?

A

the object is travelling at a steady speed

80
Q

What does a

curve represent on a velocity-time graph?

A

changing acceleration

81
Q

How could you calculate

distance travelled using a velocity-time graph?

A

by calculating the area under any section of the graph

82
Q

In what direction does

friction work?

A

the opposite direction to the movement

83
Q

When do you get

friction?

(2 cases)

A

between two surfaces or when an object passes through a fluid

84
Q

Define

drag.

A

the resistance you get in a fluid

(air resistance is a type of drag)

85
Q

What is the most important factor in

reducing drag?

A

keeping the shape of the object streamlined

86
Q

What makes an object

streamlined?

A

the object is designed to allow fluid to flow easily across it

(this reduces drag)

parachutes work in the opposite way

87
Q

What is the relationship between

friction and speed?

(from fluids)

A

frictional forces from fluids always increase with speed

88
Q

How does a

falling object reach its terminal velocity?

A
  1. When the falling object first sets off, the force of gravity is much more than the frictional force slowing it down, so it accelerates.
  2. As the speed increases the friction builds up.
  3. This gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force (so the resultant force is zero). It will have reached its maximum speed or terminal velocity and will fall at a steady speed.
89
Q

What is

Newton’s First Law?

A

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity.

(a resultant force is needed to make something start moving, speed up, or slow down)

90
Q

What is the relationship between

resultant force and acceleration?

A

they are directly proportional

91
Q

What is the relationship between

acceleration and mass of the object?

(for a fixed resultant force)

A

acceleration is inversely proportional to the mass of the object

so an object with a larger mass will accelerate less than one with a smaller mass (for a fixed resultant force)

92
Q

What equation describes

Newton’s Second Law?

(linking acceleration, resultant force and mass)

A

resultant force = mass x acceleration

F = ma

F: N
m: kg
a: m/s^2

93
Q

Define

inertia.

A

the tendency to continue in the same state of motion

94
Q

What does

inertial mass

measure?

A

how difficult it is to change the velocity of an object

95
Q

How can you calculate

inertial mass?

A

by rearranging Newton’s Second Law

m = F/a

(inertial mass is just the ratio of force over acceleration)

96
Q

What is

Newton’s Third Law?

A

when two objects interact, the forces they exert on each other are equal and opposite

97
Q

Describe how you would carry out the

trolley experiment (which tests Newton’s second law).

(7 steps)

A
  1. Set up two light gates and connect them to a computer.
  2. Set up the trolley so it holds a piece of card in the middle that will interrupt the signal on the light gates.
  3. Connect the trolley to a piece of string that goes over a pulley (at the end of the table) and is connected on the other side to a hook.
  4. Mark a starting line on the table so that the trolley always travels the same distance to the light gate.
  5. Place the trolley on the starting line, holding it so the string is taut, and release it.
  6. Record the acceleration measured by the light gate as the trolley passes through it.
  7. Repeat this twice more to get an average acceleration.
98
Q

What are the

two different variations of the trolley experiment?

A
  1. Investigating the effect of mass.
    Add masses to the trolley one at a time to increase the mass of the system. Record the average acceleration for each mass.
  2. Investigating the effect of force.
    Keep the total mass of the system the same, but change the mass on the hook. To do this, start with all the masses loaded onto the trolley, and transfer the masses to the hook one at a time, to increase the accelerating force. Record the average acceleration for each force.
99
Q

What is an

emergency stop?

(for a car)

A

where maximum force is applied by the brakes in order to stop the car in the shortest possible distance

the longer it takes to do this, the higher the risk of crash

100
Q

What is the general equation to find

stopping distance?

(for a car)

A

stopping distance = thinking distance + braking distance

101
Q

What is the

thinking distance?

(in an emergency stop)

A

how far the car travels during the driver’s reaction time

(the time between the driver seeing a hazard and applying the brakes)

102
Q

What is the

braking distance?

(in an emergency stop)

A

the distance taken to stop under the braking force

103
Q

What is a

typical car braking distance at
30mph?
60mph?
70mph?

A

30 - 14m
60 - 55m
70 - 75m

104
Q

What is

thinking distance

affected by? (2 things)

A
  1. Your SPEED - the faster you’re going the further you’ll travel during the time you take to react.
  2. Your REACTION TIME - the longer your reaction time, the longer your thinking distance.
105
Q

What is

braking distance

affected by? (4 things)

A
  1. Your SPEED - for a given braking force, the faster a vehicle travels, the longer it takes to stop.
  2. The WEATHER or ROAD SURFACE - if it is wet or icy, or there are leaves or oil on the road, there is less grip (and so less friction) between a vehicle’s tyres and the road, which can cause tyres to skid.
  3. The CONDITION of your TYRES - if the tyres of a vehicle are bald (they don’t have any tread left) then they cannot get rid of water in wet conditions. This leads them to skidding on top of the water.
  4. How good your BRAKES are - if brakes are worn or faulty, they won’t be able to apply as much force as well-maintained brakes, which could be dangerous when you need to brake hard.
106
Q

What happens when the

brake pedal is pushed?

(in a car)

A

The brake pads are pressed onto the wheels.

This contact causes friction, which causes work to be done. The work done between the brakes and the wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature.

107
Q

Why are

very large decelerations (due to a very large braking force) dangerous?

A

they may cause brakes to overheat (so they don’t work as well)
or could cause the vehicle to skid

108
Q

What is a

typical huamn reaction time?

A

between 0.2 and 0.9s

109
Q

What affects

reaction time?

(4 examples)

A
  • tiredness
  • drugs
  • alcohol
  • distractions
110
Q

Describe the

ruler drop test.

(for reaction time) (steps)

A
  1. Sit with your arm resting on the edge of a table.
  2. Get someone else to hold a ruler so it hangs between your thumb and forefinger, lined up with zero.
  3. Without giving any warning, the person holding the ruler should drop it. Close your thumn and finger to try to catch the ruler as quickly as possible.
  4. The measurement on the ruler at the point where it is caught is how far the ruler dropped in the time it takes you to react.
  5. You can calculate how long the ruler falls for (the reaction time) because acceleration due to gravity is constant.

(do a lot of repeats and calculate an average)

To calculate this you would use these two equations:
v^2 - u^2 = 2ad
a = Δv/t

111
Q

What are the

typical stopping distances at
30mph?
50mph?
70mph?

A

30mph - 21m (9 + 14)
50mph - 53m (15 + 38)
70mph - 96m (21 + 75)

112
Q

What is the relationship between

braking distance and speed?

A

as speed doubles, the braking distance increase 4-fold

The work done to stop the car is equal to the energy in the car’s kinetic energy store (1/2 mv^2). So, as speed doubles, the kinetic energy increases 4-fold , and so work done to stop the car also increases 4-fold. Since W = Fd and the braking force is constant, the braking distance increases 4-fold.

113
Q

What type of quantity is

momentum?

A

a vector

114
Q

What is the equation for

momentum?

A

momentum = mass x velocity

p = mv

p: kgm/s
m: kg
v: m/s

115
Q

What is the

conservation of momentum?

A

in a closed system, the total momentum before an event, is the same as after the event

116
Q

What equation links

time, mass, force and velocity?

(and sort of momentum)

A

force = (mass x change in velocity) / change in time

F = mΔv / t

momentum = mass x velocity
p = mΔv

so this can be rewritten as F = p / t

117
Q

Why are

cars designed to slow people down over a longer time then they have a crash?

A

The longer it takes for a change in momentum, the smaller the rate of change of momentum, and so the smaller the force. Smaller forces mean the injuries are likely to be less severe.

118
Q

What are some safety features of

cars?

and how do they work? (3)

A
  • crumple zones - crumple on impact, increase the time taken for the car to stop
  • seat belts - stretch slightly, increasing the time taken for the wearer to stop
  • air bags - inflate before you hit the dashboard, slows you down more gradually
119
Q

How does a

bike helmet reduce the severity of injury after a crash?

A

it contains a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash

this reduces the impact on your brain

120
Q

How do

crash mats reduce the severity of injury after a crash?

A

they increase the time taken for you to stop if you fall on them

this is because they are made from soft, compressible materials

GCSE AQA Physics (8 decks)

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