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GCSE Physics > Forces > Flashcards

Forces Flashcards

1
Q

Scalar quantities?

A

Quantities that only have magnitude but no direction.

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2
Q

Vector quantities?

A

Quantities that have magnitude and an associated direction.

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3
Q

How can vector quantities be represented?

A

Using an arrow.

Arrow length represents: The vector’s magnitude.
Arrow direction represents: The vector’s direction.

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4
Q

Examples of scalar quantities:

x6

A
  • Mass
  • Energy
  • Speed
  • Temperature
  • Distance
  • Time
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5
Q

Examples of vector quantities:

A
  • Displacement
  • Force (eg weight)
  • Velocity
  • Acceleration
  • Momentum
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6
Q

What is meant by a force?

A

A push or pull that acts on an object due to its interaction with another object.

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7
Q

True or false, a force is a vector quantity?

A

True.

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8
Q

Forces can be divided into what two categories?

A

Contact and Non-contact force.

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9
Q

When two objects experience a contact force due to each other ie a contact force occurs when:

A

The objects are physically touching

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10
Q

When two objects experience a non-contact force due to each other:

A

The objects are physically separated.

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11
Q

Examples of contact forces:

A
  • Tension
  • Friction
  • Air resistance
  • Normal contact force (a force acting perpendicular to a surface)
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12
Q

Examples of non-contact forces:

A
  • Gravitational force/gravity
  • Electrostatic force of repulsion and attraction (force between two charged objects due to their electric field)
  • Magnetic force (the force between two objects due to their magnetic field)
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13
Q

What is mass?

A

The amount matter an object has in it

  • The mass of an object does not depend on where the object is, unlike weight.
  • Mass is a scalar quantity bc it only has magnitude.
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14
Q

What is meant by weight?

A

The force acting on an object’s mass due to gravity.

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15
Q

Force is measured in?

A

Newtons, (N)

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16
Q

Mass is measured in?

A

Kilograms, (kg)

(grams -> kg = divide by 1000)

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17
Q

Weight is measured in?

A

Newtons, (N)

  • because it is a force.
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18
Q

What is the gravitational field strength, g, of the Earth?

A

9.8 N/kg

  • SYMBOL: (g)
  • UNIT: N/kg
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19
Q

Equation to calculate weight?

A

Weight (N) = Mass (kg) x Gravitational field strength (N/kg)
WEIGHT ACTS DOWNWARDS

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20
Q

What can we say about the relationship between weight and mass?

A

They are directly proportional.

Weight ∝ Mass

^^If we double the mass, the weight doubles.

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21
Q

How can we determine an object’s weight?

A

By using a calibrated spring balance or Newtonmeter.

NOT A NORMAL SCALE! THAT’S FOR MASS

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22
Q

Gravitational field strength is:

A

The measure of the force of gravity in a particular location.

  • This is the reason why the weight of an object varies depending on its location.
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23
Q

What is meant by the centre of mass?

A

The point where the weight of an object may be considered to act at a single point.

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24
Q

What is meant by a resultant force?

A

A single force that has the same effect as all the original forces acting together.

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25
Q

To calculate the resultant force of an object what do we do?

A

Subtract the smaller force from the larger force.

If there’s more than one force subtract the total force acting in one direction from the total force acting in the opposite direction.

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26
Q

A car has a driving force of 10 000N and there’s a force of friction of 5000N acting between the tyres and the car, what is the resultant force?

A

10 000N - 5000N = 5000N

Answer = 5000N

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27
Q

If the resultant force is 0N on a flying object in the up/downards direction and the front/backwards direction (ie the forces are balanced) describe the:

Velocity
Acceleration
Altitude (height above the ground)

A

Velocity = constant
No acceleration
Altitude = constant

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28
Q

Draw the free-body diagram for a sky-diver with a constant velocity and altitude:

A

Upward force = lift
Downward force = weight
Forward force = thrust
Backward force = Air resistance/Drag

Bc constant velocity and altitude all the arrows should have the same length.

There should be just a circle in the centre to represent the object.

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29
Q

State the forces acting on a sky-diver:

A

Upward force = lift
Downward force = weight
Forward force = thrust
Backward force = Air resistance/Drag

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30
Q

We can calculate the resultant force of forces are acting parallel to each other.

So what how do we calculate the resultant of two forces acting at an angle?

A
  • We can use the parallelogram end-to-end method (check notes)
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31
Q

Check notes on:

Drawing the resultant force of two forces acting at an angle.

Finding the horizontal and vertical components of a force acting at an angle.

A

Remember, a single force can be resolved into two components acting at right angles to each other.

The two components have the same effect as the single force.

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32
Q

When is work done….

A

Energy is transferred.

Work is done when a force causes an object to move through a distance.

This causes an energy transfer.

So ‘Work’ can be seen to be a measure of energy transfer due to a force over a distance.

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33
Q

To calculate work done:

When calculating work done the distance must be:

A

Work done (J) = Force (N) x Distance (m)

‘The perpendicular distance from the pivot to the line of action of the force’

W = Fs

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34
Q

Work done is measured in:

'’One joule of work is done when…..’’

A

Joules. (J)

One joule of work is done when a force of 1 Newton causes a displacement of 1 metre.

So 1J = 1Nm

^Use this fact to convert between J and Nm.

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35
Q

The force of friction due to two objects causes an increase in the temperature of those objects, true or false?

A

True.

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36
Q

True or false:

Elastic materials always return to their original shape or length when we take away a force acting on them.

A

True, these forces can be involved in stretching, bending or compressing the elastic material.

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37
Q

What is meant by elastic deformation?

A

A temporary change in the form (shape/length) of an object due to applied forces, which reverts to its original shape once the forces are removed.

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38
Q

In order to change a stationary object’s shape/length what do have to do?

A

We have to apply more than one force.

If only apply one force the object simply loses balance and moves.

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39
Q

Inelastic deformation occurs when:

A

Two or more forces are applied to an object resulting in the object’s change in shape and length, but when those forces are removed the object doesn’t return back to its original shape and length.

Inelastic deformation occurs with inelastic materials.

Elastic deformation occurs with elastic materials.

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40
Q

How can we calculate the force needed to stretch an elastic object?

A

Force = spring constant x extension

F = ke

spring constant, k = N/m (Newtons per metre)
extension, e = m (metres)

extension can apply to:

compression (caused due to squeezing)
extension (caused due to stretching)

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41
Q

A force that stretches (or compresses) a spring does work and elastic potential energy is stored in the spring.

Provided the spring is not ________
the work done on the spring and the elastic potential energy store in the spring are equal.

Fill the gap.

A

Inelastically deformed.

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42
Q

What is the moment of a force?

How can we calculate the moment of a force?

A

The turning effect of a force.

Moment, M (Nm) = Force, F (N) x Distance, (m).

M = Fd

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43
Q

When we calculate the moment of a force, the distance must be:

A

Perpendicular from the pivot to the line of action of the force.

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44
Q

Moments is measured in what, and not what?

A

Newton-metre (Nm)

NOT

Newtons per metre (that relates to spring constant)

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45
Q

When we are calculating the moment, what must the distance be?

A

The perpendicular distance from the pivot to the line of action of the force (the force must act at a right angle to the pivot)

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46
Q

If an object is balanced what can we say about the relationship between the total clockwise moment and total anticlockwise moment about a pivot?

A

The total clockwise moment and the total anticlockwise moment about the pivot must be equal for the object to be balanced, think of the see-saw.

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47
Q

Complete PQ on balanced moments.

A
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48
Q

Explain in terms of moments why an object may topple.

A
  • The weight of an object acts directly down the object’s centre of mass.
  • If the object is tilted the line of action of the weight will lie outside the base of the object.
  • This causes a resultant/unbalanced moment (ie the total CWM isn’t equal to the total ACWM) acting on the block
  • So the block topples.
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49
Q

What do levers do?

A

They transmit the turning effect of a force.

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50
Q

Information about levers:

A
  • Levers are force multipliers.
  • If the input force and output force are on different sides of the pivot then the forces will act in different directions (eg scissors)
  • If the input force and the output forces are on the same side of the pivot then the forces will act in the same direction (eg wheel barrow)
  • The input force is further away from the pivot than the output force is to produce a greater moment.
    ^^Force is applied further away from pivot = greater moment due to greater distance.
51
Q

Levers allow us to do what?

A

Lift a heavy object by applying a relatively small amount of force.

Force applied = effort
Object being lifted = load

(Force is applied further away from pivot = greater moment due to greater distance).

52
Q

Gears:

How do they work?

Explain in terms of gear A (smaller gear) and gear B (larger gear).

A
  • Gear A is the smaller gear and is connected to the engine of which provides it with a turning force so that it rotates.
  • Gear B is connected to the wheels and cause them to rotate.
  • Each time gear A rotates due to the engine, gear B rotates so that the wheels rotate - so gears allow the turning effect in the engine to be transmitted to the wheels.

^^The gears turn in opposite directions, if A rotates ACW then B CW.

Why are the gears different sizes?
- The radius of gear B is twice the radius of gear A.
- This allows for the moment of gear B to be twice the moment of gear A, so there’s a greater turning effect transmitted to the wheels.

  • So the turning effect of the engine has been doubled as it has been passed to the wheels.

Work done on the teeth of the gears is still the same, this is bc gear A rotates twice for every time gear B rotates once.

53
Q

What does the turning effect of a gear depend on?

A

The distance between the edge of the gear and the centre of the gear.

54
Q

A fluid can either be a liquid or a gas, is this true or false?

A

True

A fluid can refer to a liquid or a gas.

55
Q

Pressure in fluids causes:

A

A force normal (at right angles) to any surface in the fluid.

56
Q

What is the unit of pressure?

A

The Pascal (Pa).

57
Q

How can we calculate the pressure of a fluid?

A

Pressure (Pa) = The force normal to the surface (N)/ area of that surface (m^2)

UNITS:

Pressure = Pa
The force normal to the surface = N
The area of the surface = m^2

Pa = F/A

^remember to convert from km to m. (x1000)

58
Q

Why do crisp packets swell up in aeroplanes?

A
  • When aeroplanes are flying the air pressure in the cabin (atmospheric pressure) is lower than the air pressure in the bag.
  • Due to diffusion air particles move from an area conc. to higher conc.
  • So they do this in the packet causing the crisp packet to inflate.
59
Q

What is the atmosphere?

A

A thin layer of air around the Earth.

60
Q

Describe a simple model of the atmosphere and of atmospheric pressure.

A
  • The atmosphere: a thin layer of air around the Earth.

Describe atmospheric pressure:

  • Air molecules colliding with a surface create atmospheric pressure.
  • The no. of air molecules above a surface decreases as the height of the surface above ground level increases.
  • Which is why atmospheric pressure decreases with an increase in height.

^^This is why mountain climbers often take a supply of oxygen.

61
Q

When we have a column of water filled up and we poke holes in it, how come water from the lower holes squirt out further than water from the holes above?

A

Water from the bottom of the container is at a higher pressure than the water from the top of the container.

62
Q

We know how to calculate the pressure of any fluid, but how can we calculate the pressure specifically in a column eg a lake?

A

Pressure in a column = height of the column x density if the liquid x gravitational field strength.

P = Hpg

UNITS:

Pressure = Pascal, Pa
Height = metres
p (density) = kg/m^3
gravitational field strength = N/kg

63
Q

Why does the pressure of a liquid in a column increase with its depth?

A

As the depth increases, there’s a greater weight of liquid acting downwards.

64
Q

Why does the pressure of a liquid increase with its density?

A

Liquids with a greater density have a greater weight acting downwards.

65
Q

What are the factors that effect the pressure in a liquid?

A
  • The depth of the liquid
  • The density of the liquid
  • The gravitational field strength

^Think of pressure in a column equation.

66
Q

What causes a liquid to float or sink?

A
  • When an object is submerged in a liquid it will experience a force due to collisions of surrounding liquid molecules and its surface.
  • Because the bottom of the object is deeper in the liquid than the top it, the bottom now experiences a greater force exerted by particles than the top.
  • The object now experiences a larger upward force than a downward force.
  • So there is a resultant force acting upwards which pushes the box upwards (this force is called upthrust).
  • If the object’s weight is larger than the upthrust the object will sink.
  • If the object’s weight is smaller than the upthrust then the object will float.
67
Q

A more dense object will?

A less dense object will?

A

Sink

Float

68
Q

Whether an object will sink of float depends on?

A
  • The upthrust and the weight.

DOWNARD FORCE (WEIGHT) V UPWARD FORCE (UPTHRUST) TO CREATE A DOWNARD/UPWARD RESULTANT FORCE WHICH MAY CAUSE SINKING/FLOATING

69
Q

True or false, speed does not involve direction?

Is the speed of a moving object constant?

A

True, so speed is a scalar quantity.

Rarely.

70
Q

What factors affect the speed of an object?

A
  • Age (speed of a moving being)
  • Terrain
  • Fitness
  • Distance travelled
71
Q

How can we calculate speed?

A

Speed = Distance/time

s = v/t

s = m/s
v = m
t = s

^This can be rearranged to give distance

v = st

72
Q

What is the typical walking speed?

A

1.5 m/s

73
Q

What is the typical running speed?

A

3 m/s

74
Q

What is the typical cycling speed?

A

6 m/s

REMEMBER:

'’she WALKED, then RAN, then got tired so she CYCLED’’

  • walk speed 1.5
  • ran speed 3
  • cycle speed 6

(all in m/s and they double)

75
Q

What is the typical speed for a car on main road?

A

13 m/s

76
Q

What is the typical speed for a fast train in the UK?

A

50 m/s

77
Q

What is the typical speed for a cruising aeroplane?

A

250 m/s

78
Q

What is the typical speed of sound in air?

A

330 m/s

^This can very eg may travel fast on warmer days than on cooler days.

79
Q

Define velocity:

Is velocity a vector or scalar quantity?

A

Speed in a given direction.

Eg 9m/s NORTH.

Bc velocity has both magnitude and direction it is a VECTOR quantity.

80
Q

How do we calculate velocity?

A

In the same way we calculate speed, but we also add the direction (can use a compass given or may be stated).

81
Q

True or false, if an object moves in a circle its velocity is always changing even though its speed remains constant?

Explain why.

A

True.

Changing velocity is due to the object changing direction as it goes around the circle, though the speed remains constant.

So therefore its acceleration is changing.

Think of NESW example and a car on a roundabout.

82
Q

The gradient of a distance time graphs shows:

A

An object’s speed OR An object’s velocity.

So to calculate the speed of an object from a distance time graph what can we do?

Calculate the gradient of the line.

Change in y/change in x

83
Q

In a distance time graph is the slope is curved:

Upwards:
Downards:

What does the tell us?

A

The object is accelerating.
The object is decelerating.

84
Q

If an object is accelerating (shown by slope) how can we determine its speed at a given time?

A

By drawing a tangent at the time and calculating change in y/change in x.

85
Q

Acceleration:

A

The change in the velocity of an object over a given time.

measured in metres per second squared (m/s^2)

86
Q

How can we calculate acceleration?

A

Acceleration (m/s^2) = Change in velocity (m/s)/time (s)

WHAT IS ACCELERATION MEASURED IN?

m/s^2

Slowing down: deceleration.

delta v/t

87
Q

The gradient of a velocity-time graph tells us:

A

The acceleration of the object.

so just change in y/change in x

(remember: the gradient of a distance-time graph tells us the speed/velocity, the gradient of a velocity-time graph tells us its acceleration).

Horizontal line on a velocity time graph tells us there’s no change in velocity.

An upward sloping line tells us that the object is accelerating.

A downward sloping line tells us that the object is decelerating.

88
Q

How can we work out the displacement of an object from a velocity time graph?

A

From the area under the graph.

  • Can split into shapes if it shows clear shapes.
  • Count squares and estimate the parts of squares.
89
Q

How can we calculate acceleration if it is occurring at a constant rate?

A

v^2 - u^2 = 2as

v = final velocity
u = initial velocity
a = acceleration
s = distance

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

^we can rearrange this to work out whatever we need.

90
Q

Near the surface of the Earth, an object falling freely under gravity has an acceleration of:

A

9.8m/s^2

^an object falling ONLY due to gravity, no other forces acting on it :)

91
Q

Describe how terminal velocity is reached.

A
  • An object falling through a fluid initially accelerates due to the force of gravity (weight) which acts downards causing it to accelerate.
  • As it accelerates it experiences an upwards force of friction with fluid particles (air resistance if in air).
  • Eventually the friction balances out the force due to gravity acting on the object.
  • So the RF becomes 0N and the object will stop accelerating and will move at a constant velocity.
  • This is called terminal velocity.
92
Q

What does the terminal velocity of an object depend on?

A
  • The object’s shape which may cause them to have a lower/higher force of friction and so a lower/higher terminal velocity.
  • The object’s mass
93
Q

What is Newton’s first law of motion?

A
  • If the resultant force acting on an object is zero and the object is stationary then the object will remain so.
  • If the resultant force acting on an object is 0 then the object will continue to move with the same velocity (same direction w same speed)

The velocity of an object will only change if a resultant force is acting on it.

94
Q

What are the resistive forces of a vehicle moving?

A
  • Friction with the air (car body and air)
  • Friction with the road (tyres and road)
95
Q

What happens when a resultant force acts on an object?

Conditions for deceleration and accelleration.

A
  • If acts in the opposite direction at which object is moving -><- the object will decelerate.
  • If acts in the same direction&raquo_space; will accelerate.
96
Q

True or false a resultant force can change the direction and speed of an object?

A

True

97
Q

State Newton’s second law of Motion:

A

The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to the mass of the object.

98
Q

How can we calculate the force needed to accelerate an object?

A

Force = Mass x Acceleration

F = MA
N. Kg m/s^2

99
Q

QUESTIONS:

Cars travel at a speed of what on main road?

Cars travel at a speed of what on a motorway?

To accelerate from main road to motorway an acceleration of how much is involved?

For a typical family car this would require a force of how much?

A

1) 13m/s
2) 30m/s
3) 2m/s
4) 2000N

100
Q

What is meant by inertia?

A

The tendency for an object to continue in its state of rest or motion.

101
Q

What is meant by inertial mass?

A

The measure of how difficult it is to change the velocity of an object.

SO:

An object with a larger inertial mass will require a larger force to produce a given acceleration than an object with a smaller inertial mass, true or false?

TRUE.

102
Q

What is Newton’s third law of motion?

A

'’Whenever two objects interact the forces they exert on each other are equal and opposite’’

  • Equal in magnitude.
  • Opposite in direction.
103
Q

How may Newton’s third law be applied to a skater situation?

A

SKATEBOARDER:

  • When a skateboarder jumps off the skateboard they apply a push force onto the skateboard.
  • This causes the skateboard to move to the right.
  • At the same time the skateboard exerts an upward force equal in magnitude to the skater.
  • This causes the skater to move to the left.
104
Q

Describe the forces acting on a sky diver:

A
  • As soon as the sky diver jumps out the plane the only force acting on it is weight due to gravity and this does not change throughout the whole journey.
  • Due to weight the SD experiences a resultant force acting downwards so they accelerate towards the ground.
  • As they fall they experience air resistance due to friction w air particles which acts upwards.
  • Bc weight is still greater than AR the SD continues to accelerate towards the ground.
  • As the SD velocity increases, their air resistance also increases until the AR balances out the weight.
  • Now there is a RF of 0N and the SD has reached terminal velocity.
  • This velocity is extremely great and the SD would die if they hit the ground.
  • So the SD opens a parachute which causes the surface area to increase.
  • The increase in SA causes AR to increase so the air resistance is greater than the weight.
  • Now the SD decelerates (Velocity decreases) bc there is a resultant force acting upwards.
  • Bc their velocity has decreased the AR also decreases until it balances the weight and the RF becomes zero.
  • At this point the sd is now falling at a lower terminal velocity that is safe enough for them to hit the ground.
105
Q

Draw and describe a graph for the sky diver.

A
106
Q

What is the stopping distance of a vehicle?

A

The sum of the distance a vehicle travels during a driver’s reaction time and the distance the vehicle travels under a braking force.

Distance travelled in reaction time = thinking distance.

Distance travelled under braking force = braking distance.

Stopping distance = Thinking distance + braking distance.

107
Q

Stopping distance =

A

Thinking Distance + Breaking distance

108
Q

For a given braking force, the greater the speed of the vehicle:

A

The greater the stopping distance.

109
Q

What is meant by the thinking distance?

A

The distance travelled by a vehicle during the driver’s reaction time.

110
Q

What are the factors which affect reaction time and therefore thinking distance?

x3

A
  • Tiredness
  • Distractions
  • Drugs and alcohol

^This increases the thinking distance

111
Q

Watch a video on reaction time RP (bio).

A
112
Q

Typical reaction times:

A

0.2s - 0.9s

113
Q

Which factors affect the braking distance?

A
  • Adverse weather conditions eg wet/icy (reduces friction between tyres and road thus increasing the braking distance)
  • Conditions of the car’s tyres.
  • Conditions of the car’s brakes.
114
Q

Describe the energy transfers when a car is braking:

A
  • The brake presses against the wheel.
  • A force of friction acts between the brake and the wheel.
  • The kinetic energy in the moving car is now converted into thermal energy in the brakes.
  • This causes the temperature of the brakes to increase.
  • At the same time the car slows down as it loses kinetic energy.
115
Q

We know that the greater the speed, the greater the braking force needed to stop the car in a certain distance.

So if a car is travelling at high speeds and the driver needs to brake the driver needs to apply a large braking force to the car wheels.

A large breaking force will call the car to decelerate rapidly.

What are the dangers of these large decelerations?

A
  • Overheating: A large braking force will cause the car to decelerate rapidly resulting in a large amount of KE being transferred to the thermal ES of the car. This may cause overheating.
  • Loss of control
116
Q

Equation for momentum:

A

Momentum = mass x velocity

p = mv

momentum = kilograms metre per second, kg m/s.
mass = kilograms, kg
velocity = m

117
Q

True or false, all moving objects have momentum?

If an object is not moving then what can we say about its momentum?

A

True.

Stationary objects have a momentum of 0kg m/s.

118
Q

What is meant by the conservation of momentum?

A

In a closed system the total momentum before an event is equal to the total momentum after the event.

119
Q

Explain in terms of conservation of momentum what happens when a moving truck collides with a stationary car:

A
  • Moving car has momentum.
  • It collides w the stationary car which has no momentum.
  • Now both the van and car move together at a lower initial velocity of the van.
  • Now the total momentum of the car and the van is equal to the initial momentum of the van by itself.
  • So momentum has been conserved.
120
Q

True or false, rapid changes in momentum can be extremely dangerous?

A

True.

During a car crash, the momentum of the passengers falls from a large amount to zero in less than a few seconds.

This places huge forces on the passengers which could be lethal.

121
Q

True or false when a force acts on an object that is moving or able to move a change in momentum occurs?

A

True.

122
Q

The equations F = ma and a = v-u/t (aka delta v over t) combine to give:

A

F = m x change in velocity/change in time.

in this equation m x change in velocity is the same as change in momentum.

So this equation is the same as saying force = the rate of change in momentum

123
Q

How can we reduce the dangers of rapid changes in momentum?

A
  • By making the decrease in momentum occur over a longer period of time.

eg:

  • Air bags
  • Seat belts in a car
  • Crash mat
  • Cushioned surface
  • Bike helmet

^These slow down momentum change and reduce the forces acting which reduce the risk of serious injury.

GCSE Physics (4 decks)

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