Physics - P3

Front 1

What is the formula for speed, what are the units for speed, how many seconds are there in one hour and why is average speed written?

Back 1

Distance/time = speed.

Units = m/s

There are 3600 seconds in 1 hour.

Average speed is written because the speed of a vehicle changes during a journey.

Front 2

What is the speed of a car at a certain point in time called and what does increasing the speed mean for the car?

Back 2

Instantaneous speed is the name for speed at a certain point in time.

Distance travelled = average speed x time.
- (u+v)/2 x t

Increasing the speed means increasing the distance travelled in the same time.

Increasing the speed reduces the time needed to cover the same distance.

Front 3

What do distance-time graphs allow, what does the gradient tell you and what do the different gradients show?

Back 3

It allows a collection of data to be shown as it is easier to interpret data when they are plotted a graph than when listed in a results table.

The gradient of a distance-time graph tells you about the speed of the object (a higher speed means a steeper gradient).

A straight line shows that the speed is constant.

A curved line shows that the speed is changing.

Front 4

What is a change of speed per unit of time called, what does it mean if something is accelerating/decelerating and how do you work out the distance travelled from a speed-time graph?

Back 4

Change of speed per unit of time is called acceleration.

If speed is increasing the object is accelerating and if the speed is decreasing then the object is decelerating.

The area under a speed-time graph is equal to the distance travelled.

Front 5

What is the formula for measuring acceleration, what is it measured in and what does negative acceleration mean?

Back 5

Change in speed (or velocity)/time taken = acceleration.

Measured in m/s squared.

Negative acceleration shows the car is decelerating.

Acceleration is the gradient of a speed-time graph.

Front 6

What is velocity and what is relative velocity?

Back 6

Velocity is a vector because it has both size (speed) and direction.

When 2 cars are moving past each other, their relative velocity is:
- the sum of their individual velocities if they are going in opposite directions.
- the difference of their individual velocities if they are going in the same direction.

Front 7

How do vehicles go around roundabouts?

Back 7

A vehicle may go around a roundabout at a constant speed but it is accelerating and this is because its direction of travel is changing - it's not going in a straight line.

The driver needs to apply a force towards the centre of the roundabout to change direction which gives the vehicle an acceleration directed towards the centre of the roundabout.

Any object moving along a circular path moves at a tangent to the circle, or arc of a circle.

Front 8

What is the equation for force, what can this also be used for, what are the units of force and talk about balanced and unbalanced forces?

Back 8

Force = mass x acceleration (f = m x a).

It is used to find mass or acceleration if resultant force is known.

Where F = unbalanced force in N, m = mass in kg and a = acceleration in m/s squared.

If the forces acting on an object are balanced, it's at rest or has a constant speed.

If the forces acting on an object are unbalanced, it speeds up or slows down.

Front 9

What is thinking distance, what is braking distance and what is the equation concerning these two factors?

Back 9

The thinking distance is the distance travelled in between the driver realising he needs to brake and actually braking.

The braking distance is the distance taken to stop once the brakes are applied.

Stopping distance = thinking distance + braking distance.

Front 10

Why may reaction time and thinking distance increase?

Back 10

Reaction time, and therefore thinking distance may increase if a driver is:
- tired.
- under the influence of alcohol or other drugs.
- travelling faster.
- distracted or lacks concentration.

Front 11

Why may braking distance increase?

Back 11

Braking distance may increase if:

- road conditions are poor e.g. icy.
- the car has not been properly maintained e.g. worn brakes.
- the speed is increased.

Front 12

How should you drive safely?

Back 12

For safe driving it is important to be able to stop safely:
- keep an appropriate distance from the car in front.
- have different speed limits for different types of road and locations.
- slow down when road conditions are poor.

Front 13

What are the factors affecting braking distance?

Back 13

The greater the mass of a vehicle the greater its braking distance.

The greater the speed of a vehicle the greater its braking distance.

When the brakes are applied the brake pads are pushed against the disc which creates a large friction force that slows the car down:
- worn brakes reduce the friction force, increasing the braking distance.

Worn tyres with very little tread reduce the grip of the wheels on a slippery road, leading to skidding and an increase in braking distance.

Increased braking force reduces the stopping distance.

Front 14

Describe the relationship between thinking distance and speed, and braking distance and speed.

Back 14

Thinking distance increases linearly with speed.

Braking distance increases as a squared relationship e.g. the braking distance at 60mph is 9 times the braking distance at 20mph.

Front 15

When is work done, what is the equation and what are the units?

Back 15

Work is done when a force moves an object in the direction in which the force acts.

Work done = force x distance moved (in the direction of the force).

Force N, distance m, work done J.

Front 16

What is weight a measure of, what is the formula and what does a mass of 1kg have on Earth:

Back 16

Weight is a measure of the gravitational attraction on a body acting towards the centre of the Earth.

Weight = mass x gravitational field strength.

A mass of 1kg has a weight of about 10N on Earth.

Front 17

What is power, what is the formula, what are the units, how can it be rearranged and when is a person's power greater?

Back 17

Power is the rate at which work is done.

Work done/time taken = power.

Work done/power = time taken.
Work done = power x time

Time = s, power = W, work done = J

A person's power is greater when they run than when they walk.

Front 18

Why are some cars more powerful than others and what is the problem with fuel and fuel consumption?

Back 18

Some cars are more powerful than others:

- they travel faster and cover the same distance in a shorter time and require more fuel.

- the power rating of a car depends on its engine size.

Fuel:

- more powerful cars have greater fuel consumption.
- fuel is expensive and a car with high fuel consumption is expensive to run.
- fuel pollutes the environment.
- car exhaust gases, especially carbon dioxide, are harmful.
- carbon dioxide is also a major source of greenhouse gases, which contribute to climate change.

Front 19

How is power related to force and speed?

Back 19

Power = work done/time taken.

Which equals:
Power = (Force x distance)/time taken.

Which equals:
Power = Force x distance/time.

Which equals:
Power = Force x speed.

Front 20

What does the breaking distance of a car increase with, what is the formula for kinetic energy and what are the units?

Back 20

Braking distance increases with increasing speed, but not proportionally.

Kinetic energy = half mv squared.

m = mass in kg, v = velocity in m/s, kinetic energy = energy in J

Front 21

When a car stops what happens to the kinetic energy and what is this shown by?

Back 21

When a car stops, its kinetic energy changes into heat in the brakes, tyres and road, shown by the formula:

- work done by brakes = loss in kinetic energy.

Change in kinetic energy can be shown by this formula:
- braking force x braking distance = loss in KE

When the speed of a car doubles, the KE and braking distance quadruple which is why there are speed limits and penalties for drivers who exceed them.

Front 22

What is fuel consumption data based on and what are the factors that affect fuel consumption?

Back 22

Based on ideal road conditions for a car driven at a steady speed in urban and non-urban conditions.

Factors that affect the fuel consumption of a car are:
- the amount of energy required to increase its kinetic energy.
- the amount of energy required for it to do work against friction.
- its speed.
- how it is driven, such as excessive acceleration and deceleration, constant braking and speed changes.
- road conditions, such as a rough surface.

Front 23

What are the advantages of battery powered electric cars?

Back 23

Don't pollute the local environment.

Cost of recharging batteries is low.

Front 24

What are the disadvantages of electric, battery powered cars?

Back 24

The battery takes up a lot of room.

They have a limited range before recharging.

They are expensive to buy but the cost of recharging is low.

Don't pollute the environment but batteries need to be recharged which uses electricity from a power station and power stations pollute the local atmosphere and cause acid rain.

Front 25

Why have electric cars been developed, what are the pros and cons of solar powered electric cars and why aren't biofuels that good?

Back 25

Electric cars were developed because exhaust fumes from petrol-fuelled cars cause serious pollution in towns and cities.

Solar powered electric cars:

PRO: don't produce carbon dioxide emissions.
CON: rely on the Sun shining and need backup batteries.

Biofuels reduce CO2 emissions but this isn't certain because deforestation leads to an increase in CO2 levels.

Front 26

What is the formula for momentum and what are the units?

Back 26

Momentum = mass x velocity.

Units = kgm/s.

Front 27

How should injuries be reduced in a collision?

Back 27

Forces should be as small as possible:
- force = change in momentum/time.
- spreading the momentum change over a longer time reduces the force.
- to minimise injury, forces acting on the people in a car during an accident must be minimised.
- force = mass x acceleration.

Front 28

How can forces be reduced and how can that be done, also with safety features?

Back 28

Forces can be reduced by reducing the acceleration.

This can be done by:
- increasing stopping or collision time.
- increasing stopping or collision distance.

Safety features that do this include:

- crumple zones.
- seat belts.
- air bags.
- crash barriers.
- escape lanes.

Front 29

What are the safety features that modern cars have?

Back 29

Modern cars have safety features that absorb energy when a vehicle stops suddenly:
- brakes that get hot.
- crumple zones that change shape.
- seat belts that stretch a little.
- air bags that inflate and squash.

Front 30

On impact what do the safety features of a car do?

Back 30

Crumple zones at the front and rear of the car absorb some of its energy by changing shape or 'crumpling'.

Seat belts stretch a little so that some of the person's kinetic energy is converted to elastic energy.

Air bags absorb some of the person's kinetic energy by squashing up around them.

Front 31

What do all safety features on a car do?

Back 31

The safety features:

- absorb energy.
- change shape.
- reduce injuries.
- reduce momentum to zero slowly, therefore reducing the force on the occupants.

Front 32

Why don't some people like wearing seat belts?

Back 32

People don't like seat belts because:

- there is a risk of chest injury.
- they may be trapped in a fire.
- drivers may be encouraged to drive less carefully because they know they have the protection from a seat belt.

Front 33

What helps to provide safety information in crash tests?

Back 33

Despite computer modelling, crash tests using real vehicles and dummies provide more safety information.

Front 34

What is ABS and how does it work?

Back 34

ABS brakes are a primary safety feature which helps to prevent a crash.

- Anti-Lock Brake System

In the ABS system:

- wheel-speed sensors send information to a computer about the rotational speed of the wheels.
- the computer controls the pressure to the brakes, via a pump, to prevent the wheels locking up.
- this increases the braking force (F) just before the wheels start to skid.

Front 35

How do you calculate the kinetic energy lost in a car (to do with brakes) and what is the relationship between force and braking distance?

Back 35

Kinetic energy lost = work done by brake.
- half mv squared = force x distance.
- m=mass of car, v=speed of car before breaking and d=braking distance.

If the force increases, the braking distance decreases.

Front 36

What are other primary safety features in a car?

Back 36

Cruise control which stops a driver becoming tired on a long journey and pressing harder on the accelerator.

Electric windows and paddle shift controls which allow the driver to concentrate on the road.

Front 37

What do all objects fall with?

Back 37

The same acceleration due to gravity as long as the effect of air resistance is very small.

Front 38

What does the size of the air resistance force on a falling object depend on?

Back 38

Its cross-sectional area:
- the larger the area the greater the air resistance.

Its speed:
- the faster it falls the greater the air resistance.

Front 39

When does air resistance have a significant effect on motion?

Back 39

It only has an effect when it is large compared to the weight of the falling object.

Front 40

Describe what is happening to the parachutist in diagram 1.

Back 40

The weight of the parachutist is greater than air resistance so he accelerates.

The parachutist accelerates, displacing more air molecules every second.

The air resistance force increases which reduces the acceleration.

Therefore the higher the speed, the more resistance.

Front 41

Describe what is happening to the parachutist in diagram 3.

Back 41

The weight of the parachutist and air resistance are equal.

The parachutist has reached terminal speed/velocity because the forces acting on him are balanced.

He travels at a constant speed.

Front 42

Describe what is happening to the parachutist in diagram 4.

Back 42

The air resistance is larger than the weight of the parachutist so he slows down and air resistance increases.

When the parachute opens, the upward force increases suddenly as there is a much larger surface area, displacing more air molecules every second.

The parachutist decelerates, displacing fewer air molecules each second, so the air resistance force decreases.

Front 43

Describe what is happening to the parachutist in diagram 5 and what else uses parachutes.

Back 43

The air resistance and weight of the parachute are the same so he reaches a new, slower terminal speed/velocity so he lands safely.

Drag racers and the Space Shuttle use parachutes to slow them down rapidly.

Front 44

What is the force on each kilogram of mass due to gravity on Earth and what is 'g' also know as?

Back 44

The force on each kilogram of mass due to gravity (g) is approximately 10N on Earth (10N/kg).

'g' is also known as acceleration due to gravity.
- g=10m/s squared approximately.

Front 45

Give 3 facts about gravitational field strength:

Back 45

It is unaffected by atmospheric conditions.

It varies with position on the Earth's surface (9.78N/kg at the equator and 9.83N/kg at the poles).

It varies with height above or depth below the Earth's surface.

Front 46

What does an object held above the ground have, what is this shown by and what is this measured in?

Back 46

Gravitational potential energy.

GPE = mgh:
- m=mass.
- h=vertical height moved.
- g=gravitational field strength (10N/kg).

GPE is measured in joules (J).

Front 47

What does a bouncing ball do in terms of converting energy?

Back 47

Converts gravitational potential energy to kinetic energy and back to gravitational potential energy.

It does not return to its original height because energy is transferred.

Front 48

What is happening to the bouncing ball in this diagram just before it is dropped?

Back 48

The ball has GPE but no KE.

Front 49

What is happening to the ball just as it first reaches the ground?

Back 49

The ball has just reached the ground so it has KE but no GPE.

Front 50

What is happening to the ball as it just leaves the ground?

Back 50

The ball squashes and has elastic potential energy which is converted to kinetic energy as it leaves the ground.

Front 51

What is happening to the ball on its first rebound?

Back 51

The ball has GPE but no KE at the top of the bounce
(if the ball was originally dropped from a height of 2m then on the first rebound it would probably reach 1.2m).

Front 52

What happens when skydivers reach terminal speed and when terminal speed is reached what can it be shown by?

Back 52

When skydivers reach terminal speed, their kinetic energy (half mv squared) has a maximum value and remains constant.

The GPE lost as they fall is used to do work against friction (air resistance).

When terminal speed is reached it can be shown as:
- change in GPE = work done against friction.

Front 53

What does a roller coaster use to get so high and what energy do the riders at the top and when the car is released have?

Back 53

Uses a motor to haul the train up in the air.

Riders at the top have a lot of GPE.

When the train is released it converts GPE to KE as it falls which is shown by the formula:
- loss of GPE = gain in KE.

Front 54

Ignoring friction, as the train of a roller coaster falls what equations can be used and rearranged to represent the forces?

Back 54

Equations:
- mgh = half mv squared
- 2mgh = mv squared - 2gh = v squared

So this is independent of the mass of the falling object.

Front 55

Why is each peak of the roller coaster lower than the other, what formula is this shown by, what happens when mass doubles and what happens when speed doubles?

Back 55

Because some energy is transferred to heat and sound due to friction and air resistance.

Shown by the formula:
- GPE at top = KE at the bottom + energy transferred (to heat and sound) due to friction.

If speed doubles, KE quadruples (KE (is equal to) v squared).

If mass doubles, KE doubles (KE (is equal to) m).