Forces & braking

Two drivers race their cars in a straight line over a distance of 400 metres.

Figure 5 shows the cars.

Figure 6 shows a distance-time graph for one car in the race.

When was the car travelling at a constant speed?

Give a reason for your answer.

Tick () one box.

Between 2 seconds and 3 seconds

Between 3 seconds and 4 seconds

Between 4 seconds and 5 seconds

The two horizontal forces acting on the car were a driving force and air resistance.

How did the driving force compare with the air resistance when the car moved at a constant speed?

Give a reason for your answer.

Tick () one box.

The driving force was less than air resistance.

The driving force was the same size as air resistance.

The driving force was greater than air resistance.

At the end of the race, the driver applies the brakes and opens a parachute to decelerate the car.

Figure 7 shows one of the cars at the end of the race.

Why may it be dangerous for a car to have a very large deceleration?

Tick () one box.

The car engine may overheat.

The car will stop in a short distance.

The driver may lose control of the car.

The stopping distance of a vehicle depends on the thinking distance and the braking distance.

What is meant by ‘thinking distance’?

Tick (✓) one box.

The distance travelled before a vehicle stops.

The distance travelled while the driver reacts.

The time taken for a driver to react.

The time taken for the vehicle to stop.

What would increase the braking distance of a vehicle?

Tick (✓) one box.

Ice on the road surface

Sunny weather

Using a mobile phone while driving

What is the name of the force which causes the vehicle to decelerate when the brakes are applied?

Tick (✓) one box.

Friction

Upthrust

Weight

Figure 1 shows how the braking distance of two cars varies with speed.

How does the braking distance of the two cars vary with speed?

Tick (✓) one box.

The braking distance decreases as speed increases.

The braking distance is not affected by speed.

The braking distance increases as speed increases.

To make a fair comparison between the braking distance of the two cars, the mass of each car was the same.

Calculate the weight of a car.

mass = 850 kg

gravitational field strength = 9.8 N/kg

Use the equation:

weight = mass × gravitational field strength

Choose the unit from the box.

Which two variables should be kept the same to make a fair comparison of the braking distance of the two cars?

Tick (✓) two boxes.

The age of the driver

The caffeine intake of the driver

The colour of the car

The number of people in the car

The type of road surface

Table 1 shows the braking force on each car at a speed of 31 m/s.

The braking distance of car A was longer than the braking distance of car B at a speed of 31 m/s.

Explain why.

Use data from Table 1.

The stopping distance of a vehicle depends on the thinking distance and the braking distance.

What is meant by ‘thinking distance’?

What would increase the thinking distance?

Tick (✓) two boxes.

Ice on the road surface

Poor condition of the brakes

The driver being tired

Using a mobile phone while driving

Wet weather

Figure 19 shows how the braking distance of a new car in 2020 compares with the braking distance of a new car in 1975.

The two cars had the same mass and were driven along a level road in sunny weather.

Explain the difference in braking distance between the two cars.

Use Figure 19.

The 2020 car was driven at a speed of 27 m/s. The brakes were applied until the car stopped.

The speed of the car decreased from 27 m/s to 0 m/s with a constant deceleration.

The mass of the car was 850 kg.

Determine the mean braking force on the car.

Use Figure 19.

Use the Physics Equations Sheet.

The stopping distance of a vehicle depends on the thinking distance and the braking distance.

What is meant by ‘braking distance’?

The braking distance of a vehicle depends on the mass of the vehicle.

Use the Physics Equations Sheet to answer questions 09.2 and 09.3.

Write down the equation which links gravitational field strength (g), mass (m) and weight (W).

Calculate the mass of a vehicle with a weight of 14 700 N.

gravitational field strength = 9.8 N/kg

The thinking distance travelled by a vehicle depends on the reaction time of the driver.

Using a mobile phone increases a driver’s reaction time.

A mobile phone can be used in these ways:

typing a text message

making a phone call while holding the phone

making a hands-free phone call using the car’s audio system.

Figure 21 shows how different activities using a mobile phone affect a driver’s reaction time.

Typing a text Making a hand-held Making a hands-free message
phone call
phone call

Phone activity

The reaction time of a typical driver is 0.50 s.

Calculate the reaction time of a typical driver typing a text message while driving.

The legal alcohol limit is the maximum amount of alcohol a person can have in the bloodstream and still legally drive.

The reaction time of a typical driver at the legal alcohol limit is increased by 12%.

A student suggests that it should be illegal to use a mobile phone in any way while driving.

Explain how the information in Figure 21 supports the student’s suggestion.

The stopping distance of a vehicle depends on the thinking distance and the braking distance.

What is meant by ‘braking distance’?

The braking distance of a vehicle depends on the mass of the vehicle.

Use the Physics Equations Sheet to answer questions 01.2 and 01.3.

Write down the equation which links gravitational field strength (g), mass (m) and weight (W).

Calculate the mass of a vehicle with a weight of 14 700 N.

gravitational field strength = 9.8 N/kg

The thinking distance travelled by a vehicle depends on the reaction time of the driver.

Using a mobile phone increases a driver’s reaction time.

A mobile phone can be used in these ways:

typing a text message

making a phone call while holding the phone

making a hands-free phone call using the car’s audio system.

Figure 1 shows how different activities using a mobile phone affect a driver’s reaction time.

The reaction time of a typical driver is 0.50 s.

Calculate the reaction time of a typical driver typing a text message while driving.

The legal alcohol limit is the maximum amount of alcohol a person can have in the bloodstream and still legally drive.

The reaction time of a typical driver at the legal alcohol limit is increased by 12%.

A student suggests that it should be illegal to use a mobile phone in any way while driving.

Explain how the information in Figure 1 supports the student’s suggestion.

Figure 5 shows a car travelling at a constant speed on a straight, level road.

Draw an arrow on Figure 5 to show the direction of the force of air resistance on the car.

The mass of the car is 850 kg

Calculate the weight of the car.

Use the equation:

weight = mass × gravitational field strength

gravitational field strength = 9.8 N/kg

What is the direction of the normal contact force of the road on the wheels?

Tick (✓) one box.

The car is travelling at constant speed.

The resultant force on the car is zero.

How does the size of the normal contact force of the road on the wheels compare with the weight of the car?

Tick (✓) one box.

A car is travelling at a constant speed.

A constant braking force of 5100 N is applied by the brakes.

The car decelerates and stops.

The braking distance is 38 m

Calculate the work done by the braking force.

Choose the unit from the box.

Use the equation: work done = force × distance

Which two factors affect braking distance?

Tick (✓) two boxes.

The distance a car travels during the driver’s reaction time is called the thinking distance.

Which factor affects thinking distance?

Tick (✓) one box.

Figure 6 shows a sketch graph of how thinking distance varies with speed.

Which term describes the relationship between thinking distance and speed?

Tick (✓) one box.

Figure 9 shows a car travelling at a constant speed on a straight, level road.

Figure 10 shows a free body diagram for the car.

Explain how the free body diagram shows that the resultant force on the car is zero.

Table 4 shows how the braking distance of a car varies with the speed of the car.

A car decelerates from 24 m/s to 0 m/s

The work done by the brakes in decelerating the car is 210 000 J

Determine the mean braking force.

Use information from Table 4.

Give your answer to 2 significant figures.

The braking distance is directly proportional to the kinetic energy of the car.

Explain how the data in Table 4 supports this relationship.

Some new cars have an electric motor that is powered by a battery.

A battery supplies direct current.

What is direct current?

Tick (✓) one box.

There are different types of battery available.

Table 2 shows the maximum distance a car can travel before the battery needs recharging.

Complete Figure 5.

You should:

• label the x-axis

• label the y-axis

• plot the data from Table 2.

A car with a lead-acid battery travels 80% of the maximum distance it can travel before the battery needs recharging.

Determine the distance the car travels.

A lithium-ion battery is put on charge for 1800 s

The current is 40 A

Calculate the total charge flow during this time.

Use the equation:

charge flow = current × time

The driver of a car saw an obstacle in the road. He applied the brakes until the car stopped.

The thinking distance was 9.0 m

The braking distance was 13.5 m

Calculate the stopping distance of the car.

The driver had been drinking alcohol. The car had worn brakes.

Explain why these factors would increase the stopping distance of the car.

The stopping distance of a car depends on the thinking distance and the braking distance.

Thinking distance depends on the driver’s reaction time.

Give two factors that can affect reaction time.

Give one factor that can affect the braking distance.

The thinking distance is the distance travelled during the driver’s reaction time.

A car was travelling at 13 m/s

The driver’s reaction time was 0.6 s

Calculate the thinking distance.

Use the equation:

distance travelled = speed × time

The braking distance of the car was 14.0 m

What was the stopping distance of the car?

What is the link between speed and braking distance?

Complete the sentence.

The greater the speed, the

If a large braking force is applied, the car decelerates and stops in a very short distance.

Give two disadvantages of applying a large braking force.