Forces and braking

15* A person is driving a car on a motorway. It starts to rain and the driver is feeling tired.

The driver decides to leave more space between their car and the car in front of them in case they need to stop suddenly.

Why is it a good idea for the driver to leave more space?

In your answer:

• Describe other factors that affect the thinking distance and factors that affect the braking distance of the car.

• Explain how the stopping distance of the car is affected when it is raining and the driver is tired.

Crumple zones reduce the risk of injury to car passengers in collisions.

How do crumple zones reduce the forces exerted in a collision?

A They decrease the time of the collision.

B They decrease the speed of the collision.

C They increase the acceleration during the collision.

D They increase the time of the collision.

What does thinking distance mean?

A The distance travelled after pressing the brakes.

B The distance travelled while reacting.

C The time taken to react.

D The time taken to stop after pressing the brakes.

A student uses a ruler to measure a friend’s reaction time.

How does the student determine the friend’s reaction time?

A Divide the length of the ruler by the time taken to catch the ruler.

B Measure the length of the ruler where the friend catches it and convert to a time.

C Multiply the length of the ruler by the time taken to catch the ruler.

D Time how long it takes the ruler to hit the floor.

A student uses a ruler to measure a friend’s reaction time.

How does the student determine the friend’s reaction time?

A Divide the length of the ruler by the time taken to catch the ruler.

B Measure the length of the ruler where the friend catches it and convert to a time.

C Multiply the length of the ruler by the time taken to catch the ruler.

D Time how long it takes the ruler to hit the floor.

How do crumple zones make it safer for passengers in cars in a crash?

A They increase the force needed to stop the car.

B They increase the rate of change of momentum of the car.

C They increase the time taken for the car to stop.

D They increase the work done by the brakes.

Four drivers travel along the same road at 30 mph.

Each driver sees a red traffic light.

The graph shows the distance each driver travels after seeing the red light, but before pressing the brakes.

Which driver has the quickest reaction time?

A car travels along a road at 20 m / s.

Fig. 12.1 shows how the velocity of the car changes when the driver sees a hazard in the road at time = 0 seconds.

Fig. 12.1

Describe the motion of the car in part B of Fig. 12.1.

Name the distance travelled by the car in part A of Fig. 12.1. Tick (✓) one box.

Calculate the distance travelled by the car in part A of Fig. 12.1.

Use the equation: distance travelled = speed × time

Draw lines to match each situation to its correct speed.

Fig. 12.2 shows how acceleration changes with the mass of a car.

Fig. 12.2

Use Fig. 12.2 to determine the acceleration of a car with a mass of 960 kg.

Acceleration =........ m / s²

Use Fig. 12.2 to estimate the acceleration of a car with a mass of 1400 kg. Show your working on the graph.

Acceleration =........ m / s²

The table shows how braking distance changes with the speed of a car.

What is the braking distance at 80 mph?

A 48 m

B 96 m

C 110 m

D 216 m

The graph shows how acceleration changes with the mass of a car.

Use the graph to determine the acceleration of a car with a mass of 1250 kg.

Acceleration =........ m / s²

Use the graph to estimate the acceleration of a car with a mass of 700 kg.

Acceleration =........ m / s²

A car is travelling at 60 mph.

Which statement most likely describes what the car is doing?

Tick (✓) one box.

This graph shows how the velocity of a car changes when driver A sees a hazard in the road at time = 0 seconds.

Describe how the graph shows that the reaction time of driver A is 0.6 s.

Calculate the braking distance on the graph.

Driver B then drives the same car with the same road conditions and sees the same hazard.

Driver B:

• drives at the same speed as driver A

• has been drinking alcohol

• applies the brakes harder than driver A.

Draw another line on the graph to show how the velocity changes for driver B.

The thinking distance of a car at 20 mph is 6 m. The braking distance of a car at 20 mph is 6 m.

What is the thinking distance and braking distance of the car at 40 mph?

Which row in the table explains why a safety belt is a safety feature in a car?

Which factor increases both thinking distance and braking distance?

A Drinking alcohol

B Icy roads

C Increasing speed

D Worn tyres

This question is about energy changes in different situations.

Define braking distance.

• The braking distance is 6 m.

• The thinking distance is 6 m.

Calculate the stopping distance.

Explain how Fig. 12.1 shows the law of conservation of energy.

Complete these sentences about energy stores.

You can use each word once, more than once, or not at all.

chemical gravitational kinetic magnetic nuclear thermal

• A student rolls a ball up a hill. The ball stops at the top of the hill.

A kinetic store decreases. A........ store increases.

• A car accelerates.

A chemical store decreases. A........ store increases.

• Uranium fuel rods are used in a power station to produce electricity.

An electric kettle boils water.

A nuclear store decreases. A........ store increases.

A student measures the increase in the temperature of a metal block when it is heated.

Fig. 12.2 shows the apparatus that the student uses.

Complete the missing label on the diagram.

• The specific heat capacity of the block is 500 J / kg °C.

• The mass of the block is 0.5 kg.

• Its temperature increases by 20 °C.

Calculate the energy transferred to the block.

Use an equation from the Data Sheet to help you.

Which row in the table explains why a safety belt is a safety feature in a car?

Which factor increases both thinking distance and braking distance?

A Drinking alcohol

B Icy roads

C Increasing speed

D Worn tyres

12* Two students do an experiment to calculate their reaction times. They use a 30 cm ruler.

Describe, in detail, a suitable method the students can use to calculate reaction time using the ruler.

In your answer, include:

• Equations the students can use to calculate reaction time.

• How the students can obtain accurate and precise results.

A ruler can be used to estimate a student’s reaction time:

Which diagram shows the student with the shortest reaction time?

Stopping distance depends on thinking distance and braking distance.

Which row in the table is correct?

Brakes are used to decelerate a car safely in order to reduce the risk of injuries to the passengers.

Which row in the table is correct when the brakes are used safely?

Brakes are used to decelerate a car safely in order to reduce the risk of injuries to the passengers.

Which row in the table is correct when the brakes are used safely?

What is the typical value for human reaction time?

A 0.01 s

B 0.2 s

C 0.7 s

D 1.0 s

Thinking distance is directly proportional to speed.

The thinking distance at 30 mph is 9 m.

What is the thinking distance at 60 mph?

A 9 m

B 18 m

C 27 m

D 36 m

Which of the following factors affects thinking distance and braking distance for a car?

A Tiredness of the driver.

B Worn out brakes.

C Worn out tyres.

D The speed of the car.

What is the typical value for a human reaction time to a stimulus?

A 0.02 s

B 0.2 s

C 0.8 s

D 1.2 s

A car goes faster.

What effect does this have on thinking distance and braking distance?

This question is about modelling crumple zones in cars.

A student models a crumple zone by adding a spring to the front of a toy car. The spring compresses when the toy car hits a wall.

Before the toy car hits the wall, its speed is 2.2 m / s. The total mass of the toy car and spring is 500 g.

Show that the kinetic energy of the toy car and spring is about 1.2 J.

The spring constant of the spring is 4200 N / m.

Calculate the change in the length of the spring when the toy car hits the wall.

Give your answer to 2 significant figures.

* Evaluate the student’s model of the crumple zone.

Include ideas about how crumple zones make cars safer in your answer.

15* Person A and person B drive their cars along the same road on different days. Both cars travel at 25 m / s.

The traffic lights along the road change to red. Person A and person B see the red light and press the brakes in their car.

The graphs show the speed of each car after person A and person B see the red light.

The graphs are drawn using the same scale.

Describe the different ways the cars come to a stop.

In your answer:

• Suggest reasons why the cars take different times to stop

• Write about thinking and braking.

13* Person A and person B drive their cars along the same road on different days.

Both cars travel at 25 m / s. The traffic lights along the road change to red.

Person A and person B see the red light and press the brakes in their car.

The graphs show the velocity of each car after person A and person B see the red light.

The graphs are drawn using the same scale.

Explain what the graphs show you about how the cars stop.

Use values from the graphs and calculations in your answer.