Forces and braking

Which of these is a typical human reaction time?

There are several factors that affect the overall stopping distance of a van.

State two factors that may affect the overall stopping distance of a van.

Figure 7 is a velocity / time graph for a van braking from an initial velocity of 25 m / s.

On Figure 7 draw a similar graph for the same van braking from an initial velocity of 15 m / s.

Assume the thinking time and the braking force remain the same.

The van’s brakes are applied to stop a van in a distance of 21.5 m.

The work done by the brakes is 375 kJ.

Calculate the braking force in newtons.

Use the equation

braking force =_........

The van is travelling along a horizontal road when the brakes are applied.

Which of these will have the same value as the work done by the brakes?

A car is being driven at a constant velocity.

The driver sees an obstacle in the road ahead.

The driver uses the brakes to stop as quickly as possible.

Figure 9 shows the velocity/time graph for the car from the time when the driver sees the obstacle.

Which of these is the driver’s reaction time shown in Figure 9?

State one factor that might increase a driver’s reaction time.

Calculate the distance travelled between when the driver applies the brakes and when the car comes to rest in Figure 9.

Use the equation

distance = area under the sloping line of the graph in Figure 9

The stopping distance of a car is the thinking distance plus the braking distance.

A car has a device that can detect an obstacle in the road ahead.

The device is linked to a computer that can apply the brakes.

It is claimed that, in an emergency, the computer-controlled car will have a shorter stopping distance than if the car is controlled by a human driver.

Explain why this claim could be true.

A different car has a device that can detect rain.

This device is linked to a computer that can change the speed of the car.

In wet weather, the computer changes the speed of the car.

State the change in speed that the computer should make when the road is wet.

Give a reason why this change in speed is necessary when the road is wet.

Figure 15 is a speed limit sign from a European motorway.

The speeds shown are in km/h (kilometres per hour).

The sign tells drivers to drive at a slower speed in wet weather.

Explain why it is safer for drivers to drive at a slower speed in wet weather.

Show that a speed of 31 m/s is less than a speed of 130 km/h.

A car is travelling at a speed of 31 m/s.

The car travels 46 m between the driver seeing an emergency and starting to brake.

Calculate the driver’s reaction time.

Give your answer to 2 significant figures.

driver’s reaction time =_........

*(b) Figure 16 is a velocity/time graph for a toy train on a straight track for 7 seconds.

Using information from the graph, describe when and how the velocity and acceleration of the toy train change at different times during the 7 seconds shown on the graph.

Figure 5 is a speed limit sign from a European motorway.

The speeds shown are in km/h (kilometres per hour).

The sign tells drivers to drive at a slower speed in wet weather.

Explain why it is safer for drivers to drive at a slower speed in wet weather.

Show that a speed of 31 m/s is less than a speed of 130 km/h.

A car is travelling at a speed of 31 m/s.

The car travels 46 m between the driver seeing an emergency and starting to brake.

Calculate the driver’s reaction time.

Give your answer to 2 significant figures.

driver’s reaction time_........

A car is travelling at 10 m/s.

The driver sees a danger and stops the car.

The stopping distance for the car would be smaller if the car

Figure 4 shows a speed-time graph for the driver stopping the car.

Use the graph to find the driver’s reaction time.

Figure 5 shows the apparatus a student uses to investigate how the stopping distance of a toy car depends on the type of surface that it is stopping on.

toy car

Describe an experiment to find out how the stopping distance depends on the surface that stops the toy car.

Figure 6 shows a set of results used to find the average stopping distance of the toy car on a surface.

State the anomalous value of stopping distance given in the table in Figure 6.

Use the results in Figure 6 to calculate the average stopping distance.

State one way the student could increase the speed of the car as it reaches the flat surface.

A car is travelling down a slope at 2.0 m/s.

The car accelerates for 4.0 s.

The speed of the car increases to 12 m/s.

Calculate the acceleration of the car.

Use the equation

acceleration of the car =_........ ²

Figure 10 shows a small steel ball held at a height, h, above the ground.

The ball is released and allowed to fall to the ground.

The height h is 1.4 m.

Calculate the time, t, for the ball to reach the ground.

Use the equation

Two students use the arrangement shown in Figure 10.

They use a stopwatch to time the ball falling through the height of 1.4 m.

The students repeat the measurement many times, but their average value for t is different from the calculated value.

Suggest a reason why the students’ value for t is different from the calculated value.

Suggest one improvement the students could make to their procedure so that their value for t is closer to the calculated value.

Figure 11a shows a box falling towards a hard floor.

The box hits the floor and crumples a little before it comes to rest as shown in Figure 11b.

The momentum of the box just before it hits the floor is 8.7 kg m/s.

The box comes to rest 0.35 s after it first hits the floor.

Calculate the magnitude of the force exerted by the floor on the box.

Use an equation selected from the list of equations at the end of this paper.

State the magnitude and direction of the force exerted by the box on the floor.

Figure 12 shows a ball held in a clamp at R, above the ground.

The ball is released from the clamp and falls.

S is 3.8 m below R.

At S the momentum of the ball is 0.40 kg m/s.

Calculate the mass of the ball.

Acceleration due to gravity, g, = 10 m / s²

mass of the ball_........

Which of these is a vector?

State the equation that relates acceleration to change in velocity and time taken.

A van accelerates from a velocity of 2 m / s to a velocity of 20 m / s in 12 s.

Calculate the acceleration of the van.

acceleration =_........ m / s²

Figure 17 is a velocity / time graph for 15 s of a cyclist’s journey.

Calculate the distance the cyclist travels in the 15 s.

*(d) Many factors can affect the stopping distance of a car.

Some of these factors involve the driver and some of these factors involve the car or the road.

Explain how the stopping distance of a car is affected by

• factors involving the driver

• factors involving the car or the road.

You should include examples in your explanations.

The work done to bring a car to rest is given by the equation

work done = braking forcebraking distance

Which of these graphs is correct for the car if a constant braking force is applied?

Before the car brakes it has kinetic energy. The kinetic energy decreases as it brakes.

State what happens to the kinetic energy during braking.

The graph in Figure 1 shows how the braking distance, d, of a car depends on the velocity, v, of the car when the brakes are first applied.

An equation relating braking distance, d, to velocity, v, is

where C is a constant.

Use the equation and data from the graph in Figure 1 to calculate a value for C.

Give a unit for C.