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Year 11
OCR
Higher

Stopping safely

I can apply an understanding of force = change in momentum ÷ time to explain how forces and momentum can be controlled.

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New
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Year 11
OCR
Higher

Stopping safely

I can apply an understanding of force = change in momentum ÷ time to explain how forces and momentum can be controlled.

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Lesson details

Key learning points

  1. Increasing a stopping force decreases the time it takes a moving object to stop.
  2. Extending the time it takes to stop reduces the size of the stopping force.
  3. Many safety devices crumple on impact to increase the time that forces act before stopping a moving object.

Keywords

  • Stopping forces - Forces used to bring moving objects to a stop.

  • Stopping time - The time taken for a moving object to stop.

  • Momentum - The product of the mass and velocity of a moving object, p = m × v.

  • Crumple zones - Parts of a car designed to crumple during impacts, increasing the stopping time and reducing the stopping forces.

  • Airbags - These inflate rapidly in collisions and protect passengers from impacts with hard surfaces.

Common misconception

Pupils often do not understand clearly what is meant by stopping time and cannot explain why longer stopping times result in smaller forces.

Describe stopping time as the time it takes to stop ‘during which a force is acting’, and provide examples of smaller forces stopping moving objects over longer periods of time than larger forces.

The key understanding for car safety is that materials are designed to crumple or stretch before the force pushing on them becomes dangerously large. This means that a relatively safe force slows down the car for as long as possible in order to reduce the maximum force on a passenger.
Teacher tip

Content guidance

  • Depiction or discussion of sensitive content

Supervision

Adult supervision recommended

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
Match the following quantities to their unit.
Correct Answer:displacement,m

m

Correct Answer:velocity,m/s

m/s

Correct Answer:mass,kg

kg

Correct Answer:momentum,kg m/s

kg m/s

Q2.
Which of the following is the correct equation for stopping distance?
stopping distance = thinking time + braking distance
stopping distance = thinking time + braking time
stopping distance = thinking distance – braking distance
Correct answer: stopping distance = thinking distance + braking distance
stopping distance = thinking distance × braking distance
Q3.
Which of the following is the correct equation for momentum?
momentum = distance × velocity
momentum = kinetic energy ÷ mass
momentum = mass + velocity
Correct answer: momentum = mass × velocity
momentum = mass × speed$$^2$$
Q4.
A ball is dropped vertically from 10 m. As the ball falls, which of the following statements are correct?
Correct answer: A gravitational force acts on the ball.
The momentum of the ball is constant.
Correct answer: The momentum of the ball increases as the ball falls.
The mass of the ball increases as it falls.
Correct answer: The mass of the ball is constant.
Q5.
A car has a mass of 800 kg and velocity of 4.0 m/s. Calculate the momentum of the car.
200 kg m/s
800 kg m/s
Correct answer: 3200 kg m/s
6400 kg m/s
Q6.
A motorcycle of mass 200 kg, slows from 14 m/s to 12 m/s. What is the change of momentum for the motorcycle?
200 kg m/s
–200 kg m/s
400 kg m/s
Correct answer: –400 kg m/s
2400 kg m/s

Exit quiz

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6 Questions

Q1.
Match the following key terms to their definitions.
Correct Answer:stopping forces,Forces used to bring objects to a stop.

Forces used to bring objects to a stop.

Correct Answer:stopping time,The time is takes for an object to stop.

The time is takes for an object to stop.

Correct Answer:momentum,The product of mass and velocity of a moving object, $$p = m × v$$.

The product of mass and velocity of a moving object, $$p = m × v$$.

Correct Answer:crumple zones,Parts of a car designed to crumple and increase stopping time.

Parts of a car designed to crumple and increase stopping time.

Correct Answer:airbags,Inflate in a collision and protect passengers from impacts.

Inflate in a collision and protect passengers from impacts.

Q2.
A lorry is attempting to slow down to a stop. What will happen to the stopping time and stopping distance if the braking force increases?
The stopping time will increase.
Correct answer: The stopping time will decrease.
The stopping time will remain the same.
The stopping distance will increase.
Correct answer: The stopping distance will decrease.
Q3.
Match each of the following safety features to its function.
Correct Answer:seatbelts,These stretch to increase stopping times and reduce impact forces.

These stretch to increase stopping times and reduce impact forces.

Correct Answer:crumple zones,These collapse to increase stopping times and reduce impact forces.

These collapse to increase stopping times and reduce impact forces.

Correct Answer:air bags,These inflate to form a protective cushion, increasing stopping times.

These inflate to form a protective cushion, increasing stopping times.

Q4.
Why does padding decrease the size of impact forces?
It lowers the initial momentum of objects.
It produces larger forces to stop things more quickly.
It produces smaller forces to stop things more quickly.
Correct answer: It increases the duration of the impact.
It decreases the duration of the impact.
Q5.
A lorry of mass 5000 kg travels at a velocity of 18 m/s. The brakes produce a braking force of 15 000 N on the wheels. Calculate the stopping time for the lorry. Give the unit.
Correct Answer: 6.0 s, 6 s, 6 seconds, 6.0 seconds
Q6.
Two cars have the same momentum of 12 000 kg m/s. It takes one car 0.20 s to stop, and the other one stops in 0.05 s. Calculate the difference in the stopping forces acting on the cars.
60 000 N
80 000 N
Correct answer: 180 000 N
240 000 N