What is the impulse of a 20 N force acting for 0.50 s?

10 N s. Impulse = force × time.

What does the area under a force–time graph represent?

change in momentum. That is the impulse.

A 1000 kg car moving at 20 m/s is brought to rest in 4.0 s. What is the average force on it?

5000 N. 20 000 kg m/s ÷ 4.0 s.

What is true of an elastic collision?

Kinetic energy is conserved. Both momentum and kinetic energy are the same before and after.

How is momentum conserved in a collision in two dimensions?

separately in each of two perpendicular directions. Momentum is a vector, so each component is conserved.

Two objects approach each other head-on with a relative speed of 6 m/s and collide elastically. What is their relative speed of separation?

6 m/s. In an elastic collision the speed of approach equals the speed of separation.

Impulse and types of collision · Cambridge A Level Physics

In plain words

A force changes momentum, and how much depends on how long it acts: a small force for a long time can do the same job as a big force for a short time. That's why airbags work. And although momentum is conserved in every collision, kinetic energy usually isn't, which gives a way of sorting collisions into types.

4 things to know

Force = rate of change of momentum: F = Δp ÷ Δt. Impulse = FΔt = Δp, which is the area under a force–time graph.
In an elastic collision the total kinetic energy is conserved, and the relative speed of approach equals the relative speed of separation. In an inelastic collision some kinetic energy is transferred to other forms, but momentum is still conserved.
In two dimensions, momentum is conserved separately in each of two perpendicular directions.
Kinetic energy can be written in terms of momentum: E = p² ÷ 2m.

Worked example

A 0.15 kg ball hits a wall at 20 m/s and rebounds at 15 m/s. The contact lasts 0.050 s. Find the average force on the ball.

Take away from the wall as positive: the velocity is −20 m/s before and +15 m/s after.
Change in momentum = 0.15 × (15 − (−20)) = 5.25 kg m/s.
Force = 5.25 ÷ 0.050 = 105 N, away from the wall.

Tips and tricks

A ball that bounces back has changed its momentum by more than one that just stops. Watch the signs.
To test whether a collision is elastic, work out the total kinetic energy before and after and compare them.

6 questions, about 2 minutes.

It lands in your notebook with its questions as flashcards.

Impulse and types of collision: 6 questions and answers

These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.

What is the impulse of a 20 N force acting for 0.50 s?
- 0.025 N s
- 10 N s (the answer)
- 20.5 N s
- 40 N s
Impulse = force × time.
What does the area under a force–time graph represent?
- work done
- power
- change in momentum (the answer)
- acceleration
That is the impulse.
A 1000 kg car moving at 20 m/s is brought to rest in 4.0 s. What is the average force on it?
- 250 N
- 800 N
- 5000 N (the answer)
- 80 000 N
20 000 kg m/s ÷ 4.0 s.
What is true of an elastic collision?
- The objects stick together.
- Momentum is not conserved.
- Kinetic energy is conserved. (the answer)
- All the kinetic energy is lost.
Both momentum and kinetic energy are the same before and after.
How is momentum conserved in a collision in two dimensions?
- separately in each of two perpendicular directions (the answer)
- only along the first object's line of motion
- only as a total speed
- it isn't conserved
Momentum is a vector, so each component is conserved.
Two objects approach each other head-on with a relative speed of 6 m/s and collide elastically. What is their relative speed of separation?Stretch
- 0
- 3 m/s
- 6 m/s (the answer)
- 12 m/s
In an elastic collision the speed of approach equals the speed of separation.

6 questions

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Worksheet

4 questions, 10 marks. Write your answers on paper, then check them.

Impulse and types of collision

Cambridge International AS & A Level Physics 9702 · 10 marks · papermunch.org

Name ______________________________ Date ______________

A 2.0 kg trolley moving at 3.0 m/s hits a stationary 1.0 kg trolley, and they move off together at 2.0 m/s. Show that the collision is inelastic.[3]
Show answerHide answer
Kinetic energy before = ½ × 2.0 × 3.0² = 9.0 J. After = ½ × 3.0 × 2.0² = 6.0 J. Kinetic energy is not conserved, so the collision is inelastic.
I got this one
Explain why a car's crumple zone reduces the force on the passengers in a crash.[2]
Show answerHide answer
It increases the time taken to stop. The change in momentum is the same, so the rate of change of momentum, and therefore the force, is smaller.
I got this one
A 0.20 kg ball moving at 5.0 m/s in the x-direction is deflected so that it moves at 5.0 m/s in the y-direction. Find the magnitude of its change in momentum.[3]
Show answerHide answer
1.4 kg m/s. The change has components of −1.0 and +1.0 kg m/s: √(1.0² + 1.0²).
I got this one
Find the kinetic energy of a 2.0 kg object that has a momentum of 6.0 kg m/s.[2]
Show answerHide answer
9.0 J. 6.0² ÷ (2 × 2.0).
I got this one

Answers: Impulse and types of collision

1. Kinetic energy before = ½ × 2.0 × 3.0² = 9.0 J. After = ½ × 3.0 × 2.0² = 6.0 J. Kinetic energy is not conserved, so the collision is inelastic.
2. It increases the time taken to stop. The change in momentum is the same, so the rate of change of momentum, and therefore the force, is smaller.
3. 1.4 kg m/s. The change has components of −1.0 and +1.0 kg m/s: √(1.0² + 1.0²).
4. 9.0 J. 6.0² ÷ (2 × 2.0).