The product of mass and velocity, p = mv. It is a vector, measured in kg m s⁻¹.

State Newton's second law in terms of momentum.

The resultant force on an object is equal to the rate of change of its momentum, F = Δp/Δt.

The product of force and the time for which it acts, FΔt, which equals the change in momentum.

What are the units of impulse?

N s, which is equivalent to kg m s⁻¹.

How is impulse found when the force varies with time?

It is the area under the force against time graph.

How do you find the average force during a collision?

Divide the impulse (change in momentum) by the contact time.

Why is the maximum force larger than the average force in a collision?

The force rises from zero to a maximum and falls back, and the average is impulse ÷ time.

Why does an airbag reduce the force on a driver?

It increases the time over which the momentum changes, so the average force is smaller for the same change in momentum.

A ball rebounds from a wall. What is its change in momentum?

m(v + u), where u and v are the speeds before and after, as the velocity reverses.

In the core practical, how is the speed of the trolley measured?

With a light gate and an interrupt card: speed = card width ÷ time the card blocks the beam.

In the core practical, what graph supports FΔt = Δp?

Impulse against change in momentum, a straight line through the origin with gradient 1.

Name a source of error in the force-momentum core practical.

Friction on the track, uncertainty in the card width, or a sampling rate too low to record the collision.

Impulse and force-momentumEdexcel A-Level Physics: Flashcards

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Define momentum.

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Define momentum.: The product of mass and velocity, p = mv. It is a vector, measured in kg m s⁻¹.
State Newton's second law in terms of momentum.: The resultant force on an object is equal to the rate of change of its momentum, F = Δp/Δt.
Define impulse.: The product of force and the time for which it acts, FΔt, which equals the change in momentum.
What are the units of impulse?: N s, which is equivalent to kg m s⁻¹.
How is impulse found when the force varies with time?: It is the area under the force against time graph.
How do you find the average force during a collision?: Divide the impulse (change in momentum) by the contact time.
Why is the maximum force larger than the average force in a collision?: The force rises from zero to a maximum and falls back, and the average is impulse ÷ time.
Why does an airbag reduce the force on a driver?: It increases the time over which the momentum changes, so the average force is smaller for the same change in momentum.
A ball rebounds from a wall. What is its change in momentum?: m(v + u), where u and v are the speeds before and after, as the velocity reverses.
In the core practical, how is the speed of the trolley measured?: With a light gate and an interrupt card: speed = card width ÷ time the card blocks the beam.
In the core practical, what graph supports FΔt = Δp?: Impulse against change in momentum, a straight line through the origin with gradient 1.
Name a source of error in the force-momentum core practical.: Friction on the track, uncertainty in the card width, or a sampling rate too low to record the collision.

Exam questions on Impulse and force-momentum

A tennis ball of mass 0.058 kg approaches a racket horizontally at 20 m s⁻¹. It is in contact with the strings for 5.0 ms and leaves the racket in the opposite direction at 30 m s⁻¹.
The ball has the same incoming and outgoing speeds, but a different racket keeps the ball in contact for longer. Explain the effect on the average force on the ball.2 marks
A driver of mass 70 kg, wearing a seat belt, is travelling at 15 m s⁻¹ when the car hits a wall. The seat belt and an airbag bring the driver to rest in 0.12 s.
Calculate the average force on the driver while being stopped.2 marks
A student investigates the relationship between force and change of momentum. A trolley of mass 0.800 kg moves towards a force sensor at 0.80 m s⁻¹, measured by a light gate, and rebounds at 0.55 m s⁻¹. The data logger connected to the sensor shows that the collision lasts 0.045 s and gives the impulse, found from the area under the recorded force against time, as 1.05 N s.
Calculate the change in momentum of the trolley. Compare it with the impulse given by the data logger and state whether the results support the relationship FΔt = Δp.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).