The force multiplied by the time for which it acts, Ft, equal to the change in momentum.

What are the units of impulse?

N s, equivalent to kg m s⁻¹.

State Newton's second law in terms of momentum.

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

How do you get Ft = Δp from F = ma?

Use a = (v − u)/t, so F = m(v − u)/t and Ft = mv − mu.

A ball reverses direction. What is Δp?

Δp = m(v + u), adding the magnitudes because the direction reverses.

Why does an airbag reduce the force on a passenger?

The same change of momentum happens over a longer time, so F = Δp/t is smaller.

Name two features of a car that increase impact time.

Crumple zones and seat belts or airbags.

Mass multiplied by velocity, p = mv, a vector.

In Core Practical 9, what is plotted?

Change in momentum Δp against force F, for a fixed time.

What does the gradient of the Core Practical 9 graph give?

The time interval t over which the force acted.

How is friction compensated for in Core Practical 9?

Tilt the runway slightly so the trolley moves at constant speed when pushed.

Why keep the total mass constant in Core Practical 9?

Moving masses between trolley and hanger changes the force without changing the mass accelerated.

Impulse and force-momentum changeEdexcel International A Level Physics: Flashcards

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Define impulse.

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Define impulse.: The force multiplied by the time for which it acts, Ft, equal to the change in momentum.
What are the units of impulse?: N s, equivalent to kg m s⁻¹.
State Newton's second law in terms of momentum.: The resultant force is equal to the rate of change of momentum, F = Δp/Δt.
How do you get Ft = Δp from F = ma?: Use a = (v − u)/t, so F = m(v − u)/t and Ft = mv − mu.
A ball reverses direction. What is Δp?: Δp = m(v + u), adding the magnitudes because the direction reverses.
Why does an airbag reduce the force on a passenger?: The same change of momentum happens over a longer time, so F = Δp/t is smaller.
Name two features of a car that increase impact time.: Crumple zones and seat belts or airbags.
What is momentum?: Mass multiplied by velocity, p = mv, a vector.
In Core Practical 9, what is plotted?: Change in momentum Δp against force F, for a fixed time.
What does the gradient of the Core Practical 9 graph give?: The time interval t over which the force acted.
How is friction compensated for in Core Practical 9?: Tilt the runway slightly so the trolley moves at constant speed when pushed.
Why keep the total mass constant in Core Practical 9?: Moving masses between trolley and hanger changes the force without changing the mass accelerated.

Exam questions on Impulse and force-momentum change

A tennis ball of mass 0.057 kg travels horizontally at 25 m s⁻¹ towards a racket. It is in contact with the racket for 5.0 ms and leaves in the opposite direction at 30 m s⁻¹.
Calculate the average force exerted by the racket on the ball.2 marks
In a crash test a dummy of mass 75 kg is travelling at 14 m s⁻¹ in a car that stops suddenly. The seat belt brings the dummy to rest in 0.20 s.
The airbag increases the stopping time to 0.50 s. Calculate the average force on the dummy now.2 marks
In Core Practical 9 a student investigates the relationship between the force on a trolley and its change of momentum. The trolley has mass 0.80 kg and is pulled along a track by a falling weight on a string. Light gates record its speed as 0.40 m s⁻¹ at the first gate and 1.20 m s⁻¹ at the second gate, and a data logger records 2.0 s between the gates.
Calculate the change in momentum of the trolley between the gates and the average resultant force on it.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).