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Year 11
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Changing momentum

I can use the equation force = mass × change in velocity ÷ change in time to calculate the size of force involved in changing momentum in a certain amount of time.

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Year 11
Edexcel
Higher

Changing momentum

I can use the equation force = mass × change in velocity ÷ change in time to calculate the size of force involved in changing momentum in a certain amount of time.

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Lesson details

Key learning points

  1. The momentum of an object is changed by a resultant force acting over a period of time.
  2. Force = rate of change of momentum, F=Δp/t = mv – mu/t
  3. The size of an impact force can be changed when the duration of the impact changes.

Keywords

  • Momentum - is the product of the mass and velocity of an object, p = m v, it is a vector quantity

  • Initial velocity, u - is the velocity of an object at the start of a phase of motion, represented by u

  • Final velocity, v - is the velocity of an object at the end of a phase of motion, represented by v

  • Rate of change of momentum - is the change in momentum per second, this is equal to the resultant force acting on an object

Common misconception

Pupils often confuse the terms force and momentum.

Make explicit the understanding that forces change the motion of an object, and that the motion of an object (that a force can change) can be described by its momentum.

An important concept in this lesson is the understanding that a large force can change the momentum of an object quickly, a smaller force can change momentum by the same amount, but it takes a longer time for it to do so. Group mass and velocity in brackets for clarity in calculations of momentum.
Teacher tip

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.
Which of the following is the product of the mass and velocity (mass × velocity) of an object?
resultant force
acceleration
Correct answer: momentum
moment
kinetic energy
Q2.
Which of the following is the correct unit of momentum?
kg m
m/s
Correct answer: kg m/s
kg m/s$$^2$$
kg$$^2$$ m/s
Q3.
According to the principle of conservation of momentum, which of the following statements is correct?
The momentum of an individual object in the system cannot increase.
The momentum of an individual object in the system cannot decrease.
The velocity of an individual object in the system cannot increase.
The velocity of an individual object in the system cannot decrease.
Correct answer: The total momentum of all objects in a closed system cannot change.
Q4.
Starting with the greatest momentum, sort the following objects into order of decreasing momentum.
1 - a bus of mass 3000 kg travelling at 1 m/s
2 - a car of mass 800 kg travelling at 2 m/s
3 - a skater of mass 60 kg travelling at 8 m/s
4 - a stationary train of mass 300 000 kg
Q5.
A bicycle and rider have a combined mass of 100 kg. Calculate their acceleration when a resultant force of 250 N acts on them.
0.40 m/s$$^2$$
Correct answer: 2.5 m/s$$^2$$
3.5 m/s$$^2$$
150 m/s$$^2$$
400 m/s$$^2$$
Q6.
A van of mass 2000 kg accelerates in a straight line from 5.0 m/s to 8.0 m/s. Calculate the increase in momentum for the van. You do not need to give the unit.
Correct Answer: 6000, 6 000, 6000 kg m/s, 6 000 kg m/s

Exit quiz

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6 Questions

Q1.
Match the following key terms to their definition.
Correct Answer:momentum,the product of the mass and velocity of an object, $$p = m × v$$

the product of the mass and velocity of an object, $$p = m × v$$

Correct Answer:initial velocity,the velocity of an object at the start of a phase of motion, $$u$$

the velocity of an object at the start of a phase of motion, $$u$$

Correct Answer:final velocity,the velocity of an object at the end of a phase of motion, $$v$$

the velocity of an object at the end of a phase of motion, $$v$$

Correct Answer:rate of change of momentum,the change in momentum per second, equal to the resultant force

the change in momentum per second, equal to the resultant force

Q2.
A bus of mass 4000 kg accelerates from 2.0 m/s to 4.5 m/s. Calculate the change in momentum of the bus.
1 600 kg m/s
8 000 kg m/s
Correct answer: 10 000 kg m/s
18 000 kg m/s
26 000 kg m/s
Q3.
A ball of mass 0.40 kg hits a wall at a velocity of 3.0 m/s. It bounces off in the opposite direction at 2.5 m/s. Calculate the change in momentum for the ball.
–1.0 kg m/s
–1.2 kg m/s
–2.0 kg m/s
Correct answer: –2.2 kg m/s
–3.0 kg m/s
Q4.
Which of the following is equal to the rate of change of momentum of an object?
acceleration
increase in mass
rate of change of velocity
Correct answer: size of the resultant force acting on it
rate of increase of kinetic energy
Q5.
A pickup truck of mass 2000 kg is travelling at 12 m/s. It slows to 4 m/s in a time of 5 s. Calculate the size of the force needed to do this.
–1250 N
–1600 N
Correct answer: –3200 N
–4800 N
–80 000 N
Q6.
A ball with mass 0.30 kg and speed 5.0 m/s hits a wall and bounces back, at the same speed, in the opposite direction. The impact lasts for 0.10 s. Calculate the size of the force during the impact.
Correct Answer: -30 N, 30 N, -30.0 N, 30.0 N