Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Use Newton’s second law as: force (newton, N) = change in momentum…
Full specification wording
Topic 2 – Motion and forces: 2.26 Use Newton’s second law as: force (newton, N) = change in momentum (kilogram metre per second, kg m/s) ÷ time (second, s)
Newton’s second law links force to the rate of change of momentum: force in newtons equals change in momentum in kg m/s divided by the time in seconds over which the change occurs.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 25 of 32
Study notes
From the checked teaching notes for this specification point.
The idea
Newton’s second law links force to the rate of change of momentum: force in newtons equals change in momentum in kg m/s divided by the time in seconds over which the change occurs. Change in momentum is final momentum minus initial momentum, so direction matters. A 0.5 kg ball hitting a wall at 8 m/s and rebounding at 6 m/s changes momentum by 0.5 × (8 + 6) = 7 kg m/s if the rebound is opposite; over 0.1 s the force is 7 ÷ 0.1 = 70 N. The same law explains crumple zones and airbags: extending the collision time reduces the force for a given momentum change.
What earns the marks
- States the relationship force = change in momentum ÷ time and identifies the units N, kg m/s and s.
- Calculates change in momentum as final momentum minus initial momentum, using signs for direction.
- Divides the change in momentum by the contact time to obtain the force in newtons.
- Explains that increasing the time of a collision reduces the force for the same momentum change, as in crumple zones or airbags.
- Rearranges the equation to find change in momentum or time when force is known.
Common mistakes and how to put them right
Uses initial momentum only instead of the change in momentum.
Put it right: Subtract initial from final with correct signs.
Ignores direction when an object rebounds.
Put it right: Add the magnitudes when velocities are opposite.
Divides time by momentum instead of momentum by time.
Put it right: Keep force as the subject.
Forgets to convert units such as grams to kilograms or milliseconds to seconds before calculating.
Exam tips
- Write the equation, list known values with units, then substitute.
- Use a sign convention and state it before calculating change in momentum.
- Check the answer's unit is newtons and its magnitude is sensible.
- Link a large force to a short contact time when explaining safety features.
Try the lesson
The start of the Topic 2 – Motion and forces: 2.26 Use Newton’s second law as: force (newton, N) = change in momentum (kilogram metre per second, kg m/s) ÷ time (second, s) lesson, made from checked teaching notes for this specification point.
Have a go first
A ball's momentum changes by 12 kg m/s in 3 s. What is the resultant force on the ball?
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Test your understanding of Topic 2 – Motion and forces: 2.26 Use Newton’s second law as: force (newton, N) = change in momentum (kilogram metre per second, kg m/s) ÷ time (second, s)
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