OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Newton’s laws of motion: (d) impulse of a force; impulse = F t T
The impulse of a force is the product of the force and the time for which it acts: impulse = FΔt.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 4 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The impulse of a force is the product of the force and the time for which it acts: impulse = FΔt. Since F = Δp/Δt, impulse equals the change in momentum, so impulse = Δp = mΔv. Impulse is a vector with the same direction as the force, and its SI unit is N s, equivalent to kg m s⁻¹. For example, a force of 20 N acting for 0.50 s gives an impulse of 10 N s, changing the momentum of the body by 10 kg m s⁻¹ in the force's direction. Increasing the contact time for a given momentum change reduces the force, which is why crumple zones and airbags reduce injury.
What earns the marks
- Impulse is defined as the product of force and the time for which it acts: impulse = FΔt.
- Impulse equals the change in momentum: impulse = Δp = mΔv.
- The SI unit of impulse is N s, equivalent to kg m s⁻¹.
- Impulse is a vector in the direction of the applied force.
- For a fixed momentum change, a longer contact time gives a smaller average force.
Common mistakes and how to put them right
Confusing impulse with force.
Put it right: Impulse is force multiplied by time, measured in N s.
Forgetting that impulse equals change in momentum.
Put it right: Use impulse = Δp to link force and motion.
Assuming a longer contact time increases the force.
Put it right: For a fixed Δp, longer time means smaller average force.
Exam tips
- Use impulse = Δp when the force is not constant or is unknown.
- Check that time is in seconds before multiplying by force.
- Link safety features to increased contact time reducing peak force.
Try the lesson
The start of the Newton’s laws of motion: (d) impulse of a force; impulse = F t T lesson, made from checked teaching notes for this specification point.
Listen
The impulse of a force is the product of the force and the time for which it acts: impulse = FΔt. Since F = Δp/Δt, impulse equals the change in momentum, so impulse = Δp = mΔv. Impulse is a vector with the same direction as the force, and its SI unit is N s, equivalent to kg m s⁻¹. For example, a force of 20 N acting for 0.50 s gives an impulse of 10 N s, changing the momentum of the body by 10 kg m s⁻¹ in the force's direction. Increasing the contact time for a given momentum change reduces the force, which is why crumple zones and airbags reduce injury. A strong answer earns marks for these points. Impulse is defined as the product of force and the time for which it acts: impulse = FΔt. Impulse equals the change in momentum: impulse = Δp = mΔv. The SI unit of impulse is N s, equivalent to kg m s⁻¹. Impulse is a vector in the direction of the applied force. A common slip: Confusing impulse with force. Instead, impulse is force multiplied by time, measured in N s. Exam tip: Use impulse = Δp when the force is not constant or is unknown.
See it
Key words
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Impulse and change in momentum
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Force–time relationships
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Safety applications of impulse
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Units of impulse
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See every definition
- Impulse and change in momentum
- Impulse equals the change in momentum, so impulse = Δp = mΔv.
- Force–time relationships
- The impulse of a force is the product of the force and the time for which it acts: impulse = FΔt.
- Safety applications of impulse
- Increasing the contact time for a given momentum change reduces the force, which is why crumple zones and airbags reduce injury.
- Units of impulse
- The SI unit of impulse is N s, equivalent to kg m s⁻¹.
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