Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

4.2Linear Momentum

Change in Momentum and Impulse — AP PHYSICS 1 Revision Notes & Topical Questions

5 min read 8 parts Lesson 7 of 13 1 quiz

📚 AP PHYSICS 1 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

4.2 Change in Momentum and Impulse — AP PHYSICS 1 notes: Describe the impulse delivered to an object or system and its relation to the change in momentum.Impulse = the product of the average force and the time interval over which it acts, J = FavgΔt.Unit — N·s = kg·m/s.Vector — i…

  • Variable force
  • Definition
  • Rebound versus stop
  • Internal forces
  • Newton's second law
  • Momentum–time graph

Part 1 of 6Impulse

Impulse
Impulse the product of the average force and the time interval over which it acts, J = FavgΔt.
Unit
  • N·s = kg·m/s.
Vector
  • Vector it has the direction of the net force.
Variable force
  • Variable force the impulse equals the area under the force–time graph.

Part 2 of 6Change in momentum

Definition
  • Δp = pf − pi = mvf − mvi, found by vector subtraction with signs.
Rebound versus stop
  • Rebound versus stop a ball hitting a wall at v and bouncing back at v has |Δp| = 2mv, double that of stopping.
Internal forces
  • Internal forces transfer momentum between objects;
    • only a net external impulse changes the system's total momentum.

Part 3 of 6Impulse–momentum theorem

Jnet = Δp
  • Jnet = Δp the net impulse equals the change in momentum, mvf − mvi.
Newton's second law
  • Fnet = Δp/Δt, so the net external force equals the rate of change of momentum.
    • (F = ma for constant mass)
Momentum–time graph
  • Momentum–time graph its slope equals the net force.

Part 4 of 6Reducing impact force

DeviceEffect on ΔtΔpForce
AirbagIncreasesSameDecreases
Crumple zoneIncreasesSameDecreases
Bending knees on landingIncreasesSameDecreases
Padding or helmetIncreasesSameDecreases
Follow-through (tennis, golf)Longer contactLargerLarger speed given to the ball

Part 5 of 6Using the theorem

1
Choose a positive direction.
2
Write the initial and final velocities with signs.
3
Calculate Δp = mvf − mvi.
4
Set Δp equal to the impulse (FavgΔt or area under the F–t graph).
5
Solve for the unknown force, time or velocity and state its direction.

Part 6 of 6More from the specification

1Automotive safety crumple zones
  • Automotive safety crumple zones engineered vehicle crumple zones deform during frontal impacts, extending collision duration to reduce deceleration forces on passengers.
2Inertia
  • Resistance of an object to changes in its motion.
3Force-time graph
  • A graph whose area is the impulse.
4Linear momentum
  • The product of mass and velocity.
5Kinetic energy
  • K = ½mv2, energy of motion.
6Collision
  • Short interaction with large internal forces.

Key terms

TermMeaning
ImpulseJ = FΔt = Δp.
Impulse–momentum theoremNet impulse equals change in momentum.
Force–time graphGraph whose area is the impulse.
Average forceF = Δp/Δt.
ReboundBounce with reversed velocity, giving a larger Δp.

Avoid these mistakes

1
Forgetting signs, so a rebound is treated as a stop.
2
Thinking longer contact time reduces the momentum change — only the force falls.
3
Using the peak force as the average force.
4
Confusing impulse with force.

Last updated: