Newton's Second Law and Momentum
The net external force equals the rate of change of linear momentum, F = dp/dt, reducing to F = ma for constant mass.
What is Newton's Second Law and Momentum?
The net external force equals the rate of change of linear momentum, F = dp/dt, reducing to F = ma for constant mass.
Key formula / rule: F = dp/dt is general; F = ma assumes constant mass.
Key points
- Derive F = ma from the momentum form of the second law.
- Compute impulse and average force from a collision record.
- Analyse a variable-mass situation using F = dp/dt.
Common exam trap
Using F = ma for rockets or falling chains where mass varies.
Definitions
- Term
Newton's Second Law and Momentum
- Meaning
The net external force equals the rate of change of linear momentum, F = dp/dt, reducing to F = ma for constant mass.
- Term
Newton's Second Law and Momentum — explanation
- Meaning
The momentum form is the general statement and is essential for variable-mass problems such as rockets and for impulse. Impulse J = F Dt = Dp explains why cushioning reduces the force felt in a collision.
Learning objectives
Derive F = ma from the momentum form of the second law.
Compute impulse and average force from a collision record.
Analyse a variable-mass situation using F = dp/dt.
Formulae
- Key point
F = dp/dt is general; F = ma assumes constant mass.
- Key point
Impulse equals change of momentum and equals the area under an F-t graph.
- Key point
Force and acceleration are always parallel for constant mass.
Prerequisites
PHY-U1-NLM-T1-S1-C1
Common mistakes
Using F = ma for rockets or falling chains where mass varies.
Confusing impulse with force when the contact time is unstated.
Keywords
Newton
Second
Momentum
Practice preview
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