A.2 · 2 · A.2 Forces and momentum · Newton’s laws, momentum
Newton's second law
The law of acceleration: force is the rate of change of momentum.
SL+HLMedium16 minFree
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A footballer’s kick and a gentle tap can send the ball in the same direction. What is different about the push?
Newton’s second law
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The unit N s is equivalent to…
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The gradient of a momentum–time graph gives…
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A 2.0 kg trolley’s velocity changes from 1.0 m s⁻¹ to 5.0 m s⁻¹ in 4.0 s. The net force is…
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A friend says "F = ma is the whole second law". Explain what the law says in terms of momentum, and when F = ma can be used.
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Remember earlier lessons
From lesson A.2 · 1
If F_net = 0, then…
From lesson 0.6
36 km/h in m s⁻¹ is…
Summary card
Key points
- • F_net,ext = Δp / t: net force is the rate of change of momentum.
- • p = mv is a vector, unit kg m s⁻¹ = N s.
- • For constant mass, F = ma.
- • The gradient of a p–t graph is the net force.
Formulas
- p = mv
- Fnet,ext = Δp / t
- F = ma (constant mass)
Key terms
- Momentum :
- mass × velocity, a vector
- Newton second law :
- net external force = rate of change of momentum
Common errors
- • Using F = ma when the mass changes.
- • Forgetting that momentum has a direction.
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