A.2 · 4 · A.2 Forces and momentum · conservation of momentum
Momentum conservation
Newton’s second and third laws together: what one object gains, the other loses.
SL+HLHarder18 minFree
Hook
Two skaters at rest push apart. One glides left, the other right. Why do their momenta always cancel out?
Derivation
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Two skaters, 60 kg and 40 kg, push apart from rest. The 60 kg skater moves at 2.0 m s⁻¹. The 40 kg skater moves at…
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The total momentum of a system is conserved when…
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A 2.0 kg cart at 3.0 m s⁻¹ hits a stationary 1.0 kg cart and they stick together. Their speed is…
Apply
Explain why the momentum of two colliding bodies is conserved, using Newton’s second and third laws.
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Remember earlier lessons
From lesson A.2 · 3
Action and reaction act on…
From lesson A.2 · 2
Momentum is measured in…
Summary card
Key points
- • Third law: equal and opposite forces. Second law: F = Δp / t.
- • Together: Δp₁ = −Δp₂, so Δp_sys = 0.
- • Momentum is always conserved for an isolated system.
Formulas
- Δpsys = 0 if Fnet,ext,sys = 0
- m1u1 + m2u2 = m1v1 + m2v2
Key terms
- Isolated system :
- no net external force
Common errors
- • Using conservation when an external force acts (for example friction).
- • Forgetting signs for direction.
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