A.2 · 6 · A.2 Forces and momentum · change in momentum
Momentum change: four cases
When the direction reverses, treat momentum as a vector. Use a number line.
SL+HLHarder16 minFree
Hook
A ball hits a wall at 5 m s⁻¹ and bounces back at 2 m s⁻¹. Is its change in momentum 3 units, or something else?
The four cases
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Case 2: 0.2 kg ball, 5 m s⁻¹ down to 3 m s⁻¹ (same direction). Δp is…
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Case 3: the same ball is brought to rest. Δp is…
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A ball’s velocity goes from +5 m s⁻¹ to −2 m s⁻¹. The change in velocity is…
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Which mistake gives the wrong answer for a rebound?
Apply
A 0.50 kg ball hits a wall at 4.0 m s⁻¹ and rebounds at 1.0 m s⁻¹. Find the change in momentum. Show your choice of positive direction.
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Remember earlier lessons
From lesson A.2 · 5
Momentum is a…
From lesson A.2 · 2
F_net = …
Summary card
Key points
- • Δp = m(v_f − v_i), with signs.
- • If the direction reverses, one velocity is negative.
- • Use the number line: the change is the arrow from i to f.
Formulas
- Δp = pf − pi = mΔv
Key terms
- Change in momentum :
- final momentum minus initial momentum, a vector
Common errors
- • Subtracting speeds instead of velocities.
- • Dropping the sign of the final velocity after a rebound.
Checks solved: 0 of 4