Momentum Matters

Why it matters

A table-tennis ball at bullet speed or a bowling ball at walking pace — which would you rather catch? Physics has one number that answers: momentum.

The idea in plain words

Mass in motion. Tap the terms.

p = m × v

A 60 g tennis ball arrives at 30 m/s and returns at 30 m/s. Change in momentum?

3.6 kg·m/s — direction flips DOUBLE the change

• kg·m/s: p — linear momentum — a VECTOR along the velocity
• kg·m/s: Δp — change in momentum = mΔv (same mass) — what a force actually produces
• the bridge to F: rate of change — Δp/Δt = force — Newton's second law in its original clothes
• p₁ = 0.06 × 30 = 1.8, p₂ = −1.8 (direction flipped!)
• Δp = 1.8 − (−1.8)

Predict first

Which has MORE momentum: a 0.5 kg ball at 20 m/s, or a 5 kg ball at 2 m/s?

p = mv: 0.5 × 20 = 5 × 2 = 10. Momentum is the honest product of both — neither mass nor speed alone.

• equal — both are 10 kg·m/s — correct
• the fast one, always
• the heavy one, always

What you do

Build p = 12 kg·m/s twice: once heavy and slow, once light and fast.

Check yourself

Momentum is a…

Mass (scalar) × velocity (vector) = vector. Two equal-speed trains, opposite tracks: opposite momenta.

A stationary loaded truck's momentum is…

Any mass × zero velocity = zero. Momentum is mass IN MOTION.

The S.I. unit of momentum is…

kg·m/s from the formula; N·s from force × time — the same thing, two outfits.

• vector — its direction is the velocity's — correct
• scalar
• unit of force
• zero — v = 0 — correct
• huge — it's very heavy
• cannot be told
• kg·m/s (equivalently N·s) — correct
• newton
• joule