How Forces Affect Motion — Practice

Question 1

State the SI unit of force and define 1 newton.

Accepted Answer

The SI unit is the newton (N). 1 N is the force that gives a 1 kg mass an acceleration of 1 m/s² (1 N = 1 kg·m/s²).

✦ Answer: newton; 1 N = 1 kg·m/s².

Question 2

Why do passengers lurch forward when a moving bus suddenly stops?

Accepted Answer

By inertia, the body tends to continue in its state of motion; when the bus stops, the upper body keeps moving forward.

✦ Answer: due to inertia of motion.

Question 3

Find the momentum of a 4 kg ball moving at 5 m/s.

Accepted Answer

Step 1 — p = mv = 4 × 5.

Step 2 — p = 20 kg·m/s.

✦ Answer: 20 kg·m/s.

Question 4

What force gives a 5 kg body an acceleration of 3 m/s²?

Accepted Answer

Step 1 — F = ma = 5 × 3.

Step 2 — F = 15 N.

✦ Answer: 15 N.

Question 5

Explain, using Newton's third law, how a swimmer moves forward.

Accepted Answer

Step 1 — The swimmer pushes the water backward (action).

Step 2 — The water pushes the swimmer forward with an equal and opposite force (reaction).

Step 3 — The forces act on different bodies, so the swimmer accelerates forward.

✦ Answer: action on water backward → reaction on swimmer forward.

Question 6

A 0.5 kg ball's velocity changes from 4 m/s to 10 m/s in 2 s. Find the force.

Accepted Answer

Step 1 — F = (mv − mu)/t = (0.5×10 − 0.5×4)/2.

Step 2 — F = (5 − 2)/2 = 3/2 = 1.5 N.

✦ Answer: 1.5 N.

Question 7

A 2 kg trolley moving at 3 m/s collides with a stationary 1 kg trolley and they move off together. Find their common velocity.

Accepted Answer

Step 1 — Total momentum before = 2×3 + 1×0 = 6 kg·m/s.

Step 2 — After: combined mass = 3 kg moving at v.

Step 3 — 6 = 3v → v = 2 m/s.

✦ Answer: 2 m/s.

Question 8

A 4 kg gun fires a 0.02 kg bullet at 100 m/s. Find the recoil velocity of the gun.

Accepted Answer

Step 1 — Initial momentum = 0 (both at rest).

Step 2 — 0 = (0.02 × 100) + (4 × V).

Step 3 — 0 = 2 + 4V → V = −0.5 m/s.

✦ Answer: 0.5 m/s, opposite to the bullet (recoil).

Question 9

A cricketer pulls his hands back while catching a fast ball. Explain using momentum and force why this prevents injury.

Accepted Answer

Step 1 — The ball has a fixed momentum that must be reduced to zero (a fixed change in momentum).

Step 2 — By the second law, F = change in momentum / time.

Step 3 — Pulling the hands back INCREASES the time over which the ball stops.

Step 4 — A larger time means a smaller force on the hands, so injury is avoided.

✦ Answer: longer contact time → smaller force (F = Δp/t).

Practice — How Forces Affect Motion

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