Force, inertia, momentum, and why every action has a reaction.
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A force changes, or tries to change, the state of rest or motion of a body, or its shape. Newton's first law: a body stays at rest or in uniform motion in a straight line unless acted on by an external force. This property is inertia; the greater the mass, the greater the inertia. A passenger lurches forward when a bus brakes because the body tends to keep moving.
Why does dust fly off a carpet when it is beaten?
The carpet moves but the dust, by inertia, tends to stay at rest and so leaves it.
Which has more inertia: a 2 kg or a 5 kg body?
The 5 kg body; inertia depends on mass.
Why should passengers wear seat belts?
In a sudden stop inertia carries them forward; the belt applies the force that stops them.
Momentum p = mass × velocity (kg m/s). Newton's second law: the rate of change of momentum is proportional to the applied force, which gives F = ma. The unit of force is the newton: 1 N gives 1 kg an acceleration of 1 m/s². Force = change in momentum ÷ time, so a longer stopping time means a smaller force (cricketers draw their hands back when catching).
Find the force to accelerate a 5 kg body at 4 m/s².
F = 5 × 4 = 20 N.
A 2 kg body moves at 3 m/s. Find its momentum.
p = 2 × 3 = 6 kg m/s.
A 10 kg body is brought from 20 m/s to rest in 5 s. Find the force.
F = 10 × (20 − 0) ÷ 5 = 40 N.
Newton's third law: to every action there is an equal and opposite reaction, acting on different bodies. A rocket, a swimmer pushing water back, and a gun recoiling all show it. In a collision or explosion with no external force, total momentum stays constant. Before firing a gun and bullet are at rest, so total momentum is zero; afterwards the gun's momentum is equal and opposite to the bullet's.
A 50 g bullet leaves a 5 kg gun at 400 m/s. Find the recoil speed.
Bullet momentum = 0.05 × 400 = 20 kg m/s. Gun: 5 × v = 20, so v = 4 m/s backwards.
Why do action and reaction not cancel out?
They act on different bodies, so they cannot cancel each other.
How does a rocket move in space?
It throws gases backwards; the reaction pushes the rocket forwards.
1. State Newton's first law.
2. Define 1 newton.
3. Find the force on a 3 kg body with acceleration 6 m/s².
4. Define momentum and give its unit.
5. State the law of conservation of momentum.
6. Why does a gun recoil?
7. A 1000 kg car moving at 20 m/s stops in 10 s. Find the force.
8. Why is it dangerous to jump from a moving bus?
9. Why do cricketers lower their hands while catching?
10. Find the momentum of a 0.5 kg ball at 12 m/s.