Newton’s laws of motion — Physics, 14–17 years
How forces change motion, and why objects do not need a force to keep moving. Physics, 14–17 years.
The idea
An object changes its motion only when the forces on it do not balance. A bigger net force produces a bigger acceleration, while a bigger mass makes the same force less effective. Forces also come in pairs: when you push something, it pushes back with equal strength in the opposite direction.
Why we need it
Before Newton, people often thought that continuous force was needed to keep an object moving. Everyday friction makes that idea seem sensible: a bicycle slows when you stop pedalling. Newton’s laws separate the push from the resistance, so they let us explain motion on a smooth surface, in a vehicle, or in space.
Worked example
A 4 kg trolley is pulled with 12 N to the right, while friction pulls with 4 N to the left. First find the net force: 12 − 4 = 8 N to the right. Then use F = ma: a = 8 ÷ 4 = 2 m/s² to the right. The trolley speeds up in that direction.
The common trap
A common mistake is saying that a moving object must have a forward force. That feels reasonable because air resistance and friction usually slow real objects, so we keep pushing to cancel them. But if all forces balance, the object can keep moving at constant velocity; a net force is needed for acceleration, not for motion itself.
Where it appears
Seat belts rely on the first law: your body keeps moving when a car stops suddenly, so the belt supplies the force that slows you safely. Engineers use the second law to choose motors, brakes, and vehicle masses. Rockets use the third law as hot gases are pushed backwards and the rocket is pushed forwards.
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