Force, mass and acceleration — Science, 14–17 years
A net force changes an object's motion. The change is larger for a stronger force and smaller for a larger mass: F = m × a.
Idea
Acceleration means that velocity changes: an object may speed up, slow down or turn. The net force is the overall push or pull after opposing forces are combined. Newton's second law links them: force equals mass multiplied by acceleration, F = m × a.
Why it matters
People could observe that pushing changes motion, but they needed a way to predict how much. The law solves that problem by including both the push and the object's resistance to change, its mass. It explains why the same push moves an empty trolley more easily than a loaded one.
Worked example
A 4 kg trolley is pulled with a net force of 12 N. Use F = m × a. Rearrange to a = F ÷ m, then substitute: a = 12 N ÷ 4 kg = 3 m/s². Each second, its velocity changes by 3 metres per second in the direction of the net force.
Common trap
It is easy to think that a moving object must have a forward force. That feels right because pushing often starts the motion. But if the forces balance, a moving object keeps a constant velocity; only an unbalanced net force changes its speed or direction.
Outside school
This relationship helps engineers design car brakes, lifts, sports equipment and spacecraft. A car's mass and the available tyre force affect how quickly it can stop. In sport, changing the mass of a ball or the strength of a kick changes its acceleration, not just its final speed.
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