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# Definition of a force

A force is defined as something that causes acceleration.

Hitting a tennis ball causes the tennis ball to accelerate. The racquet exerts a force on the ball.

Whenever an object accelerates (changes the speed or direction of its movement), a force must be acting on it.

Nails placed near a magnet accelerate towards the magnet. The magnet exerts a force on the nails.

The unit of force is the newton $\text{(N)}$. The newton is defined in SI base units as $1\text{ N}=1\text{ kg}\times\text{ m/s}^2$.

An aircraft needs a force to accelerate on the runway. The force comes from the jet engines.