A force is a push or a pull.

The swimmers in the picture are applying force to the wall by pushing against it. This will generate a reaction force from the wall, which will push them in the opposite direction when the race starts.

Unbalanced forces cause the acceleration of an object. The swimmers will accelerate down the pool at the start of the race.

Forces are measured in Newtons (abbreviation N).

1 Newton is the force required to change the velocity of a 1 kg mass by 1 m/s every second. So, \(1 \text{ N} = 1 \text{ kg m/s}^2.\)