Firstly, if an object is acted on by an external force its velocity will change, it will accelerate, in the direction of the force.

Secondly, the change in velocity, acceleration, is directly proportional to the magnitude of the applied force.

Thirdly, the change in velocity is inversely proportional to the mass of the object. If the same force pushes on a 10 kg toy truck and a 2 kg toy truck the acceleration of the 2 kg toy truck will be five times greater than the acceleration of the 10 kg toy truck.

This law can be summarised by the formula

F = ma

F = force (Newton)
m = mass (kg)
a = acceleration (ms^-2)

Continue with F = ma