Force: any influence which tends to change the motion of an object

Force = mass x acceleration

Pressure: the force per unit area applied in a direction perpendicular to the surface of an object.

Pressure = Force ÷ Area

Stress: the average amount of force exerted per unit area.

Stress = force ÷ Area

Strain: relative longitudinal change in body length

Strain = change in length÷ original length