Forces - Biomechanics of the Human Body - page 9

1.6.2 Newton's Second Law (Mass and Acceleration)

The action of a nonzero resultant force on a body produces change in the vector
velocity, that is, produces an acceleration a . This acceleration is proportional to the
intensity of a net force F and inversely proportional to the mass m of the body, that is,

a ¼ F = m :

Then, we can write that F ¼ ma . The unit of velocity in SI is m/s, and as

acceleration is given by

a ¼ Δ v= Δ t ;

t , its unit in SI is m/s 2 . Therefore,
the unit of force is kg m/s 2 which receives the special name newton, N, to honor the
father of classical mechanics, Isaac Newton (1642-1727).

that is, the rate of change of speed

Δ v with time Δ

Exercise 1.4 What force should act on a ball of 0.6 kg mass, through a kick,
to acquire an acceleration of 40 m/s 2 ?

1.6.3 Newton's Third Law (Action and Reaction)

Force is a consequence of the interaction between two bodies. The third law states
that for each action force corresponds a reaction force of equal intensity but
opposite direction. Action and reaction act on different bodies. In the examples of
Fig. 1.1 , the forces (action forces) applied on an object were depicted. The forces of