Forces - Biomechanics of the Human Body - page 10

reaction were not drawn. The reaction forces are forces applied by the object on the
hand that pulls or pushes the object.

10 3 N 1 Exercise 1.5 In high jumping, an athlete exerts a force of 3,000 N ¼ 3

¼ 3 kN against the ground during impulsion. Find the force exerted by the ground

on the athlete. Discuss the nature of this force.

1.7 Some Specific Forces

1.7.1 Weight

The weight of a body or of any object is the force with which a body is attracted to
the earth. This explains the fact that an object always falls when released at a certain
height from the ground. This force, also called gravitational force, or simply weight,
is exerted by the earth on bodies not necessarily in contact. Other examples of force
among bodies that do not need to be in contact are electric forces and magnetic
forces (force exerted by a magnet on metallic objects, see Fig. 1.2 ). Gravitational
force is always directed toward the center of the earth, as illustrated in Fig. 1.10 .
The magnitude of weight vector W is then given by ( 1.2 ):

F ð N Þ¼ W ð N Þ¼ mg : (1.2)

m is the mass of the body, measured in kilograms (kg), and g is the acceleration
of gravity, equal to 9.8 m/s 2 in any location near the surface of the earth. As we
move toward outer space, away from our planet, the value of g decreases, which
can be observed in spaceship travels (weightlessness). In this topic, we will use
g

10 m/s 2 . The weight of a body with mass equal to 1.0 kg, where the accelera-
tion of gravity is 9.8 m/s 2 , is 9.8 N, practically equal to 10.0 N. Using this g value,
we are making an approximation of 2 % in excess.
The weight of the human body on the moon is approximately 1/6 that on the
earth, although the mass is the same, due to the low value of the acceleration
of gravity on the moon, 1.6 m/s 2 . The astronauts who walked on the moon felt
lighter, released from the weight of their bodies and while walking looked as if
they were hopping. If their total mass, including what they were wearing, was
100 kg, their weight on earth would be around 1,000 N, while on the moon, it would
decrease to 160 N.
The reaction to the weight W (action), exerted by the earth on a body, is the force
that the body exerts on the earth (reaction force R ) and acts on the center of the
earth. Its magnitude is the same as the weight and is in the opposite direction.

¼

1 Scientific notation: very large or very small numbers can be written, using powers of 10:
175,000 10 5 or 0.000175 10 4 .

¼ 1.75 ¼ 1.75