Page 73 - Schaum's Outline of Theory and Problems of Applied Physics

58 LAWS OF MOTION [CHAP. 5

Fig. 5-4

(b) We can use either of the two equations to find the tension T. From the first equation,
2
T = m A a + m A g = m A (a + g) = (5kg)(4.0 + 9.8) m/s = 69 N

THIRD LAWOFMOTION
According to Newton’s third law of motion, when one body exerts a force on another body, the second body
exerts an equal force in the opposite direction on the first body.
The third law of motion applies to two different forces on two different bodies: the action force one body
exerts on the other, and the equal but opposite reaction force the second body exerts on the first. Action and
reaction forces never balance out because they act on different bodies.

SOLVED PROBLEM 5.20
A book rests on a table. (a) Show the forces acting on the table and the corresponding reaction forces.
(b) Why do the forces acting on the table not cause it to move?

(a) See Fig. 5-5.
(b) The forces that act on the table have a vector sum of zero, so there is no net force acting on it.