4-5 Newton’s Third Law of Motion
Helpful notation: the first
subscript
is the object that the force is being exerted on; the
second is the source.

4-5 Newton’s Third Law of Motion
Conceptual Example 4-5: Third law clarification.
Michelangelo’s assistant has been assigned the task of moving a block of marble using a sled. He says to his boss, “When I exert a forward force on the sled, the sled exerts an equal and opposite force backward. So how can I ever start it moving? No matter how hard I pull, the backward
reaction force always equals my forward force, so the net force must be zero. I’ll never be able to move this load.” Is he correct?

4-6 Weight—the Force of Gravity; and the Normal ForceWeight is the force exerted on an object by gravity. Close to the surface of the Earth, where the gravitational force is nearly constant, the weight of an object of mass mis:where

4-6 Weight—the Force of Gravity; and the Normal ForceAn object at rest must have no net force on it. If it is sitting on a table, the force of gravity is still there; what other force is there?

4-6 Weight—the Force of Gravity; and the Normal ForceExample 4-6: Weight, normal force, and a box.A friend has given you a special gift, a box of mass 10.0 kg with a mystery surprise inside. The box is resting on the smooth (frictionless) horizontal surface of a table. (a) Determine the weight of the box and the normal force exerted on it by the table. (b) Now your friend pushes down on the box with a force of 40.0 N. Again determine the normal force exerted on the box by the table. (c) If your friend pulls upward on the box with a force of 40.0 N, what now is the normal force exerted on the box by the table?

4-6 Weight
—
the Force of Gravity; and
the Normal Force