Chapter 7, Problem 32A

Suppose you're weighing yourself while standing next to the bathroom sink. Using the idea of action and reaction, explain why the scale reading will be less when you push down on the top of the sink. Why will the scale reading be more if you pull up on the bottom of the sink?

Solution

To explain: The reason for less scale reading while pushing down on the top of the sink. Also, the reason for more scale reading while pulling up on the bottom of sink.

Introduction:

Newton’s thirds law states that objects exerts equal and opposite forces on each other. Also, these force are not equal if force is applied from outside the system.

Explanation:

Let a man standing near the sink and weighing without touching the sink. The scale reading shows accurate reading, since the force of gravity exerted by earth on man and the reaction force of man are equal and opposite. So these forces cancel each other.

But when man pushes on the top of the sink the less force exerted by man on the weight balance. Since the man is exerting the force on the sink. Sink will lift man off the scale, and due to this, scale reading will be less.

When man pull the sink upwards, the sinks reaction is opposite to man which pushes man downward, and due to which scale reading shows more.