Question

Ask a friend to drive a small nail into a piece of wood placed on top of a pile of books on your head. Why doesn’t this hurt you?
(A) Because the nail has a sharp point and thus, exerts less pressure on wood
(B) The force is exerted on the wood; thus, it has no effect on our head
(C) The relatively large mass of the books on your head
(D) Action-reaction forces cancel each other

Answer 1

In this problem, we are going to apply the concept of inertia which states that a body at rest tends to be at rest and if a body is in motion then it will remain in its state of motion until we applied external force on it.

Complete step by step answer:
In the given question, we are supplied with the following data:
-I have a pile of books on my head along with a piece of wood placed on the top of the pile.
The other one drives a nail into a piece of wood. We are asked to find out what should be the reason for which the first person i.e. I do not get hurt in such an incident.
-Due to inertia, the relatively large mass of the books and block on your head avoid being shifted. So, the force that pushes the nail effectively will not be as effective in speeding up the huge books and bricks, which do not move as fast when the nail is hit. According to Newton’s first law, we have that a body tends to be at rest or in motion if no force acts from outside.
-Again, we can say that the pressure applied to the object is proportional to the force acting on the object per area of the component and, in order to establish pressure, the force must be normal to the object. The pressure is inversely proportional to the area on which the force works, meaning the pressure would be lower in the impact area.
For the expression pressure we have:
$P = \dfrac{F}{A}$
Where,
$P$ indicates pressure.
$F$ indicates force applied.
$A$ indicates area.

Hence, it is quite clear that I do not get hurt due to the relatively large mass of the books and the block on the head, which resists being moved. Thus,the correct option is C.

Note: It is purely based on Newton's law of inertia, which states that a body in motion tends to be in motion and a body at rest tends to be at rest, if no external force is being applied. A passenger sitting inside a bus, leans forward when the driver pulls the brakes.