Question

A rubber sucker sticks to a surface because of
A. property of rubber
B. gravitational force
C. elastic spring force
D. atmospheric pressure

Answer 1

Hint: For the rubber sucker to stick to the surface there must be a net force that is pushing it towards the surface. Use the concept and formula of pressure i.e. $P=\dfrac{F}{A}$. When the rubber is pressed against a surface, the air molecules in the sucker escape out making the inside pressure less than the outside pressure. This statement will help you to think about a proper answer.
Formula used:

$P=\dfrac{F}{A}$

Complete step by step answer:
A rubber sucker consists of a rubber cup (hollow hemisphere) with a stick attached at the top of the cup. When the rubber sucker is pressed against a surface, maybe a wall, it sticks to the surface.

Before proceeding further, let us discuss a concept, which will help us.
Pressure is the perpendicular force exerted per unit area. In simple words, pressure (P) is the perpendicular force (F) acting on a surface divided by the area (A) of that surface.

i.e. $P=\dfrac{F}{A}$
$\Rightarrow F=PA$.

This means that force on the body is equal to the product of the pressure exerted on a body and the area of the surface of the body.

If the area is constant, then force is directly proportional to the pressure.

Air is made up of particles or molecules, which exert some pressure on the surrounding objects called atmospheric pressure. The pressure depends on the number of air molecules. More the air molecules, more is the pressure and more pressure means greater force is being exerted on the body.

When we press the rubber sucker against a surface, all the air molecules inside the rubber cup escape out. Hence, fewer air molecules are left inside in the cup than they were before. Therefore, the pressure outside is more than the pressure inside the cup.
Hence, there are two forces exerting on the cup. But the outside force is greater than the inside force because the outside pressure is greater than the inside pressure. Therefore, the net force on the rubber cup is directed towards the surface. Hence, we can see that the rubber cup is pushed against the surface.

In return, the surface also applies a force on the cup and hence, the rubber sucker is at rest. And it appears that the rubber sucker is stuck to the surface.
Therefore, the rubber sucker sticks to a surface due to the difference in the atmospheric pressure.
Hence, the correct option is D.

Note: Gravitational force may or may not help in the sticking of the rubber sucker. Since gravitational force on a body is always directed downwards, it will help the rubber sucker to stick to the surface when the sucker is on a horizontal floor.
Devices that help in suction like vacuum cleaner work on the principle of the pressure difference.