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A rubber sucker is pressed firmly against a smooth flat surface, as shown. It is now very difficult to pull it off.  The sucker sticks to the surface mainly because of:
AThe magnetic attraction between rubber and the surface
BThe frictional force between rubber and the surface
CThe atmospheric pressure pushing on the outer side of the sucker
DThe gravitational pull of the surface on the sucker
Answer & Solution
Correct answer: C. The atmospheric pressure pushing on the outer side of the sucker
When the sucker is pressed, most of the air between its cup and the surface is squeezed out. With very little air left inside, the pressure under the cup is much lower than the atmospheric pressure pushing on its outer surface. This excess outside pressure holds the sucker firmly against the surface.
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