A) Gravitational
B) Compressive
C) Frictional
D) Magnetic
Correct Answer: C
Solution :
The given boxes are kept between two vertical walls. When the first box is pulled upward along the walls, frictional force acts between the surfaces of the box and the walls, in the downward direction. When the walls are lubricated with oil, this frictional force is reduced. Oil eliminates the roughness (as a result of friction) between the surfaces of the box and the walls. This allows the box to slide over with minimum friction. Hence, Calvin had to apply less force to lift the second box. The correct answer is C.
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