# If a body exerts a force on a surface, the surface exerts a reaction force on the body. This reaction force is always along normal to the surface

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If a body exerts a force on a surface, the surface exerts a reaction force on the body. This reaction force is always along normal to the surface at that point and is popularly known as the normal force N. When the body slides on a surface, the surface exerts a force of friction on the body in the direction opposite to the direction of slipping. The magnitude of this frictional force is given by µN, where µ it is a constant (called coefficient of friction) for the given pair of surfaces in contact and N is the normal force.

Consider a block of mass 100 kg placed on a level surface. If one person tries to push it with a force F, he is unable to do so. He receives help from two of his friends and each one applies the same force F. Now the block moves with a uniform acceleration of 0.5 m/s2. After sometime, one of them happens to leave the exercise of pushing the block. This time the block moves with a uniform speed. The magnitude of force F and the coefficient of kinetic friction µ are respectively _________ .

(a) 50 N, 0.05

(b)100 N, 0.1

(c) 50 N, 0.1

(d)100 N, 0.2

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Correct option: (c) 50 N, 0.1

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