Students connect a spring scale to a block on a rough horizontal surface. The students use the spring scale to measure the magnitude of the horizontal force needed to pull the block at a constant speed. Which of the following statements explains why two forces exerted between objects are equal in magnitude?
(A) The gravitational and normal forces exerted on the block, because they are a Newton's third-law pair.
B. The frictional force and force exerted by the spring scale on the block, because they are a Newton's third-law pair.
(C) The normal force and the frictional torce exerted on the block because objects always exert forces of equal magniude on each other.
(D) The frictional forces that the block and the surface exert on each other. because objects always exert forces of equal magnitude on each other.

Question

Accepted Answer

∻Solution∻ ‖ a ⋮ According to Newton's Third Law, for every action, there is an equal and opposite reaction. ‖ ‖ b ⋮ When the block is pulled at a constant speed, the net force on the block is zero, which means the force exerted by the spring scale is equal in magnitude to the frictional force opposing the motion. ‖ ‖ c ⋮ The frictional forces that the block and the surface exert on each other are equal in magnitude and opposite in direction, satisfying Newton's Third Law. ‖ ***D*** ∻Key Concept∻ ⚹ Newton's Third Law: For every action, there is an equal and opposite reaction. Equation: $$F_{ ext{action}} = -F_{ ext{reaction}}$$ (Action and reaction forces are equal and opposite) ⚹ ∻Explanation∻ ⚹ The correct answer is D because it correctly identifies the frictional forces between the block and the surface as a Newton's third-law pair, which are always equal in magnitude and opposite in direction.⚹

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