Question

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 ar 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 force exerted on the block, because objects always exert forces of equal magnitude 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.

Answer 1

For there to be a reaction there must also be action. In the horizontal movement there is a balance in which the magnitude of the Forces in opposite directions must be in total 0. The only horizontal forces are friction and the force exerted by the spring scale. For this reason the correct answer is B:

The frictional force and force exerted by the spring scale on the block, because they are a Newton's third-law pair.