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.