Students connect a spring scale to a block on a rough horizontal surface.

Question

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?

1. A) he gravitational and normal forces exerted on the block, because they are a Newton’s third-law pair.
2. 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

Answer B because for an object to move horizontally at constant speed, the horizontal forces must be equal in magnitude and opposite in direction (no acceleration means forces are balanced, net force is zero). The only horizontal forces are the friction and the force exerted by the spring scale.