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. 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.

"The frictional force and force exerted by the spring scale on the block, because they are a Newton's third-law pair" statement explains why two forces exerted between objects are equal in magnitude.

Answer: Option B

Explanation:

Every object in this universe obeys the laws of motion proposed by Newton. According to the laws, an object must have equal and opposite directional horizontal forces acting on it to move horizontally in constant speed. Thus here for the block to move horizontally over a rough horizontal surface, the block will be exhibiting frictional force between the surface of block and the horizontal surface as well as pulling force of spring.

So, these are the two horizontal forces act on the objects. Also as per Newton's third law, every action will have equal but with opposite reaction. So the pulling force should be the acting force and it should be equal and opposite of the frictional force so that the object can move at constant speed. Thus the option stating that the frictional force and the force exerted by the spring scale on the block, because they are a Newton's third law pair is true.

The frictional forces that the block and the surface exert on each other, because objects always exert forces of equal magnitude on each other.