If you exert a horizontal force of 200 N to slide a crate across a factory floor at constant velocity, how much friction is exerted by the floor on the crate? Is the force of friction equal and oppositely directed to your 200-N push? If the force of friction isn’t the reaction force to your push, what is?
The friction on the crate is 200 N, which cancels your 200 N push on the crate to yield the zero net force that accounts for the constant velocity ( zero acceleration). Although the friction force is equal and oppositely directed to the applied force, the two don’t make an action reaction pair of forces. The frictional force is indeed equal and oppositely directed to the 200 N pushing force. That’s because both forces do act on the same object the crate. The reaction to your push on the crate is the crate push back on you. The reaction to the frictional force of the floor on the crate is the opposite friction force of the crate on the floor.