Question

make an argument. carlos gets tired of pushing and instead begins to pull with force ( f_{pull}) at an angle to the horizontal. the block slides along the rough horizontal surface at a constant speed. a free - body diagram for the situation is shown below. blake makes the following claim about the free - body diagram: blake: the velocity of the block is constant, so the net force exerted on the block must be zero. thus, the normal force ( f_n) equals the weight ( f_{wg}), and the force of friction ( f_f) equals the applied force ( f_{pull}). what, if anything, is wrong with this statement? if something is wrong, identify it and explain how to correct it. if this statement is correct, explain why. checklist: i answered the question directly. i stated a law of physics that is always true. i connected the law or laws of physics to the specific circumstances of the situation. i used physics vocabulary (force, mass, acceleration, velocity, coefficient, friction).

Explanation:

Blake is correct that the net - force is zero due to constant velocity (Newton's first law). However, the normal force does not equal the weight because the pulling force has an upward vertical component. In the vertical direction, $F_N+F_{pull,y} = F_{mg}$, so $F_N=F_{mg}-F_{pull,y}$. In the horizontal direction, the friction force $F_f$ equals the horizontal component of the pulling force $F_{pull,x}$, i.e., $F_f = F_{pull,x}$.

Answer:

The statement is wrong. The normal force $F_N$ does not equal the weight $F_{mg}$ because the pulling force $F_{pull}$ has an upward - vertical component. The friction force $F_f$ equals the horizontal component of the applied force $F_{pull,x}$, not the entire applied force $F_{pull}$.