Question

A crane lifts a carton upwards with constant speed.
If gravity and the applied force are the only two forces acting on the carton, what can you say must be true about the weight $mg$ and the applied force $F_A$?

• A. $|mg|<|F_A|$
• B. $|mg|\le |F_A|$
• C. $|mg|=|F_A|$
• D. $|mg|\ge |F_A|$
• E. $|mg|>|F_A|$

Answer 1

The correct option is C $|mg|=|F_A|$

Since the carton is lifted up at constant speed, for the entire trip $\Delta v=0$

$⟹\frac{\Delta v}{\Delta t}=0=a$

Thus the net force acting on the object must also be zero, and the object must be in equilibrium.

$⟹mg-F_A=0$

$⟹\left|mg\right|=\left|F_A\right|$

Hence correct answer is option C.