Question 5.QA.1: Pushing a box across the floor You push a box across a horiz...
Pushing a box across the floor
You push a box across a horizontal floor with a force \overrightarrow{\pmb{F}}_a that is angled below the horizontal, as shown in Figure 5.6. How does the magnitude of the normal force from the floor compare to the weight of the box as you push it?
A. The normal force from the floor is less than the box’s weight.
B. The normal force from the floor is equal to the box’s weight.
C. The normal force from the floor is greater than the box’s weight.
As the box slides horizontally across the floor, it doesn’t move vertically. Therefore, the vertical component of the acceleration of the box must be zero. This means that the upward-pointing normal force must cancel out both the weight and the vertical component of \overrightarrow{\pmb{F}}_a which point downward. Therefore, the correct answer is C. (Notice that it is not clear from the problem whether or not the horizontal component of the box’s acceleration is zero, but that doesn’t affect our answer for this problem, since we are concerned only with what is going on vertically.)
