A truck is traveling horizontally to the right (Fig.4–38). When the truck starts to slow down, the crate on the (frictionless) truck bed starts to slide. In what direction could the net force be on the crate? (a) No direction. The net force is zero. (b) Straight down (because of gravity). (c) Straight up (the normal force). (d) Horizontal and to the right. (e) Horizontal and to the left.
The obtained result is option (a) that states: No direction. The net force is zero.
In this problem, one of the essential points is that the total force on the crate will not act in the vertical direction when the crate does not accelerate either up or down.
Since the bed of the truck is frictionless and there is no other surface in contact with the crate, the horizontal force acting on the crate will be zero.
As mentioned above, there is no net force exerted on the crate because both horizontal and vertical forces are of zero magnitude. So, the force on the crate will have no direction.
Thus, option (a) is the correct answer.
(a) What is the acceleration of two falling sky divers (total mass = 132 kg including parachute) when the upward force of air resistance is equal to one-fourth of their weight? (b) After opening the parachute, the divers descend leisurely to the ground at constant speed. What now is the force of air resistance on the sky divers and their parachute?
See Fig. 4–44.
A person pushes a 14.0-kg lawn mower at constant speed with a force of directed along the handle, which is at an angle of 45.0° to the horizontal (Fig. 4–58). (a) Draw the free-body diagram showing all forces acting on the mower. Calculate (b) the horizontal friction force on the mower, then (c) the normal force exerted vertically upward on the mower by the ground. (d) What force must the person exert on the lawn mower to accelerate it from rest to 1.5 m/s in 2.5 seconds, assuming the same friction force?
FIGURE 4-58 Problem 50.
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