PS 15A (Answer on a separate sheet of paper)
Name: _________________________________
1. Suppose the acceleration of an object is zero. Does this mean that there are no forces acting on it? Give an
example supporting your answer.
No. It just means that the net force on the object is zero. Example is driving a car at a constant velocity.
Vertically  The force of gravity (Fg), weight, is equal and opposite to the normal force (FN) of the road.
Horizontally  The applied force (Fa) is equal and opposite to the force of friction and air resistance.
2. Two horizontal forces, 225 N and 165 N, are exerted in the same direction on a crate. Find the net horizontal force
on the crate. Be sure to indicate the direct direction of the net force. Draw a free body diagram to support your
answer.
If both forces are to the right, then 225 N + 165 N = 390 N to the right = Fnet.
3. If the same two forces in #2 are exerted in opposite directions, what is the net horizontal force on the crate? Be
sure to indicate the direction of the net force. Draw a free body diagram to support your answer.
If 225 N is to the right and the 165 N is to the left, then Fnet = 60 N to the right.
4. A 225 N force is exerted on the crate toward the north and the 165 N force is exerted toward the east. Find the
magnitude and direction of the net force.
Use Pythagorean Theorem to find the magnitude. Fnet = 279 N
Use tan-1 to find direction. 53.7 °
5. Your hand exerts a 10 N force upward while holding a book in your hand motionless.
a. What is the force of gravity on the book? 10 N downward
b. What is the net force on the book? 0 N
c. Draw a free body diagram to support your claim.
6. For each of the following situations, draw a free-body diagram labeling all forces with their agents and indicate
the direction of acceleration and the net force. Draw arrows the appropriate lengths.
a. A book on a desk when your hand is pushing down on it.
b. A skydiver falls downward through the air at constant velocity (air drag/resistance is important).
c. A cable pulls a crate at constant speed across a horizontal surface (there is friction).
d. A rope lifts a bucket upward at constant speed (ignore air drag/resistance).
e. A rocket blasts off and its vertical velocity increases with time (ignore air drag/resistance).
There is a net force upward that is accelerating the rocket.
7. The force of gravity, Fg, can be calculated using the following equation.
Fg = mg
where m = mass of the object and g = magnitude of acceleration due to gravity = 9.9 m/s2
Calculate the force of gravity (weight) of a person with a mass of 76 kg on the surface of the Earth.
744.8 N
8. Calculate the weight of a 1.0 kg mass near the surface of the Earth.
9.8 N
9. A moving object is in equilibrium if:
a. All forces act on it in the same direction.
b. The resultant force can be resolved into vertical and horizontal components.
c. The resultant force on the object is opposite the direction of motion.
d. Its velocity is constant.
e. Its acceleration is constant.
10. Your hand exerts a 6.5 N upward force on a pound of sugar as you hold it motionless. Considering the force of
gravity on the sugar, what is the net force on the sugar? Give the magnitude and direction.
The net force is 0N. There is a 6.5 N force of gravity downward that cancels out the 6.5 N force upward.
11. The sketch shows a painter’s staging in mechanical equilibrium. The person on the left weighs 250 N, the person
on the right weighs 300 N, the staging weighs 300 N, and the tension in the left rope is 400 N. What is the tension
in the right rope?
450 N