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Review Test #3: Forces & Motion
Reading: Ch. 4, p.p. 86 – 111, Ch. 5 p.p. 126-130 only
Topics: Forces, Mass & Weight, Newton’s Three Laws
Labs & Activities: Mass vs. Weight Handout, 2nd Law Lab, Parachute, Friction Investigations
Equations:
Fg= mg
a = Fnet
m
Ff = μFN
Objectives:
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Be able to identify and diagram the forces on an object.
Know what net force means and understand the direction it points relative to a and v for different
types of motion.
Know the differences between mass and weight.
Be able to calculate weight given the mass and vice versa.
Be able to apply Newton’s three laws correctly to a variety of situations.
first law- law of inertia- objects tend to keep doing what they are doing especially if they are more massive
second law- equation with a, Fnet and m
third law- law of action/reaction- if A pushes B, then B pushes A equally in the other direction
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Know what factors affect the amount of air resistance and friction on an object.
Be able to distinguish between static and kinetic friction.
Be able to solve problems.
Solutions should include:
a force diagram for each object in the problem
a list of known and unknown information with symbols and units
the formulas used in equation form
final answers with correct sign and units
Review Questions/Problems
1. An unbalanced force will make an object…
a.
b.
c.
d.
accelerate in the direction of the unbalanced force.
accelerate in the opposite direction of the unbalanced force.
move with a constant velocity.
remain at rest.
2. A deep space probe suddenly loses power while traveling through space.
a. The probe will suddenly come to a stop.
b. The probe will gradually slow to a stop
c. The probe will continue moving with constant speed.
d. The probe will speed up.
3. What is the net force on a car traveling northward with a constant velocity of 20 m/s?
4. Draw a force diagram for each of the situations. Include the direction of a, Fnet and v.
A ball is at max.
height. (ignore air)
A skydiver just
opened parachute.
An air hockey puck
moves freely.
A carriage is pushed
so it speeds up.
5. What is the reaction force to a rock hits a window?
6. A bug hits the windshield of a car. Compare
a. the force of the bug on the windshield to the force of the windshield on the bug.
b. the acceleration of the bug to the acceleration of the windshield (car).
7. Mary is 72 kg. Is this her mass or her weight? Explain how you know. Then calculate the
other quantity.
8. The force diagram below shows the forces acting on a carton that is being pulled along a
surface. Calculate the acceleration of the carton.
120 N
52 N
24 N
v
120 N
9. A 750 kg elevator that is moving upward contains 150 kg of mass. At the time it is
speeding up at 3.5 m/s². Draw a force diagram for the elevator described. Calculate the
magnitude of each force in your diagram.
10. A 30 kg crate initially at rest on a horizontal surface requires an 80 N horizontal force to
set it into motion. Find the coefficient of static friction.
11. A 2500 kg car accelerates at a rate of 4 m/s² as it slows to a stop. Find the coefficient of
kinetic friction.
12. A 6 kg box is pushed across a horizontal surface that has a coefficient of kinetic friction
of 0.306. If the acceleration of the box is 0.5 m/s², what is the force pushing the box
forward?