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Physics
Unit 3 Review
Use GUESS method on problems.
1.
Name:_____________________
Date:_________ Period:____________
A sheet of paper can be withdrawn from under a container of milk without toppling it if the paper is jerked
quickly. The reason this can be done is that _____.
a. the milk carton has very little weight
b. there is an action-reaction pair operating
c. gravity pulls very hard on the milk carton
d. the milk carton has inertia
e. none of the above
2. Compared to its weight on earth, a 50 kg object on the Moon will weigh _____.
a. less
b. more
c. the same amount
3. The mass of a dog that weighs 100 N is about _____.
a. 1 kg
b. 10 kg
c. 100 kg
d. 1000 kg
e. none of the above
4. Force is to Newtons as weight is to _________________.
a. kg
b. m/s2
c. N
d. kg m/s
5. You would have a larger mass of gold if your chunk of gold weighed 1 N on _____. (THINK!!)
a. the moon b. Earth
c. it doesn’t matter which planet you are on.
6. Which has more mass, a kilogram of feathers or a kilogram of iron?
a. the feathers
b. the iron
c. same masses
7. According to Newton’s Second Law of Motion, the acceleration of an object ______ its mass.
a. is directly proportional to
b. is inversely proportional to
c. doesn’t depend on
8. Which force is one of the four fundamental forces?
a. gravitational
b. weak normal
c. frictional
d. strong normal
9. If an object weighs 49 N on Earth, how much does it weigh on Mars if the gravitational acceleration on Mars
is approximately 3.71 m/s2?
a. 5.0 N
b. 13.2 N
c. 18.6 N
d. 49.0 N
10. An object has a constant mass. A constant force on the object produces constant _____.
a. velocity
b. acceleration
c. both A and B
d. none of the above
11. A force of 100 N accelerates a mass of 100 kg at the rate of 1 m/s2. The acceleration of a mass of 2 kg
acted upon by a force of 2 N is _____.
a. half as much
b. twice as much
c. the same d. none of the above
12. Weight is equal to____________
a. mass (kg)
b. 9.8 m/s2
c. force due to gravity
d. density
e. volume
13. A 15 N object is moving along a frictionless surface with a 20 N force to the right and a 20 N force to the
left. The resulting net force in the x-direction causes :
a) a balanced force
b) constant velocity
c) an unbalance force to the right
d) both a and
e) none of the above
14. A tennis ball and a solid steel ball are sitting on a
table. Which ball has the greatest net force acting
on it?
a. the tennis ball
b. the steel ball
c. neither they are the same
15. A student hits a nail with a hammer. During the
collision, there are forces _____.
a. on the hammer but not the nail
b. on the nail but not the hammer
c. on the nail and the hammer
17. Complete the following symbol & units table:
Physical Quantity
Symbol
SI Unit
Time
t
s
Acceleration
a
m/s2
Speed
v
m/s
Velocity
v
m/s
Displacement
Δd
m
Distance
d
m
Mass
m
kg
Force
F
N
Weight
Fg
N
16. Action/reaction forces always occur _____.
Torque
ζ
N∙m
a. by themselves
b. in pairs on the same object
c. in pairs on different objects
d. in octets
18. If the torque required to loosen a nut on a wheel of a car has a magnitude of 40.0 N∙m, what is the minimal
force must be exerted by a mechanic at the end of a .30 m wrench to loosen the nut?
Diagram (Givens + Unknowns)
Equation:
ζ = (F)r
F= ζ / r
Torque
ζ = 40.0 N m
Substitute: F = 40 / .30
Force
F = ????? N
Level Arm
r = 0.30 m
Solve: F = 133.3 N
19. To tighten a bolt, you push with a force of 80 N at the end of a wrench handle that is 0.25 m from the axis of
the bolt What torque are you exerting?
Diagram (Givens + Unknowns)
Equation:
ζ = (F)r
Torque
ζ = ???? N m
Substitute: ζ = 80(.25)
Force
F = 80 N
Level Arm
r = 0.25 m
Solve: ζ = 20 N m
20. An unfortunate bug splatters against the windshield of a moving car. Compared to the acceleration of the
car, the acceleration of the bug is _____.
a. larger
b. smaller
c. the same
21. An unfortunate bug splatters against the windshield of a moving car. Compared to the force of the car on the
bug, the force of the bug on the car is _____.
a. larger
b. smaller
c. the same
22. An unfortunate bug splatters against the windshield of a moving car.
in velocity?
a. the car
b. the bug
c. both the same
Which undergoes the greater change
23. A block is being pushed to the right with a force of 10 N along a flat table.
10.0 N
??? N
a. What direction and magnitude of force must be applied to produce a net force of zero? ____10 N left__
b. What direction and magnitude of force must be applied to produce balance forces? ______10 N left___
c. What direction and magnitude of force must be applied to have an unbalanced force that slows down the
block? ___anything greater than 10 N left_________
d. What direction and magnitude of force must be applied to have an unbalanced force that speeds up the
block? _______anything less than 10 N left_____
24. What is the resultant of a person running with an average velocity of 5.0 km/hr south and then running with
an average velocity of 3.0 km/hr west? Draw a diagram and then be sure you add vectors properly.
Diagram: (Given
Equation:
a2 + b2 = c2
Substitute:
??? km/hr
(5)2
5.0 km/hr
+ (3)2
c2
36
5.8 km/hr
=
Solve:
3.0 km/hr
= c2
= c
c =5.8 km/hr
25. A certain unbalanced force gives a 5 kg object an acceleration of 2.0 m/s2. What acceleration would the same
force give a 30 kg object?
FBD (Givens and Unknowns)
Equation: F =ma and F= ma
a = 2.0 m/s2
m= 5 kg
F = ?? N
F = ??? N
m = 30 kg
a = ??? m/s2
Substitute:
Obj 1
F = ??? N
m = 5 kg
a =2.0 m/s2
Obj 2
F ??? N
m = 30 kg
a = ??? m/s2
Solve
F = (5)(2)
F = 10 N
F = ma
(10) = (30) a
a = 0.33 m/s2
26. Suppose you exert 400 N horizontally on a 50 kg crate on a factory floor. The friction between the crate and
the floor is 200 N. What is the acceleration of the crate? Draw a FBD.
FBD (Givens and Unknowns)
Equation:
Fx = Fa – Ff and Fx = ma
Fa = 400 N
Ff = 200 N
m = 50 kg
Find anet = ?? m/s2
Fx = 400 N – 200 N = 200 N
Horiz
Fx = 200 N
Fx = ma
a = Fx / m
m = 50 kg
a = 200 N / 50 kg
a = ??? m/s2
Solve a = 4.0 m/s2 to the right
Substitute:
27. Lupita pushes a table a with a 400 N force to the right while Sven pushes the table with a 1600 N force to the
left. If the net acceleration is 6.0 m/s2 to the left, what is the weight of the table?
FBD (Givens and Unknowns)
anet = 6 m/s2 to the left
Fa = 400 N
Fa = 1600 N
Fg = ?? N
a = 9.8 m/s
Equation:
Fx = Fa + Fa and Fx = ma Fg = ma
Substitute:
Fx = 400 – 1600 = -1200 N
Horiz
Vert
Fx = -1200 N Fg = ??? N
Fx = ma
m = ??? kg
m = ??? kg
(-1200) = m(-6.0)
a = -6.0 m/s2 a = 9.8 m/s2
200 kg = m
2
Fg = ma
Fg = (200)(9.8)
Fg = 1960 N
Solve: Fg = 1960 N
28. A 40.0 kg block is pushed along the floor with a constant applied force of 85.0 N. The block moves with
constant velocity. Draw a FBD. Do the x forces balance? Do the y forces balance? What is the force due to
friction? Calculate the normal force.
FBD (Givens and Unknowns)
Eq
Fg = ma
Fn = Fg
Fa = Ff
Fn = ?? N
Ff = ?? N
m = 40.0 kg
ag = 9.8 m/s2
Fa = 85.0 N
Fg = ?? N
Fx = 0 N
Fy = 0 N
Substitute:
Vertical
Fg = ??? N
m = 40 kg
a = 9.8 m/s2
Fg = (40 kg)(9.8 m/s2) =
Fg = 392 N
Ff = 392 N
Ff = 85.0 N
Solve: x forces balance and y forces balance
since the is no net force because the block is
moving with constant velocity.
Fn = 392 N
Ff = 85.0 N
29. A 500 kg bear grasping a vertical tree slides down at a constant velocity. What is the friction force between
the tree and the bear? Draw a FBD.
Diagram (Givens + Unknowns)
Equation:
Fg = ma
Ff = ?? N
m = 500 kg
ag = 9.8 m/s2
Fg = ?? N
N
Substitute:
Vertical
Fg = ??? N
m = 500 kg
a = 9.8 m/s2
Fg = (500 kg) (9.8 m/s2)
Fg = 1960 N
Ff = Fg
(no net force since constant velocity)
Solve:
Ff = 4900 N
30. Find the net force in the x-direction for the following scenario. Use the GUESS method and be sure to
properly bubble your answer.
Fn = 147 N
Solve:
a= 8.0 m/s2 to
the right
Horiz
F = ??? N
m = ??? kg
a = 8.0 m/s2
Vert
Fg = 147 N
m = ???? kg
a = 9.8 m/s2
F = ma
147 = m (9.8)
15 kg = m
Fg = 147 N
F =ma
F = (15) (8)
F = 120 N