SPH3U0 Review - Forces
1. Inertia is the tendency of an object to resist a change in its state of motion.
2. The amount of inertia depends on the volume of the object.
3. Weight is the gravitational force that is acting on an object.
4. The acceleration due to gravity is a constant value on the surface of Earth.
5. Objects that fall close to Earth's surface accelerate at 9.8 m/s2. On a more massive planet,
the acceleration is greater.
6. The mass of an object does not change at different locations, but the weight of an object
often does.
7. A static frictional force exists when an object is moving.
8. Kinetic frictional forces are usually greater than static frictional forces.
9. The force of friction depends on only the two surfaces that are in contact.
10. If an object is in equilibrium, the forces that are acting on it are balanced.
11. An observer in an accelerating frame of reference is considered to be in a non-inertial
frame of reference.
12. Newton's laws are valid in all frames of reference.
13. The net force is the vector sum of all the acting forces.
14. If a net force is acting on an object, the object continues to move at a constant speed.
15. Newton's third law states that forces always act in pairs.
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SPH3U0 Review - Forces
16. Common Forces
A
____ i. force that acts perpendicular to a contact surface
____ ii. gravitational force that acts near a planet's surface
____ iii. force that results when a soccer ball is kicked
____ iv. force that keeps snow from sliding down a roof
____ v. force that opposes a toboggan as it slides down a hill
B
a. static friction
b. kinetic friction
c. weight
d. normal force
e. applied force
17. Newton's Three Laws of Motion
A
____ i. You lose speed as you ride a bicycle uphill.
____ ii. A parachutist falls faster and faster to Earth.
____ iii. A moving car tries to stop at an icy intersection.
____ iv. A standing person stumbles forward as a bus
suddenly stops.
____ v. Earth attracts the Moon, and the Moon attracts Earth.
B
a. Newton's first law
b. Newton's second law
c. Newton's third law
18. Forces and Motion
A
____ i. A parachutist falls at her terminal velocity.
____ ii. A cyclist travels at a constant speed.
____ iii. A car increases in speed when a stoplight turns green.
____ iv. A textbook is at rest on a desk
____ v. A water balloon is dropped from a window.
B
a. net force = 0
b. net force  0
19. Fundamental Forces
A
____ i. holds the nucleus of an atom together
____ ii. causes like charges to repel each other, and
unlike charges to attract each other
____ iii. is involved in radioactive decay
____ iv. causes matter to attract matter
____ v. is the strongest of the four fundamental forces
20. An object that has balanced forces acting on it is said to be in
_______________________.
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B
a. gravity
b. strong nuclear force
c. weak nuclear force
d. electromagnetic force
SPH3U0 Review - Forces
21. A _______________________ _______________________ _______________________
is used to represent all the forces that are acting on one object.
22. Weight is measured in _______________________.
23. A sled is being pulled at a constant speed. Therefore, the applied force must be
_______________________ _______________________ the frictional force.
24. Newton's _______________________ law describes how objects in equilibrium behave
(net force = 0).
25. An accelerating frame of reference is considered to be a(n) _______________________
frame of reference.
26. Newton's _______________________ law describes how an object accelerates if it is not
in equilibrium (net force  0).
27. A parachutist jumps from a plane and accelerates toward the ground. The force of gravity
must be _______________________ _______________________ the force of air
resistance.
28. No unbalanced force is acting on a body that has a(n) _______________________ of
zero.
29. Each of the four fundamental forces interacts with matter through a(n)
_______________________ particle.
30. What does a force of 1.0 N equal?
A) 1.0 kg/s B) 1.0 kg m2/s2 C) 1.0 kg m/s2 D) 1.0 kg m/s
E) 1.0 kg m2/s
31. Which factor increases the force of friction that acts on a brick sliding on a horizontal
surface?
A) increasing the mass of the brick
D) lubricating the surfaces in contact
B) increasing the surface area in contact
E) decreasing the mass of the brick
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SPH3U0 Review - Forces
C) decreasing the surface area in contact
32. Three books  X, Y, and Z  rest on a table, as shown in the diagram below. The
weight of each book is given. What is the net force (unbalanced force) that is acting on
book Y?
A) 10 N [down]
B) 5 N [down]
C) 4 N [up]
D) 5 N [up]
E) zero
33. An airplane flies in straight, level flight at a constant velocity. The mass of the airplane is
1000 kg. The frictional drag of the air on the airplane is 1500 N. What is the net force
that is acting on the airplane?
A) 11 500 N
B) 10 000 N
C) 1500 N
D) 0 N
E) cannot be determined from the information given
34. In what direction is the acceleration of a moving object?
A) in the same direction as the final velocity
B) in the same direction as the frictional force
C) in the opposite direction to the frictional force
D) in the same direction as the initial velocity
E) in the same direction as the net force
35. A golfer pulls a golf bag behind him. The golf bag has a mass of 8.0 kg. It accelerates at
4.0 m/s2. What is the net force that is causing its motion?
A) 2.0 N B) 4.0 N C) 12 N D) 32 N E) 80 N
36. A hockey player winds up to make a shot. She strikes the puck with a net force of 30 N.
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This gives the puck an acceleration of 300 m/s2. What is the mass of the puck?
A) 0.1 kg B) 0.9 kg C) 1.0 kg D) 0.09 kg E) 0.01 kg
37. An elevator that weighs 12 000 N is accelerating upward. The tension in the cable is 20
000 N. The frictional resistance to motion is 5000 N. What is the net force on the
elevator?
A) 37 000 N [up]
D) 8000 N [up]
B) 27 000 N [up]
E) 3000 N [up]
C) 13 000 N [up]
38. A crane is used at a building site to move large materials from the ground to the top of
the building. While moving a crate of blocks, there are two force vectors acting on the
crane: the force of tension and the force of gravity. If the force of tension equals the force
of gravity, which motion(s) of the crate of blocks is (are) possible?
I. The crate is moving upward at a constant speed.
II. The crate is moving downward at a constant speed.
III. The crate is stationary.
A) I only B) II only C) III only D) I and II only E) I, II, and III
39. A fisher suspends a fish on a newton spring scale in an elevator. When does the scale
show the highest reading?
A) when the elevator moves upward with increasing speed
B) when the elevator moves upward with decreasing speed
C) when the elevator remains stationary
D) when the elevator moves downward with increasing speed
E) when the elevator moves downward at a constant speed
40. To cut a lawn, a force, F, is applied to the handle of a lawn mower. The angle between
the handle and the horizontal is 45. When the force vector is resolved into horizontal and
vertical components, how do these components compare?
A) The horizontal component is greater than the vertical component.
B) The vertical component is greater than the horizontal component.
C) Each component has the same magnitude as F.
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SPH3U0 Review - Forces
D) Each component is smaller than F.
E) Each component is greater than F.
41. A picture frame has a mass of 10 kg. It is being held against a wall at an angle of 60 to
the vertical. What force, F, is needed to stop the frame from sliding down the wall?
Assume that the surface is frictionless.
A) 400 N
B) 200 N
C) 115 N
D) 100 N
E) 50 N
42. Two boxes are placed side by side on a frictionless surface so that they touch each other.
Box A has a mass of 4.0 kg. Box B has a mass of 12 kg. A constant force of 24 N [right]
is applied to box A. What force does box A exert on box B?
A) 1.5 N [right]
B) 6.0 N [right]
C) 18 N [right]
D) 11 N [right]
E) 24 N [right]
43. According to Newton's third law, action and reaction forces always occur as pairs of
forces. Why do they not cancel each other?
A) The action force happens first, and then the reaction force happens.
B) The action force is greater in magnitude than the reaction force.
C) The reaction force is greater in magnitude than the action force.
D) The action force and the reaction force both act in the same direction.
E) The action force and the reaction force act on different objects.
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SPH3U0 Review - Forces
44. In 1976, the province of Ontario made seat belt use mandatory for people in the front seat
of a car. In 1983, both front and rear seat belt use was made mandatory. Using physics
terms or laws, explain the importance of seat belts.
45. Indicate whether each situation has a net force of 0 N. If the net force is not 0 N, identify
the source of the unbalanced force.
a) A fish tank is at rest on a table
b) An apple falls from a tree
c) A cyclist coasts down a long hill
d) A car travels at a constant velocity.
46. A child accidentally releases a helium balloon into the air on a windy day. Draw a freebody diagram to show the forces that act on the balloon.
47. Draw a free-body diagram to show the forces that are acting on a skydiver at each point
in the skydiver's fall.
a) immediately after jumping out of the airplane
b) at terminal velocity (falling at a constant maximum speed)
c) immediately after opening the parachute
48. A wagon is being pulled at an angle of 25 to the horizontal. Draw a free-body diagram to
show all the forces that are acting on the wagon.
49. The Sun attracts Earth with a force of 3.6 × 1022 N [toward the centre of the Sun]. With
what force does Earth attract the Sun?
50. According to Newton's third law, when a child pulls on a wagon, the wagon pulls back on
the child with equal force. If this is true, how can the child move the wagon?
51. The following diagram shows a block on a flat surface. The block is attached to another
block that is suspended over the side of the flat surface. Draw a free-body diagram to
show all the forces that are acting on each block.
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SPH3U0 Review - Forces
52. The following force-acceleration graph shows the results of a laboratory experiment.
a) The graph does not pass through the origin. Explain the significance of this.
b) Determine the force of friction that was acting on the object.
c) How would the graph change if the experiment were performed on a frictionless
surface?
d) How could you determine the mass of the object?
e) Determine the mass of the object.
53. The weight of a space probe on Jupiter is 1.62 × 104 N. The weight of the same space
probe on Earth is 6.13 × 103 N. Find the acceleration due to gravity on Jupiter.
54. A 40.0 kg anchor is released from rest underwater. The water provides a resistance of 375
N. How long does the anchor take to reach the bottom of the lake, which is 5.2 m down?
55. A 930 kg piano needs to be moved into an auditorium for a concert. An applied force
moves the piano from rest to 0.800 m/s [forward] in 6.00 s.
a) Calculate the acceleration of the piano.
b) Determine the force of friction that is opposing the motion of the piano. The
coefficient of friction between the two surfaces is  = 1.12
c) Find the applied force that is needed to move the piano with this acceleration.
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SPH3U0 Review - Forces
56. Several children jump on a toboggan and ask for a ride. The coefficient of friction
between the snow and the toboggan is 0.15. The total mass of the toboggan is 66 kg. The
rope makes an angle of 35 with the horizontal. What applied force is needed to move the
toboggan with an acceleration of 0.27 m/s2?
57. A hockey player has a mass of 105 kg. He is skating at a constant speed. He needs to
exert a force of 10.3 N to maintain his constant speed.
a) Determine the coefficient of friction between the player's skates and the ice surface.
b) The player picks up a stack of pylons, which have a mass of 10.0 kg. What effect does
this have on the coefficient of friction?
c) What force does the player need to exert to skate at a constant speed with the pylons?
58. A wagon has a mass of 34 kg. It is being pulled with an applied force of 24 N at an angle
of 30° to the horizontal. A force of friction, equal to 14 N, is opposing the motion of the
wagon. Find the acceleration of the wagon.
59. A toboggan has a mass of 52 kg. It is being pulled with an applied force of 75 N at an
angle of 50° to the horizontal. It is accelerating at 0.74 m/s2? What is the force of friction
that is opposing the motion.
60. A skydiver, who has a mass of 82 kg, reaches terminal velocity.
a) What is the net force that is acting on the skydiver?
b) Calculate the force of air resistance that is opposing the motion of the skydiver.
61. A 2000 kg elevator has an acceleration of 1.1 m/s2 [up]. Calculate the force of tension in
the supporting cable.
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SPH3U0 Review - Forces
Answer Key -- forces review
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
True
False - The amount of inertia depends on the mass of the object.
True
False - The acceleration due to gravity varies with location and altitude.
True
True
False - A static frictional force exists when an object is at rest. OR A kinetic frictional
force exists when an object is moving.
False - Static frictional forces are usually greater than kinetic frictional forces. OR
Kinetic frictional forces are usually less than static frictional forces.
False - The force of friction depends on the two surfaces that are in contact and the
normal force that supports the object.
True
True
False - Newton's laws are valid in an inertial frame of reference. OR Newton's laws are
not valid in a non-inertial frame of reference.
True
False - If a net force is acting on an object, the object accelerates in the direction of the
net force. OR If there is no net force acting on an object, the object continues to move at a
constant speed.
True
i. d; ii. c; iii. e; iv. a; v. b
i. b; ii. b; iii. a; iv. a; v. c
i. a; ii. a; iii. b; iv. a; v. b
i. b; ii. d; iii. c; iv. a; v. b
equilibrium
free-body diagram
newtons
equal to
first
non-inertial
second
greater than
acceleration
exchange
C
A
E
D
E
D
A
E
E
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SPH3U0 Review - Forces
39.
40.
41.
42.
43.
44.
A
D
B
C
E
Based on Newton's first law, a passenger keeps moving at a constant speed unless an
external force changes her or his motion. The external force that is applied by a seat belt
is meant to slow down the motion of the passenger and reduce injury by preventing the
passenger from striking the dashboard or windshield.
45. a) net force = 0 N
b) gravity
c) gravity
d) net force = 0 N
46. The diagram should show three forces: Fg, Fa, and Fw.
47.
a) Fg  Fr
b) Fg = Fr
c) Fg  Fr
48.
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SPH3U0 Review - Forces
49. 3.6 × 1022 N [toward the centre of Earth]
50. To analyze the motion of the child and the wagon, consider each as a separate object.
Draw a free-body diagram of each object, with only the forces that are acting on the
object. The child can move the wagon as long as the applied force is equal to or greater
than the forces that resist the child's motion.
51. The block on the flat surface should have four force vectors drawn: Ff, Fa, Fg, and FN.
The suspended block should have two force vectors drawn: Fg and FT.
52. a) The object did not start to accelerate until the applied force was greater than the force
of friction. The point where the graph intersects the force axis represents the amount of
friction.
b) 1.0 N
c) The graph would pass through the origin.
d) To find the mass, calculate the slope of the line.
Force
Mass =
 2.0 kg
Acceleration
e)
53. Recall: Weight = ¢g


FgJupiter  mg
Solve for the mass of the space probe on Earth.

F 613
.  103 N
m  
 6.25  102 kg
g
9.81 N/kg
Solve for the acceleration due to gravity on Jupiter.

.  104 N
 F 162
g 
 25.92 N/kg  25.9 m/s2
m
625 kg
54. Given: ²1 = 0 m/s, d = 5.2 m, m = 40.0 kg, ¢resistance = 375 N [up]
Calculate the force of gravity.
¢g = m+ = (40.0 kg)(9.81 N/kg) = 392 N [down]
Use [down] as positive.
¢net = ¢resistance + ¢g = –375 N + 392 N = 17 N [down]
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SPH3U0 Review - Forces
Determine
the acceleration of the anchor.

17 N
 F
a  net 
 0.425 m/s2  down
m
40.0 kg
Calculate the time to fall.
 
1
d  v1 ( t )  a ( t ) 2
2

2d
2(5.2 m)

Since v1  0 m / s, t   
 4.95 s or 5.0 s
a
0.425 m/s2
55. Given: ²1 = 0 m/s, ²2 = 0.800 m/s [forward], t = 6.00 s, m = 930 kg
 
 v2  v1 0.800 m/s  0 m/s
a

 0133
.
m/s2  forward 
t
600 s
a)
b) ¢f = ¢N = m+ = (1.12)(930 kg)(9.81 N/kg) = 1.02 × 104 N [backward]
c) ¢net = mv = (930 kg)(0.133 m/s2 [forward]) = 124 N [forward]
¢net = ¢applied + ¢friction
Consider forward to be positive.
Fnet = Fapplied – Ffriction
Fapplied = Fnet + Ffriction
Fapplied = 124 N + (1.02 × 104 N) = 1.03 × 104 N [forward]
56. Draw a free-body diagram of the toboggan:
¢net = mv = (66 kg)(0.27 m/s2) = 17.8 N [forward]
¢net = ¢horizontal + ¢f
Consider the applied force to be positive. Then the force of friction is negative, since it is
acting in the opposite direction.
¢net = Fhorizontal – Ff
¢net = Fa (cos 35) – (FN)
¢f = FN = (Fg – Fvertical) = (0.15) [(66 kg)(9.81 N/kg) – Fa(sin 35)]
¢net = mv
Fa (cos 35) – (¢N) = 17.8 N
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SPH3U0 Review - Forces
Fa (cos 35) – 0.15 [647 N – Fa(sin 35)] = 17.8 N
Fa (cos 35) + Fa(0.15)(sin 35) = 17.8 N + 97.1 N
Fa [cos 35 + (0.15)(sin 35)] = 115 N
(0.905)Fa = 115 N
Fa = 127 N
57. a) The hockey player is skating at a constant speed. Therefore, the forces that are acting
on him are balanced.
¢applied = ¢friction

Ff
10.3 N
  
 0.009 999 5 or 0.010
FN (105 kg)(9.81 N/kg)
b) The coefficient of friction remains the same. The coefficient depends on the surfaces
in contact.
c) Determine the applied force that is needed for the player to skate at a constant speed.
This is equal to the force of friction.
¢f = ¢N = (0.0100)(105 kg + 10 kg)(9.81 N/kg) = 11.3 N [forward]
58. Find the horizontal component of the applied force.
¢horizontal = (24 N)(cos 30°) = 20.8 N [forward]
¢net = ¢horizontal + ¢friction = 21 N – 14 N = 7.0 N [forward]

 F 7.0 N
a 
 0.206 m/s2  forward  or 0.21 m/s2  forward 
m 34 kg
59. Find the horizontal component of the applied force.
¢horizontal = (75 N)(cos 50°) = 48 N [forward]
¢net = mv = (52 kg)(0.74 m/s2) = 38 N
¢net = ¢horizontal + ¢friction
¢net = ¢horizontal – ¢friction
¢net = ¢horizontal – ¢friction = 48 N – 38 N = 10 N [backward]
60. a) A skydiver that is falling at terminal velocity is falling at a constant speed. If the
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SPH3U0 Review - Forces
skydiver is falling at a constant speed, then the acceleration is zero.
¢net = mv = (82 kg)(0) = 0
b) If the net force that is acting on the skydiver is zero, then the forces that are acting on
the skydiver are balanced.
¢resistance = ¢gravity = m+ = (82 kg)(9.81 m/s2) = 804 N [up] or 8.0 × 102 N [up]
61. Use Newton's second law to determine the net force that is acting on the elevator.
¢net = mv = (2000 kg)(1.1 m/s2 [up]) = 2.2 × 103 N [up]
¢net = ¢tension + ¢gravity
Calculate the force of gravity acting on the elevator.
¢net = m+ = (2000 kg)(9.81 N/kg) = 1.96 × 104 N [down]
¢net = ¢tension – ¢gravity
¢tension = ¢net + ¢gravity = 2.2 × 103 N + 1.96 × 104 N = 2.18 × 104 N [up] or 2.2 × 104 N
[up]
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