PHYSICS UNIT 3 TEST REVIEW – Forces and Circular Motion
1) List the SI base units used to measure
mass kg
time s
length m
KEY
temperature K or °C
2) Define FORCE. any push or pull that tends to accelerate an object, measured in
newtons (N), a vector quantity
3) If there is a net force, there must be an acceleration, and therefore a change in velocity
4) What does it mean if the forces acting on an object are in equilibrium?
a state of a body on which no net force acts, a state of balance
5) What is equilibrium? when all the forces that act upon an object are balanced
6) Is a net force experienced when an object is in equilibrium? If an object is in
equilibrium, the net force is 0
7) What is a net force? sum of all of the applied forces acting on an object
8) If there is no net force, must that also mean that there are no forces acting on an object at all?
net force being zero may mean that
- there are no forces acting on an object, but also could mean that
- there are forces acting on an object, but they are in equilibrium
9) What forces are acting on an object that is in free fall? gravity
10) What is TENSION? equals the weight of the object
11) What is the tension in a rope that is holding a 75 kg box in equilibrium?
Fg = mg = (75kg) (9.81m/s2) = 735 N
12) A girl whose weight is 600 N hangs from the middle of a bar supported by two vertical strands of
rope. What is the tension in each strand?
600N ÷ 2 strands of rope = 300N on each strand
13) A 250.0 kg fireman slides down a fire pole at constant velocity. What is the friction force
between the fireman and the pole?( g=10 m/s2)
Ff = Fg = mg = 250 kg x 10 m/s2 = 2500 N
14) Define TERMINAL VELOCITY. the velocity at which a falling body moves through a
medium, as air, when the force of resistance of the medium is equal in
magnitude and opposite in direction to the force of gravity
15) What is the acceleration of a person once terminal velocity has been reached? 0 m/s2
16) Define WEIGHT. the force of gravity on the object
17) How is it calculated? Fg = mg
18) What will change an object’s weight? change in gravity
19) Would your weight be the same on earth as it is on the moon?
No, weight varies with gravity
20) What is mass? how much matter an object has
21) What units do we measure it in? kilograms (kg)
22) If an object has a weight of 100 N, what is its mass?
m = Fg / g = 100 N ÷ 9.81m/s2 = 10.2 kg
23) Would your mass be the same on earth as it is on the moon?
mass does not change with location
24) What is the mass of a Chuck Norris that weighs 200 N? m = Fg/g = 200N / 9.8m/s2 = 20kg
25) List Newton’s 1st Law of Motion. an object at rest will remain at rest, an object in
motion will remain in motion, unless a force acts upon it
26) What is another word used to describe this law? Law of Inertia
27) What is inertia? A property of matter by which it continues in its existing state of
rest or uniform motion in a straight line
28) Does this law apply to objects that are moving, objects that are at rest, or all objects? ALL
29) A push on a 1-kilogram brick accelerates the brick. Neglecting friction, to equally accelerate a
10-kilogram brick, one would have to push 10 times as hard
30) If you push horizontally on a car with a force of 500 N and the car doesn't move, the friction
force must be __500__N. Now if you push with 750 N so the car rolls at constant velocity, the
friction force is ___750_N.
31) What is the normal force?
Any force coming from the surface and acting at a right angle to the surface
32) What direction will it always act in? perpendicular to the surface of contact
Normal Force
Applied force
Friction force
Weight
33) Label the forces shown in the free-body diagram above.
34) Draw a free-body diagram for a box sitting on a ramp. Be sure
to include the following forces: gravity, normal force, friction,
gravity (weight) down the incline.
35) A man pulls on a lapdog with a force directed 40.0º above the horizontal. If the horizontal
component of the force on the dog is 30 N, what is the upwards force exerted on the dog?
Fx = F cos Θ = cos (40.0) · 30N = 22.98 N
23 N
36) A late traveler rushes to catch a plane, pulling a suitcase with a force directed 30.0º above the
horizontal. If the horizontal component of the force on the suitcase is 60.6 N, what is the force
exerted on the handle?
Fx = F cos(Θ)
60.6N = cos(30)• F
60.6 / cos(30) = 70N = F
37) A 15-N force and a 45-N force act on an object in opposite directions. What is the net force on
the object?
15N
45N
45N - 15N = 30N
38) If two teams playing tug-of-war pull on a rope with equal but opposite forces, what is the net
force on the rope?
0N
39) A box is pulled at a constant velocity by a rope across a horizontal carpeted surface. If a force
of 25N is being applied to the box rope at an angle of 35º to the ground, what is the force of
friction between the box and the carpet?
Horizontal Force = Ff = F cos(Θ) = cos (35) · 25N = 20.5 N
21 N
40) A car with a weight of 5000.0 N is accelerated across a level surface at 2.5 m/s2. What net force
acts on the wagon? (g = 9.81 m/s2)
m = Fg/g = 5000.0N/9.81m/s2 = 509.68 kg
Fnet = ma = (509.68)(2.5) = 1274.2 N
1300 N
41) A wagon with a weight of 300.0 N is accelerated across a level surface at 0.50 m/s 2. What net
force acts on the wagon? (g = 9.81 m/s2)
m = Fg /g = 300.0N /9.81 m/s2 = 30.6 kg
Fnet = ma = (30.6)(50) = 15.3 N
42) A sled is pulled at a constant velocity across a horizontal snow surface. If a force of 8.0 x 101 N
is being applied to the sled rope at an angle of 53º to the ground, what is the force of friction
between the sled and the snow?
53°
Ff = Fapp cos(Θ)
= cos(53)• 80 N = 48N
43) A box weighing 1.0 x102 N is held in place on a frictionless 20.0° slope by a rope attached to a
stake at the top. The rope is parallel to the slope. What is the normal force of the slope acting
on the sled?
FN = Fg cos(Θ)= cos(20)•100N = 94N
44) What is Newton’s Second Law of Motion? states that the acceleration of an object is
dependent upon two variables - the net force acting upon the object and the
mass of the object
45) How are force and acceleration related to each other? directly proportional
46) How are force and mass related to each other? more Force for more mass
less Force for less mass
47) How are acceleration and mass related to each other? inversely proportional (opposites)
48) A 400,000 kg airplane in takeoff uses a thrust force of 160,000 N. What is the acceleration of
the plane during takeoff?
a =
F
160 000 N
m = 400 000 kg = .4 m/s2
49) A girl pulls on a 10-kg wagon with a constant force of 20 N. What is the wagon's acceleration?
a =
F
m =
20 N
10 kg = 2 m/s2
50) You pull horizontally on a 50-kg crate with a force of 450 N and the friction force on the crate is
250 N. The acceleration of the crate is
Fapp = 450 – 250 = 200 N
a = F/m = 200 N / 50 kg = 4 m/s2
51) An elevator weighing 2.00 x 105 N is
supported by a steel cable. What is the
total tension in the cable when the
elevator is accelerated upward at a
of 1.00 m/s2? (g = 9.81 m/s2)
F = ma = (20387.36kg)(1.00m/s2) = 20387.36 N
m = Fg / g
rate
= 200 000N / 9.81 m/s2
= 20387.36 kg
52) What is Newton’s 3rd Law of Motion? for every action there is an equal and opposite reaction
53) What is an Action-Reaction Pair? a pair of objects that interact to produce a force on each
other of equal magnitude but of opposite direction
54) How do the forces exerted by each object compare? equal but opposite
55) What is the action-reaction pair when a hammer hits a nail?
hammer exerts force on nail --- nail exerts force equal and opposite to hammer
56) What is the action-reaction pair when a tennis ball hits the wall?
tennis ball exerts force on wall --- wall exerts force equal and opposite to ball
57. A box weighing 20 N is held in place on a frictionless 30.0º slope by a rope attached to a stake
at the top. The rope is parallel to the slope. What is the normal force of the slope acting on the
sled?
FN = Fg cos(Θ) = cos(30)•20 N = 17.3N
58. A box (shown above) is pulled at constant velocity by a rope with a tension of 50N . The rope
makes an angle of 35.0º above the horizontal. What is the normal force of the floor on the box?
FN = Fg for horizontal surfaces
Fg = FN =25 N
59. A book with a mass of 1.5 kg is held in equilibrium on a board with a slope of 35.0º by a
horizontal force. What is the normal force exerted by the book?(g=9.8 m/s2)
Fg = mg = (1.5kg)(9.81m/s2) = 14.7N
FN = Fg cos(Θ) = cos(35)•14.7 N = 12N
60. A 3-N force and a 35-N force act on an object in opposite directions. What is the net force on
the object?
Fnet = 35 N - 3 N = 32 N
61. A 5.0 kg squirrel grasping a vertical tree slides down at constant velocity. What is the friction
force between the tree and the squirrel?( g=9.8 m/s2)
Fg = mg = (5.0 kg) (9.81m/s2) = 49 N
62. A 0.2 kg sweaty gym towel hangs on the center of a 6m long clothesline forming a 0.8 degree
angle to the horizontal to dry. This causes the clothesline to drop by 4 cm (0.04 m). What is
the tension in the clothesline?
FT = Fg / sin (Θ)
1.96 N
FT = 1.96 / sin (0.8)
FT = 140 N
Fg = mg = (0.2kg)(9.81m/s2) =
63. A box with a weight of 80 N hangs from the middle of two vertical strands of rope. What is the
tension in each strand?
80 N ÷ 2 = 40 N
64. A 10 kg box is pushed with a constant force of 8 N. What is the acceleration of the box?
a =
F
m =
8N
10 kg = .8 m/s2
65. A box is pushed horizontally with a force of 80 N and the box doesn't move, so the friction force
must be 80 N. If the box is pushed with a force of 125 N so that it slides at constant velocity,
the friction force is ______
- constant velocity
- net force = zero
- no acceleration
125 N
66. You pull horizontally on a 75-kg donkey on ice with a force of 120 N and the friction force on
the donkey is 33 N. What is the acceleration of the donkey?
Fnet
m =
120 N – 33N
87
75 kg = 75
= 1.16 m/s2
67. The centripetal force always acts in what direction? toward the center of the circle
a =
A space station has a radius of 1.25 km. If an 80 kg astronaut stands on the outermost level of
the space station, what tangential velocity must he have in order for gravity on Earth to be
simulated?
𝑣𝑡 2
𝑚
𝑚
𝑎𝑐 =
⇒ 𝑣𝑡 = √𝑎𝑐 𝑟 = √9.81𝑠2 × 1250𝑚 = 111 𝑠
𝑟
69. A space station has a radius of 1.25 km. If an 80 kg astronaut stands on the outer level of the
space station, what centripetal force will he experience if the acceleration at the edge of the
space station is 9.8 m/s2?
68.
Fc = mac = 80 kg x 9.81m/s2 = 7841 N
70. A 1.0 kg bucket of water being swung in a circle from a 0.75 m long rope at a speed of 2.75
m/s. What is the centripetal force acting on this bucket?
Fc = mac = 1.0 kg x 10.08 m/s2 = 10 N
ac = v2/r
= 2.75m/s / .75 s
= 10.08 m/s2
71. What is the gravitational force between two asteroids, one with a mass of 8000 kg and the other
with a mass of 7550 kg that are separated from each other by a distance of 82 m? (G = 6.67 x
10-11 Nm2/kg2)
82m
8000kg
Fg =
=
=
=
=
7550kg
G(m1m2/r2)
6.67x10-11 (8000)(7550) / 822)
6.67x10-11 (60400000)/6724)
6.67x10-11 (8982.748364069)
6 x 10-7N
Remember: Anything from previous tests (conversions, significant digits, metric prefixes,
acceleration, velocity, displacement, projectile motion) may show up on this test!!!!