Physics 20 Unit
2: Dynamics
1
Physics 20

STA
Work Booklet
Unit I
–
Dynamics
Chapter
3
–
Forces
Name: ______________________________________________________________________________
Physics 20 Unit
2: Dynamics
2
Physics 20 Unit
2: Dynamics
3
3.1
–
The Nature of Force
1.
2.
Shana, Kelsey, and Amanda pull on a toy exerting forces described in the diagram below. Let North
be positive and south Negative. Determine the net force acting on the toy.
3.
Three forces act on an object. Two are pulling to the south, one
with a magnitude of 35N and the other
with a
magnitude
of 23N. The third force is pulling towards the north with a magnitude of 29N.
Determine the net force acting on the object.
Physics 20 Unit
2: Dynamics
4
4.
Two forces pull on an object. One force pulls with 30N to the west. The second force is unknown.
Determine the unknown force if the net force is 40 N to the East?
5.
A car
is speeding up
. The weight of the car is 12 000 N [down]. The force o
f air resistance is 3600 N
[backward]. With the engine engaged, the
engine applies a
20 000
N [forward]
force to the axels
. Draw a
free

body diagram for this situation.
6.
Two people, A and B, are dragging a canoe out of a
lake onto a beach using light ropes. Each person
applies a force of 60.0 N [forward] on the rope. The
force of friction exerted by the beach on the canoe is
85.0 N [backward]. Starting with a free

body
di
agram, calculate the net force on the canoe.
7.
Two dogs, A and B, are pulling a sled across a horizontal, snowy
surface. Dog A exerts a force of
200
N [forward] and dog B a force
of 150 N [forward]. The force of
friction exerted by the snow on
the sled is
60 N [backward]. The
driver attempts to slow down the
sled by pulling on it with a force
of 100 N
[backward]. Starting with
a free

body diagram, calculate the
net force on the sled.
Physics 20 Unit
2: Dynamics
5
8.
In a tractor pull, four tractors are
connected
by strong chains to a
heavy load. The load is initially
at
rest. Tractors A and B pull with
forces of 5000 N [E] and 4000 N [E]
respectively. Tractors C and D pull
with forces of 4500 N [W] and
3500 N [W] respectively. The
magnitude of the force of frictio
n
exerted
by the ground on the load
is 1000 N.
(a) Starting with a free

body
diagram, calculate the net force
on the load.
(b) If the load is initially at rest,
will it start moving? Explain.
9.
The total weight of a biker and her
motorbike is 1800 N [down].
The engine applies a force of
500 N
[forward]. The air resistance
and friction
acting on the
biker and bike is 200 N [backward]. The
normal
force exerted by the road on the
biker and bike is 1800 N [up].
(a) Consider the biker
and bike as a single
object. Draw a free

body diagram for
this object.
(b) Calculate the net force.
Answers
1)
FBD
2)
140N [North]
3)
29N [South]
4)
70N [East]
5)
FBD
6)
35.0N
7)
190N [forward]
8)
a)
0N
b)
No, not moving.
9) a)
FBD
b)
300N [forward]
Physics 20 Unit
2: Dynamics
6
3.2
–
Newton’s First and Second Law
1.
Use Newton’s first law to explain why
(a) steel barriers usually separate the cab of a truck from the
load
(b) trucks carrying tall loads navigate corners slowly, and
(c) customers
who order take

out drinks are provided with lids.
2.
Give two examples
that illustrate the property of
inertia for
both a stationary and a moving object.
3.
Use Newton’s first law to describe the
motion of
(a) a
car that attempts to go around an icy
curve too quickly
(b) a lacrosse ball after leaving the
lacrosse stick.
Physics 20 Unit
2: Dynamics
7
4.
Apply Newton’s first law and the concept
of inertia to each of these situations.
a)
How could you remove the newspaper
without
toppling the plastic beaker?
b)
While moving at constant speed on a
level, snowy surface, a snowmobiler
throws a ball vertically
upward. If the
snowmobile continues moving at
constant velocity, the ball returns to
the driver.
Why does the ball land
ahead
of the driver if the snowmobile
stops? Assume that the air
resistance
acting on the ball is negligible.
5.
Determine the accelerations which result when a 12

N net force is applied to a 3

kg object and then to
a 6

kg object.
6.
A net force of 15
N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s
2
.
Determine the mass of the encyclopedia.
7.
Suppose that a sled is accelerating at a rate of 2 m/s
2
. If the net force is tripled and the mass is
doubled, then what is the ne
w acceleration of the sled?
Physics 20 Unit
2: Dynamics
8
8.
A lacrosse player exerts an average
net horizontal force of 2.8 N
[forward] on a 0.14

kg lacrosse
ball
while running with it in the
net of his stick. Calculate the average horizontal
acceleration of the ball
while in
contact with the lacrosse net.
9.
The net force acting on a 6.0

kg grocery cart is 12 N [left]. Calculate
the acceleration of the cart.
10.
A net force of 34 N [forward] acts
on a curling stone causing it to
accelerate at 1.8 m/s2 [forward] on
a
frictionless icy surface. Calculate
the mass of the curling stone.
Answers
1)
Answers Vary
2)
Answers Vary
3)
Answers Vary
4)
Answers Vary
5)
a=4
m/s
2
and a=2
m/s
2
6)
3kg
7)
3m/s
2
8
)
20 m/s
2
9)
2.0 m/s
2
10
)
19 kg
Physics 20 Unit
2: Dynamics
9
3.3
–
Multi
Force Analysis
1.
Two forces act on an object. The first force pulls 400N to the East while the second pulls 600 N to
the West. The object accelerates 5.00 m/s
2
to the West. Determine the mass of the object.
2.
A ball has a mass of 0.240 kg and is thrown straight up into the air, the initial acceleration of the ball
is 2.69 m/s
2
. What is the applied force on the ball?
3.
Bryce applies a horizontal force of 120 N to the East on her 10.0kg toboggan causing it
to accelerate
2.00 m/s
2
to the East. Determine the force of friction opposing the toboggan’s motion.
4.
Three forces act simultaneously on an
object:
F
1
is 65 N [30.0
°
],
F
2
is 80 N [115
°
],
and
F
3
is 105 N
[235
°
]. Calculate the net
force
and direction
acting on the object.
Physics 20 Unit
2: Dynamics
10
5.
While in the water, a force of 320 N [up] exerted by the water on a dolphin causes the dolphin to
jump up. The acceleration of the dolphin is 2.6 m/s
2
[up].
a) Calculate the mass of the dolphin.
b) What
would be the acceleration of the dolphin if it had the same strength but half the mass?
6.
Two people, A and B, are dragging a sled on a horizontal, icy surface
with two light ropes. Person A
applies a force of 65.0 N [30.0
°
] on one
rope. Person B applies a force of 70.0 N [300
°
] on the other
rope. The force of friction on the sled is negligible and the ropes
are parallel to the icy surface.
Calculate the net force on the sled.
Physics 20 Unit
2: Dynamics
11
7.
An ice hut used for winter fishing
is resting on a level patch of snow. The combined mass of the hut
and angler inside is 80 kg. A wind exerts a horizontal force of 205 N on the hut, and causes it to
accelerate. While in motion, the magnitude of the force of friction acting on the hut is 19
6 N. What is
the acceleration of the hut?
8.
A 25
.0

kg store sign is suspended
as shown in Figure 3.18.
Calculate the forces
F
T1
and
F
T2
exerted at
the point at which
the sign is suspended.
Answers
1)
40.0kg
2)
3.00 N
3)
100N
4)
42N [27° E of S] or [63° S of E]
5)
a=5.2m/s
2
6)
95.5N [17.1° S of E] or [72.9° E of S]
7)
0.12 m/s
2
(forward)
8
) FT
1
=
299N [125°] and FT
2
=172N [0°]
Physics 20 Unit
2: Dynamics
12
3.
4
Force of Friction
1.
A 2.6 kg sled is pulled along a horizontal surface at a constant velocity to the south. If the
coefficient
of kinetic friction between the surface
s
is 0.17, determine the force of friction acting
on the sled?
2.
Due to a frictional force
of 3.30 N, a 20 kg curling rock slides to rest along a horizontal sheet of
ice. Calculate the coefficient of kinetic friction between the curling rock and the ice.
Physics 20 Unit
2: Dynamics
13
3.
The coefficient of static friction can be calculated knowing the foce applied at the
threshold of
motion. (i.e.) just as the object beging to move. A 220 g block is placed on a horizontal surface.
If a force of 1.2 N is applied and just starts the object in motion, determine the coefficient of
static friction between the two objects. [
0.56]
4.
A larger force is required to start an object moving than is needed to keep the object moving. A
250 g glass block is set on a glass plate and a horizontal force is applied. The force is gradually
increased in magnitude. When the applied force reaches a
value of 2.3 N the block starts to
move. The force required to keep the block moving at constant velocity is 0.98 N. Deteremine
the
coefficient
of :
a)
Static friction between the block and the surface [0.94]
b)
Kinetic Friction between the block and the surf
ace [0.40]
5.
The driver of a 2600 kg truck travelling at 13.1 m/s along a level, but icy road steps on the brakes.
The wheels lock and the tires skid along the icy surface for a distance of 26.2 m before coming to
a stop. Calculate the coeffic
ient of kinetic friction between the tires’ surface and the ice [0.33]
Physics 20 Unit
2: Dynamics
14
6.
A 100N force is applied to accelerate a 20 kg box along a horizontal sidewalk where the kinetic
coefficient of friction between the box and sidewalk is 0.40. Determine the
magnitude of the
box’s acceleration.
7.
A force of 31 N [forward] is needed to
start an 8.0

kg steel slider moving along
a horizontal steel
rail. What is the
coefficient of static friction?
8.
A biker and his motorcycle have a weight
of
2350 N [down]. Calculate the force of
kinetic
friction for the rubber tires and dry
concrete
(u=0.70
)
if
the motorcycle skids.
Answers
1)
4.3N
2)
0.017N
3)
0.56
4)
a) 0.94
b) 0.40
5)
0.33
6)
1.1 m/s
2
7)
0.40
8
)
1.6 X 10
3
N
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