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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
Chapter 4
Forces and
Newton’s Laws
of Motion
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4 Forces and Newton’s Laws of Motion
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Weight is actually not the only one…
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Reading Quiz
1. A “net force” is
A. the sum of the magnitudes of all the forces acting on an
object.
B. the difference between two forces that are acting on an
object.
C. the vector sum of all the forces acting on an object.
D. the force with the largest magnitude acting on an object.
Slide 4-7
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Answer
1. A “net force” is
A. the sum of the magnitudes of all the forces acting on an
object.
B. the difference between two forces that are acting on an
object.
C. the vector sum of all the forces acting on an object.
D. the force with the largest magnitude acting on an object.
Slide 4-8
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Reading Quiz
2. Which of the following is NOT one of the steps used to identify
the forces acting on an object?
A. Name and label each force the object exerts on the
environment.
B. Name and label each contact force acting on the object.
C. Draw a picture of the situation.
D. Identify “the system” and “the environment.”
E. Name and label each long-range force acting on the
object.
Slide 4-9
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Answer
2. Which of the following is NOT on of the steps used to identify
the forces acting on an object?
A. Name and label each force the object exerts on the
environment.
B. Name and label each contact force acting on the object.
C. Draw a picture of the situation.
D. Identify “the system” and “the environment.”
E. Name and label each long-range force acting on the
object.
Slide 4-10
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Reading Quiz
3. Which of these is not a force discussed in this chapter?
A. The tension force.
B. The normal force.
C. The orthogonal force.
D. The thrust force.
Slide 4-11
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Answer
3. Which of these is not a force discussed in this chapter?
A. The tension force.
B. The normal force.
C. The orthogonal force.
D. The thrust force.
Slide 4-12
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Reading Quiz
4. An action/reaction pair of forces
A. point in the same direction.
B. act on the same object.
C. are always long-range forces.
D. act on two different objects.
Slide 4-13
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Answer
4. An action/reaction pair of forces
A. point in the same direction.
B. act on the same object.
C. are always long-range forces.
D. act on two different objects.
Slide 4-14
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What Causes Motion?
Slide 4-15
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What Causes Motion?
Slide 4-15
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What Causes Motion?
In the absence of any forces acting on it, an object will continue
moving forever. Motion needs no “cause.”
Slide 4-15
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Seat Belts: An Application of Newton’s First Law
Slide 4-16
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What Is a Force?
A force...
... is a push or a pull. ... acts on an
object.
... requires an
agent.
... is a vector. ... is a contact force or a
long-range force. Slide 4-17
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Force Vectors
Slide 4-18
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A Short Catalog of Forces: Weight w
Slide 4-19
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Spring Force Fsp
Slide 4-20
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Tension ForceT
Slide 4-21
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Normal Force n
Slide 4-22
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Friction fk and fs
Slide 4-23
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Drag D and Thrust Fthrust
Slide 4-24
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Identifying Forces
Slide 4-25
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Example Problem
A block is dragged uphill by a rope. Identify all
forces acting on the block.
Slide 4-26
w
kf
Tn
weight
tension
normal force
kinetic friction forcek
w
T
n
f
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Example Problem
Block A hangs from the ceiling by a rope. Another block B hangs
from A. Identify the forces acting on A.
Slide 4-27
Aw
ABT
Block A
Block B
ACT
These are the forces acting on Block A:
weight of Block A
tension force on Block A due to Block B
tension force on Block A due to the ceiling
A
AB
AC
w
T
T
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Example Problem
Block A hangs from the ceiling by a rope. Another block B hangs
from A. Identify the forces acting on A.
Slide 4-27
Aw
ABT
Block A
Block B
ACT
These are the forces acting on Block A:
weight of Block A
tension force on Block A due to Block B
tension force on Block A due to the ceiling
But there are other forces in the system...
A
AB
AC
w
T
T
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Example Problem
Block A hangs from the ceiling by a rope. Another block B hangs
from A. Identify the forces acting on A.
Slide 4-27
Aw
Bw
BAT
ABT
Block A
Block B
ACT
CABT
The other forces are:
weight of Block B
tension force on Block B due to Block A
tension force on the ceiling due to Blocks A and B
Since the system is at rest, 0.
B
BA
CAB
net
w
T
T
F
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A ball, hanging from the ceiling by a string, is pulled
back and released. Identify the forces acting on it just
after its release.
Example Problem
Slide 4-28
w
T
the ball's weight
tension
There are no other forces such as air resistanceacting on the ball, because "just after its release" it has not yet started moving
w
T
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Newton’s Second Law
Slide 4-29
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An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator.
• Is T greater than, equal to, or less than w? Or is there not
enough information to tell?
Example Problem
Slide 4-30
w
T the elevator's weight
tension
Since the elevator is moving upward at a"steady speed", it is not accelerating. Thenet force must therefore be zero, and the
magnitude of and must be equal.
w
T
w T
In
other words, .w T
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An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator.
• Is T greater than, equal to, or less than w? Or is there not
enough information to tell?
Example Problem
Slide 4-30
w
T
If the elevator moves fast enough, there could beother downward forces, such as the drag force
due to air resistance, shown as in the figure. At the slow speeds most elevators travel it would be u
D
nlikely to be significant, but if present, the
magnitude of would have to exceed that of to maintain a steady speed.
T w
D
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Free-Body Diagrams
Slide 4-31
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Newton’s Third Law
Slide 4-32
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Checking Understanding
An object, when pushed with a net force F, has an
acceleration of 2 m/s2. Now twice the force is applied to an
object that has four times the mass. Its acceleration will be
A. ½ m/s2.
B. 1 m/s2.
C. 2 m/s2.
D. 4 m/s2.
Slide 4-33
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Answer
An object, when pushed with a net force F, has an
acceleration of 2 m/s2. Now twice the force is applied to an
object that has four times the mass. Its acceleration will be
A. ½ m/s2.
B. 1 m/s2.
C. 2 m/s2.
D. 4 m/s2.
Slide 4-34
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A 40-car train travels along a straight track at 40 mph. A
skier speeds up as she skis downhill. On which is the
magnitude of the net force greater?
A. The train.
B. The skier.
C. The net force is the same on both.
D. There’s not enough information to tell.
Checking Understanding
Slide 4-35
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Answer
A 40-car train travels along a straight track at 40 mph. A
skier speeds up as she skis downhill. On which is the
magnitude of the net force greater?
A. The train.
B. The skier.
C. The net force is the same on both.
D. There’s not enough information to tell.
Slide 4-36
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Checking Understanding
10-year-old Sarah stands on a skateboard. Her older brother
Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
A. greater than the force of Sarah on Jack.
B. equal to than the force of Sarah on Jack.
C. less than the force of Sarah on Jack.
Slide 4-37
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Answer
10-year-old Sarah stands on a skateboard. Her older brother
Jack starts pushing her backward and she starts speeding up.
The force of Jack on Sarah is
A. greater than the force of Sarah on Jack.
B. equal to than the force of Sarah on Jack.
C. less than the force of Sarah on Jack.
Slide 4-38
Then why is it that Sarah speeds up?
It is because the force of Jack on Sarah exceeds the friction force due to the ground on Sarah, so the net force on Sarah is not zero
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Summary
Slide 4-39
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Summary
Slide 4-40
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Summary
Slide 4-41