Chapter 5
Applying
Newton’s
Laws
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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
5 Applying Newton’s Laws
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Slide 5-2
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Slide 5-3
1
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Slide 5-4
Reading Quiz
1. Which of the following statements about mass and weight is
correct?
A.
Your mass is a measure of the force gravity exerts on
you.
B. Your mass is the same everywhere in the universe.
C. Your weight is the same everywhere in the universe.
D. Your weight is a measure of your resistance of being
accelerated.
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Slide 5-5
Answer
1. Which of the following statements about mass and weight is
correct?
A.
Your mass is a measure of the force gravity exerts on
you.
B. Your mass is the same everywhere in the universe.
C. Your weight is the same everywhere in the universe.
D. Your weight is a measure of your resistance of being
accelerated.
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2
Reading Quiz
2. The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
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Slide 5-7
Answer
2. The apparent weight of an object is
A. the pull of gravity on the object.
B. the object’s mass times the acceleration of gravity.
C. the magnitude of the contact force that supports the
object.
D. the pull of gravity on an object that is accelerating
upward.
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Slide 5-8
Reading Quiz
3. The coefficient of static friction is
A.
B.
C.
D.
smaller than the coefficient of kinetic friction.
equal to the coefficient of kinetic friction.
larger than the coefficient of kinetic friction.
not discussed in this chapter.
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3
Answer
3. The coefficient of static friction is
A.
B.
C.
D.
smaller than the coefficient of kinetic friction.
equal to the coefficient of kinetic friction.
larger than the coefficient of kinetic friction.
not discussed in this chapter.
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Slide 5-10
Reading Quiz
4. The force of friction is described by
A.
B.
C.
D.
the law of friction.
the theory of friction.
a model of friction.
the friction hypothesis.
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Slide 5-11
Answer
4. The force of friction is described by
A.
B.
C.
D.
the law of friction.
the theory of friction.
a model of friction.
the friction hypothesis.
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4
Equilibrium
An object is in equilibrium when
the net force acting on it is zero.
In component form, this is
The net force on each
man in the tower is
zero.
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Slide 5-13
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Example Problem
A 100.-kg block with a weight of 980 N hangs on a rope.
Find the tension in the rope if
A.
the block is stationary.
m
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5
Example Problem
A 100.-kg block with a weight of 980 N hangs on a rope.
Find the tension in the rope if
A.
the block is stationary.
m
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Slide 5-15
Example Problem
A 100.-kg block with a weight of 980 N hangs on a rope.
Find the tension in the rope if
A.
B.
the block is stationary.
it’s moving upward at a steady speed of 5.0 m/s.
m
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Slide 5-15
Example Problem
A 100.-kg block with a weight of 980 N hangs on a rope.
Find the tension in the rope if
A. the block is stationary.
B. it’s moving upward at a steady speed of 5.0 m/s.
C. it’s accelerating upward at 5.0 m/s2.
m
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6
Example Problem
A wooden box, with a mass of 22 kg, is pulled at a constant
speed with a rope that makes an angle of 25° with the wooden
floor. What is the tension in the rope?
m
Slide 5-16
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Checking Understanding
A rod is suspended by a string as shown. The lower end of
the rod slides on a frictionless surface. Which figure correctly
shows the equilibrium position of the rod?
Slide 5-17
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Answer
A rod is suspended by a string as shown. The lower end of
the rod slides on a frictionless surface. Which figure correctly
shows the equilibrium position of the rod?
B
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Example Problem
A ball weighing 50 N is pulled back by a rope to an angle of 20°.
What is the tension in the pulling rope?
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Slide 5-19
Example Problem
A ball weighing 50 N is pulled back by a rope to an angle of 20°.
What is the tension in the pulling rope?
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Slide 5-19
Using Newton’s Second Law
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Example Problem
A sled with a mass of 20 kg slides along frictionless ice at 4.5 m/s.
It then crosses a rough patch of snow which exerts a friction force
of 12 N. How far does it slide on the snow before coming to rest?
Slide 5-21
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Example Problem
Macie pulls a 40 kg rolling trunk by a strap angled at 30° from the
horizontal. She pulls with a force of 40 N, and there is a 30 N
rolling friction force acting on trunk. What is the trunk’s
acceleration?
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Mass and Weight
–w = may = m(–g)
w = mg
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Apparent Weight
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Apparent Weight
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Example Problem
A 50 kg student gets in a 1000 kg elevator at rest. As the elevator
begins to move, she has an apparent weight of 600 N for the first 3
s. How far has the elevator moved, and in which direction, at the
end of 3 s?
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Example Problem
Find the x- and y-components of w in each of these three
coordinate systems.
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Slide 5-26a
Example Problem
Find the x- and y-components of w in each of these three
coordinate systems.
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Slide 5-26b
Example Problem
Find the x- and y-components of w in each of these three
coordinate systems.
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Example Problem
A 75 kg skier starts down a 50-m-high, 10° slope on frictionless
skis. What is his speed at the bottom?
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Example Problem
Burglars are trying to
haul a 1000 kg safe
up a frictionless ramp
to their getaway truck.
The ramp is tilted at
angle θ. What is the
tension in the rope if
the safe is at rest? If
the safe is moving up
the ramp at a steady 1
m/s? If the safe is
accelerating up the
ramp at 1 m/s2? Do
these answers have
the expected behavior
in the limit θ → 0° and
θ → 90°?
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Slide 5-28
Example Problem
The same burglars push the 1000 kg safe up a 20° frictionless
slope with a force of 4000 N. What is the safe’s acceleration?
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12
Static Friction
fs max = µsn
Slide 5-30
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Kinetic Friction
fk = µkn
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Working with Friction Forces
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Example Problem
A car traveling at 20
m/s stops in a distance
of 50 m. Assume that
the deceleration is
constant. The
coefficients of friction
between a passenger
and the seat are μs =
0.5 and μk = 0.3. Will a
70 kg passenger slide
off the seat if not
wearing a seat belt?
Slide 5-33
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Drag
An object moving in a gas or liquid experiences a drag force
Drag coefficient. Depends on
details of the object’s shape.
“Streamlining” reduces drag by
making CD smaller. For a typical
object, CD 0.5.
Density of gas or liquid. Air has
a density of 1.29 kg/m 3.
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A is the object’s cross section area
when facing into the wind.
Drag depends on the square of the speed.
This is a really important factor that limits the
top speed of cars and bicycles. Going twice
as fast requires 4 times as much force and,
as we’ll see later, 8 times as much power.
Slide 5-34
Cross-Section Area
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Terminal Speed
A falling object speeds up
until reaching terminal speed,
then falls at that speed
without further change.
If two objects have the same
size and shape, the more
massive object has a larger
terminal speed.
At terminal speed, the net
force is zero and the object
falls at constant speed with
zero acceleration.
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Slide 5-36
What is the terminal speed of a lacrosse ball with a
diameter of 6.4 cm and a mass of 150 g?
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Applying Newton’s Third Law: Interacting Objects
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15
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Slide 5-38
Example Problem
Block A has a mass
of 1 kg; block B’s
mass is 4 kg. They
are pushed with a
force of magnitude
10 N.
a. What is the
acceleration of
the blocks?
b. With what force
does A push on
B? B push on
A?
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Slide 5-39
Checking Understanding
Which pair of forces is an action/reaction pair?
A.
B.
C.
D.
The string tension and the friction force acting on A.
The normal force on A due to B and the weight of A.
The normal force on A due to B and the weight of B.
The friction force acting on A and the friction force acting on
B.
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Example Problem
What is the acceleration of block B?
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Slide 5-42
Example Problem
What is the acceleration of block B?
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Ropes and Pulleys
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17
Example Problem
Block A, with mass
4.0 kg, sits on a
frictionless table.
Block B, with mass
2.0 kg, hangs from a
rope connected
through a pulley to
block A. What is the
acceleration of block
A?
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Summary
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Slide 5-45
Summary
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Summary
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Slide 5-47
Additional Example Problem
A wooden box, with a mass of 22 kg, is pulled at a constant
speed with a rope that makes an angle of 25° with the wooden
floor. What is the tension in the rope?
m
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