Reading Quiz
Chapter 5
Physics 111, Concordia College
1. The coefficient of static friction is
A. smaller than the coefficient of kinetic friction.
B. equal to the coefficient of kinetic friction.
C. larger than the coefficient of kinetic friction.
D. not discussed in this chapter.
Physics 111, Concordia College
1. The coefficient of static friction is
A. smaller than the coefficient of kinetic friction.
B. equal to the coefficient of kinetic friction.
C. larger than the coefficient of kinetic friction.
D. not discussed in this chapter.
Physics 111, Concordia College
2.
The drag force pushes opposite your motion as
you ride a bicycle. If you double your speed,
what happens to the magnitude of the drag
force?
A.  The drag force increases.
B.  The drag force stays the same.
C.  The drag force decreases.
Physics 111, Concordia College
2.
The drag force pushes opposite your motion as
you ride a bicycle. If you double your speed,
what happens to the magnitude of the drag
force?
A.  The drag force increases.
B.  The drag force stays the same.
C.  The drag force decreases.
Physics 111, Concordia College
3.
You are standing on a scale in an elevator that
is accelerating upward. The reading on the
scale is
A.  Larger than your true weight.
B.  The same as your true weight.
C.  Smaller than your true weight.
3.
You are standing on a scale in an elevator that
is accelerating upward. The reading on the
scale is
A.  Larger than your true weight.
B.  The same as your true weight.
C.  Smaller than your true weight.
Chapter 5
Applying Newton’s Laws
Topics:
•
•
•
•
•
Equilibrium
Using Newton’s second
law
Mass, weight, and
apparent weight
Static and kinetic friction
Applying Newton’s third
law
Physics 111, Concordia College
Equilibrium
An object is in equilibrium when
the net force acting on it is zero.
In component form, this is
If the net force is zero then the
object does not accelerate. It
must be at rest or have a
constant velocity.
Physics 111, Concordia College
Equilibrium Problem Hints
•  If the acceleration is zero the net force must be
zero.
•  Draw a Free-Body diagram of the object.
•  Break the forces into x and y components.
•  The sum of the x-components must be zero and
the sum of the y-components must be zero.
Physics 111, Concordia College
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 m/s.
Physics 111, Concordia College
A wooden box, with a weight of 220 N, 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?
Physics 111, Concordia College
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?
Physics 111, Concordia College
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?
Physics 111, Concordia College
A ball weighing 50 N is pulled back by a rope to an angle of 20°.
What is the tension in the pulling rope?
Physics 111, Concordia College
Dynamical Systems
Physics 111, Concordia College
2nd Law Hints
•  Draw a free body diagram of the object.
•  Find the components of the forces.
•  Determine if any of the acceleration
components are known.
•  Write out the 2nd law in component form.
Physics 111, Concordia College
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?
Physics 111, Concordia College
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?
Physics 111, Concordia College
Mass, Weight and Apparent
Weight
Physics 111, Concordia College
Mass and Weight
–w = may = m(–g)
w = mg
Physics 111, Concordia College
Mass and Weight Units
MKS
English
Mass
kilogram (kg)
slug (sl)
Weight
Newton (N)
pound (lb)
•  An object with a mass of 1.0 kg has a weight of 9.8 Newtons on the
surface of Earth.
•  9.8 Newtons is the equivalent of 2.2 lbs.
•  1.0 kg is the equivalent of 6.9 x 10-2 slugs.
Physics 111, Concordia College
Find the x- and y-components of w in each of these three
coordinate systems.
Physics 111, Concordia College
A 75 kg skier starts down a 50-m-high, 10° slope on
frictionless skis. What is his speed at the bottom?
Physics 111, Concordia College
The same burglars push the 1000 kg safe up a 20° frictionless
slope with a horizontal force of 4000 N. What is the safe’s
acceleration?
Physics 111, Concordia College
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°?
Physics 111, Concordia College
Measuring Weight
Physics 111, Concordia College
Apparent Weight
•  When a person stands on a scale the scale
reads the normal force that the scale exerts
on the person.
•  If the person is not accelerating than the
normal force equals the weight and the scale
reads correctly.
Physics 111, Concordia College
Apparent Weight
Physics 111, Concordia College
Apparent Weight
•  If the person is acceleration the scale will
not read the correct weight.
∑F
y
= n − w = ma
n = mg + ma
•  n is what the scale reads. This is not mg
and is called the apparent weight.
Physics 111, Concordia College
Elevator Rides
•
Consider a person standing in an elevator that is
accelerating upward. The magnitude of the normal
force on the person exerted by the floor of the
elevator is
A.  larger than
B.  the same as
C.  smaller than
the magnitude of the weight force of the person.
Physics 111, Concordia College
Elevator
•  A 200 kg box is placed on a scale in a 2000
kg elevator. The elevator accelerates
downward at 2.4 m/s2. What does the scale
read while the elevator is accelerating?
Physics 111, Concordia College
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?
Physics 111, Concordia College
Friction Models
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Types of Friction
•  Static - object at rest with respect to the
surface.
•  Kinetic- object sliding on surface
•  Rolling- object rolling along surface
Physics 111, Concordia College
Static Friction
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fs max = µsn
Kinetic Friction
fk = µkn
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Friction
Physics 111, Concordia College
Sliding Away
•
A.
B.
C.
D.
An object is held in place by friction on an inclined
surface. The angle of inclination is increased until the
object starts moving. If the surface is kept at this
angle, the object
slows down.
moves at uniform speed.
speeds up.
You can't tell.
Physics 111, Concordia College
A 75 kg skier starts down a 50-m-high, 10° slope on skis. The
coefficient of kinetic friction between her skis and the snow is
0.06. What is her speed at the bottom?
Physics 111, Concordia College
Sliding on your seat
•  A car traveling at 20 m/s stops in a distance of 50
m. Assume that the acceleration is constant. The
coefficients of friction between the seat and the
passenger are µs = 0.5 and µk = 0.3. Will a 70 kg
passenger slide off the seat if not wearing a seat
belt?
Physics 111, Concordia College
Third Law Problems
Physics 111, Concordia College
Acceleration Constraints
Physics 111, Concordia College
Multiple Object Hints
•  Draw two (or more) free-body diagrams:
one for each object.
•  Identify acceleration constraints.
•  Identify action/reaction force pairs.
•  Apply Newton’s 2nd Law to each object in
component form.
Physics 111, Concordia College
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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What is the acceleration of block B?
40 N
Physics 111, Concordia College
Ropes and Pulleys
Physics 111, Concordia College
Pulleys
•  A pulley is used to change the direction of a string
or rope. If we treat the pulley as massless and
assume there is no friction in the bearings the
tension in the string is unchanged in magnitude.
Physics 111, Concordia College
Bob the Window-washer
•  Bob the window-washer
hangs from a pulley and
rope arrangement attached
to his window-washer
basket as shown on the left.
The rope will take a
maximum tension of 1000 N
before breaking. Bob and
the basket together weigh
1500 N. One day Bob
decides to tie one end of the
rope off to a nearby flagpole
as shown on the right.
Physics 111, Concordia College
Bob the Window-washer
The rope will take a maximum
tension of 1000 N before
breaking. Bob and the basket
together weigh 1500 N. One day
Bob decides to tie one end of the
rope off to a nearby flagpole as
shown on the right. Which of the
following is true for this new
arrangement?
A.
B.
C.
The rope is less likely to break
than before.
The rope is just as likely to break
as before.
Bob will need a good health plan.
Physics 111, Concordia College
Multiple Pulleys
A.
B.
C.
D.
E.
Sally uses the pulley arrangement below to lift a 24 kg box. With
what force must Sally pull on the rope to lift the box at constant
velocity? (Assume the rope and pulley are massless and ignore
friction. Use g = 10 m/s2)
240 N
180 N
120 N
100N
60 N
Physics 111, Concordia College
Atwood’s Machine
Two blocks are connected by a
string that passes over a pulley as
shown at the right. Block A has a
mass of 100 kg and block B has a
mass of 115 kg. Find the
acceleration of block A and the
tension in the string.
a = 0.68 m/s2
T = 1050 N
Physics 111, Concordia College
A
B
Crate Pulling
•  Bob pulls a pair of identical 10.0 kg crates across the floor
with a rope that makes an angle of 30 degrees with respect
to the horizontal. If the crates are accelerating at 2.0 m/s2,
(a) what is the tension T1 and (b) the tension T2?
Physics 111, Concordia College
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?
Physics 111, Concordia College