Kinds of Forces
Lecturer:
Professor Stephen T. Thornton
Reading Quiz: A hockey puck is sliding at
constant velocity across a flat horizontal ice surface
that is assumed to be frictionless. Which of these
sketches is the correct free-body diagram for this
puck?
A
B
C
Reading Quiz: A hockey puck is sliding at
constant velocity across a flat horizontal ice surface
that is assumed to be frictionless. Which of these
sketches is the correct free-body diagram for this
puck? No net force, because of constant velocity.
A
B
C
Last Time
Forces
Newton’s First Law
Newton’s Second Law
Newton’s Third Law
Today
Forces
Free body diagrams
Weight and mass
Normal force
Tension
Lots of conceptual quizzes
Conceptual Quiz
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down, and
eventually stops. You
conclude that:
A) the force pushing the stone forward
finally stopped pushing on it
B) no net force acted on the stone
C) a net force acted on it all along
D) the stone simply “ran out of steam”
E) the stone has a natural tendency to
be at rest
Conceptual Quiz
You kick a smooth flat
stone out on a frozen
pond. The stone slides,
slows down, and
eventually stops. You
conclude that:
A) the force pushing the stone forward
finally stopped pushing on it
B) no net force acted on the stone
C) a net force acted on it all along
D) the stone simply “ran out of steam”
E) the stone has a natural tendency to
be at rest
After the stone was kicked, no force was pushing
it along! However, there must have been some
force acting on the stone to slow it down and stop
it. This would be friction!!
Short review from last time:
Forces on assistant.
Forces on sled.
Weight
Weight is a force
W = mg called gravitational force
Note that weight is not a mass!
W  mg
Weight and Mass
The Normal Force
May Equal the Weight
Fg
FN
Is the normal
force always
equal to the
weight?
NO!
An Object
on an
Inclined
Surface
W
Force pushing downhill
Weight—the Force of
Gravity; and the Normal
Force
a)  Fy  FN  mg  ma  0
b)  Fy  FN  mg  40.0 N  0
c)  Fy  FN  mg  40.0 N  0
What happens when a
person pulls upward on the
box in the previous
example with a force
greater than the box’s
weight, say 100.0 N?
There is no normal
force!
Conceptual Quiz
A. The normal force from the table.
B. The gravitational force the apple
exerts on the Earth.
C. The gravitational force the apple
exerts on the table.
D. The normal force the apple
exerts on the table.
B)
A. The normal force from the table.
B. The gravitational force the apple
exerts on the Earth.
C. The gravitational force the apple
exerts on the table.
D. The normal force the apple
exerts on the table.
Tension in a Heavy Rope
Heavy rope:
T T T
3
2
1
Light rope:
T T T
3
= mg
mass m
g
2
1
We usually
consider light ropes.
A Pulley Changes
the Direction of a
Tension
Notice that the
tension is constant
throughout.
Elevator and counterweight (Atwood’s machine)
Magnitudes equal
aE  aC
I prefer T rather than FT
What is wrong with right diagram?
FT must be greater than mcg.
Tension in a String
Conceptual Quiz:
We have a 1.0 kg mass hanging from the string. The
string is wrapped around a pulley so the string is
horizontal. If we separate the horizontal string and
insert a spring scale, what will the scale read?
A) 0
B) 1 N
C) 5.9 N
D) 9.8 N
Answer: D
The gravitational force pulling on the
string is (1 kg)(9.8 m/s2) = 9.8 N.
The tension in the string must equal
this, and it is constant throughout.
We measure the tension by inserting
the spring. It must measure 9.8 N.
Conceptual Quiz: We have 1.0 kg masses
hanging from two pulleys. We unhook the
horizontal string and insert a spring scale.
What will the spring scale read for the
tension in the horizontal string?
A) 0 N
B) 9.8 N
C) 19.6 N
Answer: B
It doesn’t matter whether the right
hand string is attached to the pole
or to a pulley with a hanging 1.0
kg mass.
Solving Problems with Newton’s
Laws:
Free-Body Diagrams
1. Draw a sketch.
2. For one object, draw a free-body
diagram, showing all the forces acting
on the object. Make the magnitudes
and directions as accurate as you
can. Label each force. If there are
multiple objects, draw a separate
diagram for each one.
3. Resolve vectors into components.
4. Apply Newton’s second law to each
component.
5. Solve.
Let’s look more carefully
at Free-Body Diagrams
Free-Body Diagrams
Box slides down an incline.
Hanging Object. An object is hanging by
a string from your rearview mirror. While
you are accelerating at a constant rate from
rest to 28 m/s in 6.0 s, what angle does the
string make with the vertical?
Pulley. Suppose the
pulley in the figure is
suspended by a cord
C. Determine the
tension in this cord
after the masses are
released and before
one hits the ground.
Ignore the mass of the
pulley and cords.