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© 2012 Pearson Education, Inc. Slide 1-1
PreClass Notes: Chapter 4, Sections 4.5,4.6
• From Essential University Physics 3rd Edition
• by Richard Wolfson, Middlebury College
• ©2016 by Pearson Education, Inc.
• Narration and extra little notes by Jason Harlow,
University of Toronto
• This video is meant for University of Toronto
students taking PHY131.
© 2012 Pearson Education, Inc. Slide 1-2
Outline
• 4.5 Solving Force Problems
• 4.6 Newton’s Third Law,
• The Normal Force
• Hooke’s Law
“Using Newton’s second law
with multiple forces is easier if
we draw a free-body diagram.”
– R.Wolfson
Image from http://physicstasks.eu/380/a-girl-pulls-a-sledge-on-some-snowy-pavement
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© 2012 Pearson Education, Inc. Slide 1-3
Drawing a Free-Body Diagram
• When solving a problem in which multiple forces act
on an object, you should:
1. Identify the object of interest.
2. List all the forces that are acting on that object.
3. Represent the object as a dot.
4. Draw the vectors for only those forces acting on
the object, with their tails all starting on the dot.
© 2012 Pearson Education, Inc. Slide 1-4
Example 4.3
A 740-kg elevator accelerates
upward at 1.1 m/s2, pulled by a
cable of negligible mass. Find the
tension force in the cable.
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© 2012 Pearson Education, Inc. Slide 1-5
Forces and Interactions
Interaction
• is between one thing and another.
• requires a pair of forces acting on two objects.
Example: interaction of hand and wall pushing on each other
Force pair—you push on wall; wall pushes on you.
© 2012 Pearson Education, Inc. Slide 1-6
Newton’s Third Law of Motion
If object A exerts a force on object B, then object B
exerts an oppositely directed force of equal
magnitude on A.
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© 2012 Pearson Education, Inc. Slide 1-7
Action and reaction forces
• one force is called the action force; the other
force is called the reaction force.
• are co-pairs of a single interaction.
• neither force exists without the other.
• are equal in strength and opposite in direction.
• always act on different objects.
© 2012 Pearson Education, Inc. Slide 1-8
• Reexpression of Newton’s third law:
To every action there is always an
opposed equal reaction.
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© 2012 Pearson Education, Inc. Slide 1-9
Got it?
• If you exert a force 𝐹 on a bookcase, the force
which the bookcase exerts on you
A. is always − 𝐹 .
B. depends on your mass.
C. depends on the mass of the bookcase.
D. depends on the acceleration of the bookcase.
E. depends on the speed of the bookcase.
© 2012 Pearson Education, Inc. Slide 1-10
• Not every pair of equal and opposite forces form a third-law
pair.
• The two forces on the block act on the same object, so
they cannot be a third-law pair.
• They happen to be equal and opposite, but not because of
Newton’s Third Law.
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© 2012 Pearson Education, Inc. Slide 1-11
• Below is the same block on a tilted table.
• In this case those same two forces are clearly not equal
and opposite.
• The forces do not cancel, and the block accelerates.
© 2012 Pearson Education, Inc. Slide 1-12
• The gravitational forces on you and
Earth do form a third-law pair.
• [Figure not to scale.]
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© 2012 Pearson Education, Inc. Slide 1-13
Example
• What is the net force on the 3 kg block?
© 2012 Pearson Education, Inc. Slide 1-14
Elasticity
Hooke’s law: The extension of a spring is directly
proportional to the force applied to it.
extension~Force or x~F D
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© 2012 Pearson Education, Inc. Slide 1-15
Tension and Compression
When something is
• pulled it is in tension.
• squashed it is in compression.
[Image retrieved Jan.11 2013 from
http://busstop.typepad.com/blog/2007/07/the-blonde-and-.html ]
© 2012 Pearson Education, Inc. Slide 1-16
Hooke’s Law
• A stretched or compressed spring produces a restoring
force in a direction is opposite that of the stretch or
compression.
• In an ideal spring, the stretch or compression is directly
proportional to the force exerted by the spring:
• k is called the spring constant of the spring [N/m]
• The negative sign is there to remind you that the force 𝐹𝑠exerted by the spring on the object is opposite in direction to
the displacement 𝑥 of the end of the spring from its normal
position.
• Springs provide convenient devices for measuring force.
𝐹𝑠 = −𝑘𝑥
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© 2012 Pearson Education, Inc. Slide 1-17
Chapter 4 Big Ideas
• Force causes change in motion, not motion itself.
• Uniform motion (straight line, constant speed) needs no
cause or explanation.
• Any deviation in speed or direction requires a net force.
• Forces always come in pairs: Every interaction between two
objects consists of two forces which are equal in magnitude
but opposite in direction.