Transcript Powerpoint for today

Chapter 4
Forces and Newton’s Laws of Motion
Topics:
•
Motion and Newton’s first
law
• What is a force?
• Identifying forces
• Newton’s second law
• Free-body diagrams
• Newton’s third law
Sample question:
These ice boats sail across the ice at great speeds. What gets the
boats moving in the first place? What keeps them from going even
faster?
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Example
An elevator, lifted by a cable, is going up at a steady
speed.
•
•
Identify the forces acting on the elevator.
Is FT,cable => elevator greater than (A), equal to (B), or less than
Fg,Earth => elevator (C)? Or is there not enough information to tell
(D)?
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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.
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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
B. equal to than the force of Sarah on Jack.
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
A Short Catalog of Forces: Weight w

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
Spring Force Fsp
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Tension Force T
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Normal Force
n
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
Friction fk and fs
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Drag D and Thrust Fthrust
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Example 1
A block is dragged uphill by a rope. Identify all
forces acting on the block.
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Example 2
The cable at left exerts a -30 N force.
• Write the equation for the sum of the forces in the x-direction.
What is the value of FT2?
• Write the equation for the sum of the forces in the y-direction.
What is the force of gravity acting on the ball?
30

T2
T1
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Exercise
A ball, hanging from the ceiling by a string, is pulled
back and released. Identify the forces acting on it just
after its release.
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Newton’s Second Law
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Newton 2 Demonstrations
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Newton’s Third Law
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Newton’s 3rd Law Demonstrations
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Newton’s Laws of Motion
Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the
future. Objects only know what is acting directly on them right now
Newton's 1st Law
An object that is at rest will remain at rest and an object that is moving will
continue to move in a straight line with constant speed, if and only if the sum of
the forces acting on that object is zero.
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
Newton's 3rd Law
Recall that a force is an interaction between two objects. If object A exerts a
force on object B then object B exerts a force on object that is in the opposite
direction, equal in magnitude, and of the same type.
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