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 speeding up while moving
upward.
•
•
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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Nellie Newton hangs motionless by one hand
from a clothesline as shown —which is on the
verge of breaking. Which side of the line is
most likely to break?
1. Left side
2. Right side
3. 50/50 chance
of either side
breaking
Nellie hangs motionless, which means all the
forces acting on her equal zero:
The force due to gravity acting downward, her
weight, is shown by the bold vector in A.
Equilibrium dictates an equal force upward,
supplied by the ropes, indicated by the dashed
vector. This dashed vector has to be the resultant
of tensions in the left and right sides of the rope.
Their relative sizes are found by constructing a
parallelogram, with the dashed vector as its
diagonal (B). Aha, the relative magnitudes of
these tensions are shown in C. The right side is
under greater tension, and therefore is the most
likely to break.
F  0
Nellie Newton hangs motionless by one hand
from a clothesline as shown —which is on the
verge of breaking. Which side of the line is
most likely to break?
1. Left side
2. Right side
3. 50/50 chance
of either side
breaking
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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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 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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Newton’s 3rd Law Demonstrations
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