Physics 2211: Lecture 7

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Physics 2211 Lecture 7
Newtons 2nd Law of Motion Newtons 1st Law of
Motion
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Isaac Newton 1643-1727
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What do Forces Do?
EXAMPLE Block on frictionless surface pulled
with constant force
OBSERVATION 1 Object moves with constant
acceleration
when pulled with constant force.
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Effect of Increasing the Force
ADD RUBBER BANDS
OBSERVATION 2 Acceleration is directly
proportional to force
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Effect of Adding Objects
ADD MORE BLOCKS
constant stretch
Three Rubber Bands
rubber bands
OBSERVATION 3 Slope is inversely proportional to
the amount of matter accelerated
mass
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Review

• When several forces act on an object, they add.

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Three forces act on an object as shown to the
left. In which direction does the object
accelerate?

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How does an object move when it is left alone?

• Aristotle Objects are naturally at rest.

Galileo Objects are naturally in
motion (at constant velocity).
Newton Galileo was right.
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Newtons First Law
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Vocabulary

• A Reference Frame is the (x,y,z) coordinate
  system you choose for making measurements.
• An Inertial Reference Frame is a frame where
  Newtons Laws
• are valid.
• USE NEWTONS LAWS TO TEST
• FOR INERTIAL REFERENCE FRAMES

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Test for Inertial Reference Frame
EXAMPLE Airplane parked on runway A ball is
placed on the floor of the plane no net forces
act on the ball.
Less obvious a plane cruising at constant
velocity is also an inertial
reference frame!

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Test for Inertial Reference Frame

• EXAMPLE Airplane taking off.
• Ball placed on floor of plane no net forces act
  on ball.
• .

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We live in a World of Approximations . . .
Strictly speaking, an Inertial Reference Frame
has zero acceleration with respect to the
distant stars.
The Earth accelerates a little (compared to the
distant stars) due to its daily rotation and its
yearly revolution around the Sun. Nevertheless,
to a good approximation, the Earth is an
Inertial Reference Frame.
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Recap
Newtons 2nd Law Newtons 1st Law
Next Lecture
Free Body Diagrams Knight Section 4.7