Inertia, Forces and Acceleration:
The Legacy of Sir Isaac Newton
Objects in Motion
Physics 1100 – Spring 2012
Chapter 2 - Newton’s First Law of Motion - Inertia
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Aristotle on Motion
Copernicus and the Moving Earth
Galileo and the Leaning Tower
Galileo's Inclined Planes
Newton’s First Law of Motion !!!
Net Force
The Equilibrium Rule
Support Force
Equilibrium of Moving Things
Physics 1100 – Spring 2012
A Little History
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Aristotle (~320 BC)
– Air, earth, fire, water
– Unnatural motion – Horizontal motion
– Testing of theories not considered needed so his ideas lasted for 2000
years.
Galileo Galilei (~1640)
– Provided new theories based on observation and experiment.
– Stones twice as heavy do not fall twice as fast as Aristotle said
– Experimented with balls and incline planes
– Inertia!
Isaac Newton (~1700)
– Three laws of motion
– Laws of gravitation
– Greatest scientific book: Principia Mathematica Philosophiae Naturalis
- 1687
Physics 1100 – Spring 2012
Galileo and Falling Bodies
Physics 1100 – Spring 2012
Galileo and Inclined Planes
Physics 1100 – Spring 2012
Inclined Planes and Inertia
Physics 1100 – Spring 2012
Newton’s 1st law - Inertia
If the total “resultant” force acting on an object is zero,
the object will either remain at rest or it would move
along a line with a constant velocity.
Physics 1100 – Spring 2012
Newton Says
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A ball sitting still will stay that way, unless acted upon by a force.
Inertia
Mass
An object that is not subjected to any outside forces moves at constant
velocity, covering equal distances in equal times, along a straight path,
x(t) = x(0) + vt
Newton’s 1st Law
• This is not intuitively obvious.
Physics 1100 – Spring 2012
Examples of Inertia
Physics 1100 – Spring 2012
Force Has Direction!
Physics 1100 – Spring 2012
Net Force
Physics 1100 – Spring 2012
Stable Equilibrium, SF = 0
Physics 1100 – Spring 2012
A package falls off a truck that is moving at 30 m/s.
Neglecting air resistance, the horizontal speed of the
package just before it hits the ground is:
A) more than 30 m/s.
B) zero.
C) about 30 m/s.
D) less than 30 m/s but larger than zero.
E) More information is needed for an estimate.
Physics 1100 – Spring 2012
Whirl a rock at the end of a string and it follows circular
path. If the string breaks, the tendency of the rock is to
A) follow a straight-line path.
B) continue to follow a circular path.
C) revolve in a smaller circle
D) increase its speed
Physics 1100 – Spring 2012
If no external forces are acting on a moving object it will
A) continue moving at the same velocity.
B) continue moving at the same speed.
C) move slower and slower until it finally stops.
Physics 1100 – Spring 2012