1. Newton’s First Law of Motion Inertia
Chapter 3 Notes
Newton’s First Law of Motion
Inertia

2. 3.1 Aristotle on Motion Aristotle (384 – 322 B.C.)
It was natural to think that the natural state of all objects was to be at rest.
Aristotle, the formost Greek scientist , studied motion and divided it into two types: natural motion and violent motion.
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3. 3.1 Aristotle on Motion Natural Motion Straight up or down Circular
Rock
Smoke
Circular
Moon
Stars
Did not require force
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4. 3.1 Aristotle on Motion Violent Motion = Imposed motion
The result of forces (push or pull)
External cause
Object in their natural resting place could not move by themselves : they had to be pushed or pulled.
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5. 3.2 Copernicus and the Moving Earth
Nicolaus Copernicus (1473 – 1543)
Copernicus reasoned that the simplest way to interpret astronomical observations was to assume that the Earth and other planets move around the sun.
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6. 3.3 Galileo on Motion Galileo Galilei (1564 – 1642)
Supported Copernicus
Force was not necessary for motion
Friction is the force that acts between materials that touch as they move past each other.
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7. 3.3 Galileo on Motion
Galileo argued that only when friction is present-as it usually is-is a force needed to keep an object moving.
Rolling Balls on tilted surfaces
If friction were entirely absent, a ball moving horizontally would move forever.
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8. 3.3 Galileo on Motion
In the absense of friction, the moving ball would naturally keep moving. Galileo stated that this tendency of a moving body to keep moving is natural and that every material object resists change to its state of motion.
The property of a body to resist changes to its state of motion is called inertia.
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9. 3.3 Galileo on Motion
Roman Catholic Church
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10. 3.4 Newton’s Law of Inertia
Sir Isaac Newton ( )
Principia Mathematica Philosophiae Naturalis
Knighted by Queen Anne in 1705
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11. 3.4 Newton’s Law of Inertia
Newton’s First Law – The law of inertia
Restating Galileo
Newton’s first law states that every object continues in a state of rest, or of uniform speed in a straight line, unless acted on by a nonzero net force.
Things keep on keeping on.
Demo
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12. 3.5 – Mass-A Measure of Inertia
The more mass an object has the greater its inertia and the more force it takes to change its state of motion.
Mass vs. Volume
Kilogram (kg) – the fundamental unit of Mass
Mass vs. Weight
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13. 3.5 – Mass-A Measure of Inertia
Mass is the quantity of matter in an object.
Mass is a measure of the inertia that an object exhibits.
Weight is the force of gravity acting on an object.
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14. 3.5 – Mass-A Measure of Inertia
1 kg ≈ 2.2 lbs (approximately equal to)
Weight is a force. The standard unit of force is the Newton (N). 1N ≈ ¼ lb
1 kg weighs 10 (9.8 really) Newtons.
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15. 3.6 The Moving Earth Again Earth in space 30 km/s Bird catches worm
Cart & Ball Demonstration
The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them.
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