Ch. 3: Newton's First Law of Motion: Inertia

1. Aristotle was an ancient Greek philosopher (388 - 322 B.C.) • He studied motion, and divided it into two types: • Natural motion: On Earth, straight up or down. • e.g. a rock falling straight down, or smoke rising • He thought objects were seeking their natural state. • he did not believe they were caused by any force. • Violent motion: imposed motion, the result of a force that pushed or pulled. • e.g. throwing a rock, a horse pulling a cart, etc. • Believed it had an external cause. • Objects in their natural state could not move by themselves: they had to be pushed or pulled. Ch. 3: Newton's First Law of Motion: Inertia I. Aristotle on Motion

2. Until Copernicus' time, most people believed that if there was no force, there was no motion. • Many people also believed that the Earth was at rest: they couldn't believe a force could be so strong as to move the entire earth. II. Copernicus and the Moving Earth • Copernicus was one of the first Western thinkers to think that the Earth was moving around the Sun. • heliocentrism. • This idea was very controversial at the time, so he did his research in secret, and only published when he was about to die, to avoid punishment. Ch. 3: Newton's First Law of Motion: Inertia I. Aristotle on Motion

3. Galileo was responsible for showing that force is NOT required to keep things moving. • Frictionis the force that acts between materials that touch as they move past one another. • Caused by irregularities in the surfaces of objects. • Galileo argued that a force is requried to keep an object in motion only when friction is present (which it usually is). • He performed experiments with rolling balls along inclined surfaces. Ch. 3: Newton's First Law of Motion: Inertia III. Galileo on Motion

4. Ch. 3: Newton's First Law of Motion: Inertia III. Galileo on Motion

5. If a ball is dropped down an inclined plane, it will travel up a similar plane until it achieves almost the same height. • The smoother the surface, the closer to its orginal height it gets. • If the ball goes up another slope, even at a different angle, it will still achieve a similar height. • If the ball rolled down a ramp and onto a horizontal surface, Galileo reasoned that it would keep rolling, slowed down only by friction. • He realized that Aristotle was wrong: an object that is moving has a tendency to keep moving. • He posited that every object resists a change to its state of motion. • This is Inertia: the property of a body to resist changes to its motion. • Galileo, though, was concerned with how objects move, not why. IV. Newton's Law of Inertia • Newton was an English scientist who was born in the same year that Galileo died (1642-1727) Ch. 3: Newton's First Law of Motion: Inertia III. Galileo on Motion

6. Newton's first law of motion is also called the Law of Inertia • It's basically a restatement of Galileo's idea of motion. • ***Newton's 1st Law: every object continues in a state of rest, or of uniform speed in a straight line, unless acted upon by a nonzero net force. A. Objects at Rest • Objects at rest tend to stay at rest. • This is why you can pull a table cloth out from under place settings without having plates and silverware and glasses fly everywhere. Ch. 3: Newton's First Law of Motion: Inertia IV. Newton's Law of Inertia

7. Ch. 3: Newton's First Law of Motion: Inertia IV. Newton's Law of Inertia

8. B. Objects in Motion • If you've ever played air hockey, you have probably noticed that you can slide the puck with apparently no loss in speed. • This is because friction is very low when a puck rests on a bed of air--but there is still friction. • In the absence of opposing forces, a moving object tends to move in a straight line indefinitely. Forces are only needed to set objects into motion initially, and to overcome any friction that might be present. No friction--no force needed to maintain motion. Ch. 3: Newton's First Law of Motion: Inertia IV. Newton's Law of Inertia

9. The amount of inertia an object has depends on its MASS. • A balloon and a bowling ball can have the same volume, but the bowling ball has more mass, therefore more inertia. A. Mass is NOT Volume • Mass is the amount of matter an object contains. • The base unitof mass is the kilogram (NOT the gram). • Objects with high mass don't necessarily have high volume. B. Mass is NOT Weight • Mass is often confused with weight, but they are not the same thing. • Weight is measured in newtons, or in pounds. • Mass is measured in kilograms. • The weight of an object changes in space, but not its mass. Ch. 3: Newton's First Law of Motion: Inertia V. Mass, a Measure of Inertia

10. C. Mass is Inertia • mass is also a measure of the inertia of an object. • Mass and weight aren't the same, but they are proportional to each other. • Objects with high mass also have high weight. • Weight is the acceleration due to gravity times the mass of an object. • On Earth, the acceleration of gravity is about 10 m/s2 D. One Kilogram Weighs 10 Newtons on Earth • If an object has a mass of 10 kg, then its weight is the mass (10 kg) times gravity (10 m/s2) = 100 n • An object that has a mass of 50 kg has a weight of 500 N. • The acceleration due to gravity is actually 9.8 m/s2, but we'll use 10 m/s2 in this class. Ch. 3: Newton's First Law of Motion: Inertia V. Newton's Law of Inertia

11. If the Earth is moving (quickly) around the Sun, then why do objects fall straight down? • This question was asked by individuals during Copernicus' time who did not believe the Earth was revolving around the Sun. A. Objects Move With Earth • Objects fall straight down because *they* are moving along with the Earth. • The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them. B. Objects Move With Vehicles • The ancients did not have high speed vehicles like cars and planes. • Today, we can very easily see that, if you throw a ball up in a moving car, it goes straight up, even though it is traveling with the car. Ch. 3: Newton's First Law of Motion: Inertia VI. The Moving Earth (Again)