Ch. 3: Newton's First Law of Motion: Inertia I. Aristotle on Motion Aristotle was an ancient Greek philosopher ( B.C.) He studied motion, and divided it into two types: o 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. o 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 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. o 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 III. Galileo on Motion Galileo was responsible for showing that force is NOT required to keep things moving. Friction is the force that acts between materials that touch as they move past one another. o 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). o He performed experiments with rolling balls along inclined surfaces.

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

Ch. 3: Newton's First Law of Motion: Inertia III. Galileo on Motion If a ball is dropped down an inclined plane, it will travel up a similar plane until it achieves almost the same height. o 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 ( )

Ch. 3: Newton's First Law of Motion: Inertia IV. Newton's Law of Inertia Newton's first law of motion is also called the Law of Inertia o It's basically a restatement of Galileo's idea of motion. o ***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 o Objects at rest tend to stay at rest. o 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

Ch. 3: Newton's First Law of Motion: Inertia IV. Newton's Law of Inertia B. Objects in Motion o If you've ever played air hockey, you have probably noticed that you can slide the puck with apparently no loss in speed. o This is because friction is very low when a puck rests on a bed of air--but there is still friction. o 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 V. Mass, a Measure of Inertia The amount of inertia an object has depends on its MASS. o 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 o Mass is the amount of matter an object contains.  The base unit of mass is the kilogram (NOT the gram). o Objects with high mass don't necessarily have high volume. B. Mass is NOT Weight o Mass is often confused with weight, but they are not the same thing. o Weight is measured in newtons, or in pounds. o Mass is measured in kilograms. o The weight of an object changes in space, but not its mass.

Ch. 3: Newton's First Law of Motion: Inertia V. Newton's Law of Inertia C. Mass is Inertia o mass is also a measure of the inertia of an object. o 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/s 2 D. One Kilogram Weighs 10 Newtons on Earth o If an object has a mass of 10 kg, then its weight is the mass (10 kg) times gravity (10 m/s 2 ) = 100 n o 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/s 2, but we'll use 10 m/s 2 in this class.

Ch. 3: Newton's First Law of Motion: Inertia VI. The Moving Earth (Again) o If the Earth is moving (quickly) around the Sun, then why do objects fall straight down? o 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 o Objects fall straight down because *they* are moving along with the Earth. o The law of inertia states that objects in motion remain in motion if no unbalanced forces act on them. B. Objects Move With Vehicles o 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.