3.1 Aristotle on Motion

Aristotle, the foremost Greek scientist, studied motion and divided it into two types: natural motion and violent motion

Natural motion on Earth was thought to be either straight up or straight down

• Ex. a boulder falling toward the ground, a puff of smoke rising in the air, etc.

Objects would seek their natural resting places: boulder on the ground and smoke in the air. It was “natural” for heavy things to fall and for light things to rise.

Aristotle claimed that circular motion was for the heavens—they were both without beginnings or ends.

• Ex. planets and stars moved in perfect circles around Earth

These motions were considered natural, they were not thought to be caused by forces

Violent motion was imposed of motion, it was the result of forced that pushed or pulled.

• Ex. a cart being moved because it was pulled by a horse, tug of war, a ship being pushed by the force of wind, etc.

All violent motions have external causes, objects in natural resting places could not move by themselves; they had to be pushed or pulled.

3.2 Copernicus and the Moving Earth

Copernicus reasoned that the simplest way to interpret astronomical observations was to assume that Earth and the other planets move around the sun.

3.3 Galileo on Motion

A force is a push or pull. Friction is the name given to the force that acts between materials that touch as they move past each other. Friction is caused by irregularities in the surface of objects that are touching. If friction were absent, a moving object would need no force to remain in motion.

Galileo argued that only when friction is present—as it usually is—is a force needed to keep an object moving.

• Ex. He tested a rolling ball along three different angles. The ball rolling down the inclined plan speeds up, but the ball rolling up the inclined plane slows down because its going in the opposite direction of gravity’s pull. The ball rolling on the horizontal plane doesn’t roll against gravity.

  • The ball rolling on the smooth horizontal plane has almost constant velocity. If friction were entirely absent, a ball moving horizontally would move forever. Only friction would keep it from rolling forever.

Galileo stated that this tendency of a moving body to keep moving is natural and that every material object resist change to its state of motion.

The property of a body to resist changes to its state of motion is called inertia.

3.4 Newtons Law of Inertia

Newtons first law, usually called the law of inertia, is a restatement of Galileo’s idea that force is not needed to keep an object moving.

Newtons first law states that every object continues in a state of res, or of uniform speed in a straight line, unless acted on by a nonzero net force.

Objects at Rest

• Ex. Dished on a tabletop are in a state of rest. They tend to remain at rest if you snap a tablecloth from beneath them. If you do it properly, you’ll find the brief and small forces of friction are not significant enough to move the dishes.

Objects in Motion

Whereas the ancients thought continual forces were needed to maintain motion, we now know that objects continue that may be present and to set objects in motion initially. Once the object is moving in a force-free environment, it will move in a straight line indefinitely.

3.5 Mass—A Measure of Inertia

The amount of inertia an object has depends on its mass—which is roughly the amount of material present in an object.

The more mass an object has, the greater its inertia and the more force it takes to change its state of motion.

Mass is measure in the fundamental unit of kilograms. Mass is a measure of the amount of materials in an object and depends on the number of and kind of atoms that compose it. Weight on the other hand is a measure of the gravitational force acting on the object, it depends on an objects location.

The amount of material in a particular stone is the same whether the stone is located on Earth, on the moon, or in outer space. The stone’s mass is the same in all locations. But the weight would be very different on Earth and on the moon, the stone would have only one-sixth the weight it has on earth. (this is because the force of gravity on the moon is only one-sixth as strong as it is on Earth.

If the stone were in a gravity-free region of space, its weight would be zero but its mass would remain the same. Mass is not weight.

Mass is the quantity of matter in an object. Mass is a measure of inertia, or “laziness”, than an object does in response to any effort to start, stop, or otherwise change its state of motion.

Weight is the force of gravity on an object. Even though mass and weight are not the same, they are proportional to each other. Objects with greater mass have great weight; object with little mass have little weight. They are proportional but not equal.

1kg = 10 Newtons

1kg = 2.2lbs

If you have the mass of something in kilograms and want its weight in newtons at earths surface, multiply the number of kilograms by 10 or if you know the weight in newtons divide by 10 and you’ll have the mass in kilograms.

3.6 The Moving Earth Again

Objects Move With Earth

The law of inertia states that object in motion remain in motion if no unbalanced forces act on them.

So objects on Earth move with Earth as Earth moves around the sun.

• Ex. stand next to a wall. Jump up so that your feet no longer tough the ground. Does the 30 km/s wall slam into you? Why not? Because you are also traveling at 30 km/s, before, during, and after the jump.

Practice Problems

3.1

1. What were the two classifications of motion according to Aristotle?

   1. Natural motion and Violent motion

2. According to Aristotle what kinds of motion require no forces?

   1. Natural motion: a boulder falling to the ground, smoke rising into the air

3.2

1. What simple way of interpreting astronomical observations did Copernicus advocate?

   1. Assume that Earth and the other planets move around the sun

3.3

1. How did Galileo discredit Aristotle’s assertion that a force is needed to keep objects moving?

   1. Galileo did an experiment of a ball rolling down an inclined plane and observed that without friction or air resistance, it could keep moving at a constant speed without an external/continuous force

2. Galileo let a ball roll down one incline and then up another. Compare with its initial height, how high did the ball roll up the second incline?

   1. The ball rolled up the second incline at the same height it started with when rolling down

3. What name is given to the property of an object to resist changes in motion?

   1. Inertia

3.4

1. What is the tendency of an object at rest when no forces act on it?

   1. The object at rest remains at rest

2. What is the tendency of a moving object when no forces act on it?

   1. The moving object with no forces would move indefinitely

3.5

1. What relationship does mass have with inertia?

   1. The greater the mass, the greater the inertia. The smaller the mass, the smaller the inertia.

2. What does it mean to say mass and weight are proportional to each other?

   1. Objects with great mass have great weight and vise versa for smaller weight, they’re proportional. But just because an object weighs a lot doesn’t mean the mass is a lot, they’re proportionate not equal. Mass has to do with matter, weight has to do with gravity.

3. What is the SI unit of the measurement for mass?

   1. Kilogram

4. What is the SI unit of the measurement for weight?

   1. Newtons

5. What is the weight of a 1kg brick?

   1. 10 Newtons

3.6

1. If you’re smooth-riding a bus that is going 40 km/s and you flip a coin vertically, how fast is the coin moving horizontally midair?

   1. The coin is going 40km/s because of the law of inertia which states that objects in motion remain in motion if no unbalanced forces acts on them. The coin moves with us as the bus goes 40 km/s, its moving 40 km/s before, during, and after its been flipped. Only vertical force of gravity affects vertical motion.

Plug & Chug + Think & Explain

• If a woman has a mass of 50kg, calculate her weight in newtons?

  • W = m x g —> W = 50KG X 9.8 —> 490N

• Calculate in newtons the weight of a 2000kg elephant

  • W = 2000 × 9.8 —> W = 19,600N

• Calculate in newtons the weight of a 2.5kg melon. What is its weight in pounds?

  • 24.5N and approximately 5.50lb

• An apple weighs 1N. What is its mass in kilograms? What is its weight in pounds?

  • 0.2248lb

• Susie Small finds she weighs 300N. Calculate her mass

  • 30kg

• When a ball rolls down an inclined plane, it gains speed because of gravity. When rolling up it loses speed because of gravity. Why doesn’t gravity play a role when it rolls on a horizontal surface?

  • Because gravity acts vertically, not horizontally. The only thing that plays a roll when the ball is moving is friction.

• When a junked car is crushed into a compact cube, does its mass change? its volume? its weight?

  • The mass remains the same, the volume decreases, and the weight only changes if the car is in a different location (space, the moon, etc.) but if its still on earth the weight will remain the same.

• Suppose you place a ball in the middle of a wagon that is at rest and then abruptly pull the wagon forwards, describe the motion of the ball relative to the ground and the wagon.

  • Initially, as the wagon abruptly moves the ball would go forward for a split second but would be dragged bag because of how quick the pull was.