Motion: Inertia -- Physics Ch 3
Aristotle on Motion
Aristotle, the foremost Greek scientist, studied motion and divided it into two types.
Natural Motion – either straight up or straight down
It was “natural” for heavy things to fall and for very light things to rise.
Violent Motion – imposed motion as the result of being pushed or pulled.
The proper state of objects was thought to be one of rest, unless they were being pushed or pulled or were moving toward their natural resting place.
Copernicus and the Moving Earth
Most people at the time of Copernicus believed that the Earth was the center of the universe.
Copernicus believed that the Sun was the center of the universe. He worked on his ideas in secret to escape persecution.
Galileo on Motion
One of Galileo’s greatest contributions to physics was demolishing the idea that a force is necessary to keep an object moving.
Galileo found that a ball rolling on a smooth horizontal plane has almost
a constant velocity.
If friction were entirely absent, the ball would move without force
Galileo also stated that the tendency of a moving body to keep moving is natural
and that every object resists change to its state of motion.
Friction – the force that acts between materials that touch as they move past each other.
Inertia – the property of a body to resist changes to its state of motion.
Newton’s Law of Inertia
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 (unbalanced force).
Usually called the law of inertia
Forces are needed to overcome any friction. Forces are also needed to set
objects in motion initially.
Once an object is moving in a force-free environment, it will move in a straight
line indefinitely.
Examples of Newton’s first law of motion:
Throwing something in space: it will keep moving at a constant rate
Mass – A Measure of Inertia
Mass – the quantity of matter in an object.
SI Base unit: Kg (Kilogram)
The mass of an object is the same regardless of location
Weight – the force of gravity on an object.
Depends on an object's location
An object's weight will be less on the moon because gravity is less, but mass
will be the same
SI unit for weight: N (Newtons)
Mass and weight are proportional to each other in a given place.
Objects with great mass will have great weight.
Objects with less mass have less weight.
Gravity on the Moon is ⅙ of gravity on Earth.
Earth Moon
Mass 55 kg 55 kg
Weight 120 lbs 20 lbs
Weight 540 N 90 N
The Moving Earth Again
Copernicus’s announcement in the 16th century about the idea of a moving Earth stimulated much argument and debate.
Newton’s work showed that objects on Earth move with the Earth as it moves
around the Sun, just as the law of inertia shows that objects in a moving vehicle move with the vehicle.
Notions of motion today are very different from those of our distant ancestors.
Mass:
Depending only on the internal makeup of an object and the number and kind of atoms that compose it. The “laziness” of an object.
Weight:
Relates to the gravitational force acting on an object; it depends on the object’s location.
W = mg (W – weight; m – mass; g – gravity)
Mass and weight are not the same, but they are directly proportional. In the same location, something with twice the mass would have twice the weight.
On the moon’s surface, where gravity is ⅙ that of Earth’s, the mass of a stone would be the same as if it were on Earth. However, the weight would be less.
Use W=mg for the following; g acts as a constant 10 N/kg
Felecia has a mass of 45.0 kg.
What is her weight, in newtons, on Earth? 450. N
1 kg of mass corresponds to 2.2 pounds on Earth. What is her weight in pounds
on Earth? 99 lbs
What is Felecia’s mass on Jupiter’s surface? 45.0 kg
What would be her weight on Jupiter with an acceleration of gravity of
25.0 m/s/s? 1130 N
If an astronaut throws a rock in space, it will… Keep moving in a straight line
at a constant speed
The tendency of the rock to do this is called inertia.
A bus is traveling at 100 km/h; you are standing in the bus holding a pencil.
Relative to the bus, the pencil has a horizontal velocity of 0 km/h
Relative to the road, the pencil has a horizontal velocity of 100 km/h
If you drop the pencil, it will have a horizontal velocity of 100 km/h
The pencil will hit the floor… at your feet, below your hand