Introduction To Physical Science

By Princessa

Newton's Laws Of Motion:

1. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

2. The relationship between an object's mass M, its acceleration A, and the applied force F is F = MA. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

This we recongnize as essentially Galileo's concept of inertia, and this is often termed simply the "Law of Inertia".

3. For every action there is an equal and opposite reaction.

This is the most powerful of Newton's Three Laws, because it allows quantitative calculations of dynamics: how do velocities change when forces are applied. Notice the fundamental difference between Newton's 2nd law and the dynamics of Aristotle: according to Newton, a force causes only a change in velocity (an acceleration); it does not maintain the velocity as Aristotle held.

This is sometimes summarized by saying that under Newton, F=MA, but under Aristotle F=MV, where is V is the velocity. Thus, according to Aristotle there is only a velocity if there is a force, but according to Newton an object with certain velocity maintains that velocity unless a force acts on it to cause an acceleration (that is, a change in the velocity). As we have noted earlier in conjunction with the dicussion of Galileo, Aristotle's view seems to be more in accord with common sense, but that is because of a failure to appreciate the role played by frictional forces. Once account is taken of all forces acting in a given situation it is the dynamics of Galileo and Newton, not of Aristotle, that are found to be in accord with the observations.

Gravity:

The easiest way to think of gravity is that it's the thing that makes you stick to the earth. It's like a giant hand that reaches up from the ground, grabs your ankle and holds you down. Gravity affects the amount of force that you exert on the ground. It's like you are pushing on the ground.

Have you ever wondered why you don't just float off into space? It's because of gravity.

When you see movies with astronauts in space and they are floating all around, it's because they aren't close enough to Earth (or to another planet) to get pulled down by gravity. When you see the astronauts on the moon (or on another planet in a movie), they are getting held down by that thing's gravity. In fact, it's the Earth's gravity that holds our moon in it's orbit... Without gravity, the moon would just go flying out into space.

Mass is just the amount of 'stuff' you are made of... No matter where you go in the Universe, you are still made of the same amount of stuff, so your mass is always the same. (Unless, of course, you grow. Then your mass increases.)

The force of gravity that you push ground with is called weight. You might weigh 66 pounds on the Earth... But you would weigh 1/6 of this on the moon (only 11 pounds) because the moon's gravity is 1/6 as strong as the Earths.

Inertia:

This is a really important thing. It's so important we call it "The First Law Of Motion", it's a pretty easy idea...

If something is moving, it will keep moving until something stops it. If something is not moving, it will just sit there until something comes along to move it.

An object in motion will continue to stay in motion unless acted upon by an outside force. An object at rest will continue to stay at rest unless acted upon by an outside force.

What if you put a book on a table... It would just sit there unless you moved it. You could pick it up... Or push it... Or lift one side of the table so it would side off. That's Inertia!

Notes:

Sir Issac Newton's law of universal gravitation was published in 1687.

Gravity is what holds objects on Earth's Surface.

An object's mass is the same, regardless of where the object is in the Universe.

Weightlessness is a condition experienced when an object is in free fall, which means that the only force acting on the object is Earth's Gravity.

The weight of an object varies and depends on where the object is in the Universe.

The strength of gravitation between any two objects is related to the product of their masses divided by the square of distance between their centers.

Gravitation is the attractive force between any two objects.

An objects weight depends on where it is in the Universe.

The weight of an object on Earth is equal to its mass times gravity.

A brief period of microgravity lasting less than 30 seconds occurs at the peak.