Force and Motion   Jan 15, 2008   2:00/1:00 MT/PT
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The world is filled with motion. Some motions just happen: the Earth revolves around the Sun, snowflakes fall to the ground, waves surge across the sea.

Other motions are under our control. We walk. We pick up things. We ride our bikes. But whether the movement is a planet revolving or you playing a game, the motion happens because of force.

Just what is Force?

Force is just a fancy word for pushing or pulling. If I push on something or pull on it, then I am applying a force to it. Forces make things move or, more accurately, makes things change their motion. Two natural forces that affect most of the motion we are aware of are the force of gravity and electromagnetic forces. Gravity produces a force that pulls objects towards each other, like a person towards the ground. And magnetism produces a force that can either pull two magnets together or push them apart, depending on how they’re lined up.



Normal forces
A book resting on a table has the force of gravity pulling it toward the Earth. But the book is not moving or accelerating, so there must be opposing forces acting on the book. This force is caused by the table and is known as the normal force. You can "see" the normal force in some situations. If you place a thin piece of wood or plastic (a ruler works) so that it is supported by both ends (by books perhaps) and place a small heavy object in the center, the piece of wood will bend. Of course it wants to straighten out so it exerts an upward force on the object. This upward force is the normal force. You can feel the force yourself if you push down in the center of the piece of wood. The harder you push, the more the wood bends and the harder it pushes back.

normal force

One way to illustrate this concept is with one of these paper bridge experiments from
or Building Big!

Retarding forces or resisting forces

Resisting or retarding forces like air resistance or friction change motion. Whether the forces actually stop or slow something depends upon your point of view. Air friction makes a leaf travel along in the wind. When you pick up a pencil, it’s friction with your fingers that gets the pencil in motion. In each case, the friction makes the two things (like the air and the leaf) move together. Newton's Second law states:

What is Inertia?

Inertia is actually not a force at all, but rather a property that all things have due to the fact that they have mass. The more mass something has the more inertia it has. You can think of inertia as a property that makes it hard to push something around.

Newton’s first law of motion describes this best: "A body in motion tends to remain in motion, or to remain stopped if stopped, except if it’s acted on by an outside force"

So, if an object is moving - its inertia (mass) will tend to keep it in motion, and if something is at rest, its inertia will tend to keep it at rest.

What is Friction?
Friction is a force that makes the objects move together or at least have more nearly the same velocity. Say you were pushing a toy train across the floor. It doesn't take much effort or force, because the toy is light. Now say you try to push a real train. You probably can't do it because the force of friction between the train and the ground. The heavier the object, the stronger the force of friction.

What is Gravity?

Gravity produces a force that pulls objects toward one another. It is the force that keeps the Earth revolving around the sun and it's what pulls you to the ground when you trip.

newtonNewton’s laws of Motion

Some consider Sir Isaac Newton to be the greatest English mathematician of his time and perhaps one of the greatest scientists the world has known. According to a story, Newton saw an apple fall to the ground and he figured out that the same force which caused the apple to fall also governed the motion of the Moon and the planets. In 1687 Newton published his three laws of motion in the "Principia Mathematica Philosophiae Naturalis."His three laws explained how the concepts of force and motion work.

laws of motion text
Check out NASA's resources:
newtons 1st Law
Newton's 2nd law Poster
Newton's 3rd Law

Force & Motion Links

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