WHY THINGS MOVE...

All changes in motion are due to forces--friction, gravity, and pushes and pulls. Friction is a force that makes moving objects slow down. Gravity is a force that which pulls things to the earth. Pushes and pulls are forces exerted by people and machines or magnets. While a force is required to move an object, a force does not always produce motion. You could push down on a desk and the desk will not move. Energy has been expended and motion has not occurred.





Things move because observable motion is associated with balanced and unbalanced forces. Your hand exerts an upward force on a ball as you throw it toward the sky. BUT there is no force that keeps it going after it leaves your hand. The unbalanced force of gravity and air resistance prevent the ball from going on forever into the sky. Forces usually cause changes in motion. The forward force from the car engine is necessary to keep the car going at a constant speed because it balances the backward forces of friction and air resistance. (WRESA 1995 HSPT)




Newton used these principles to guide his thinking about why things move the way they do:

Inertia = whatever an object is doing in terms of motion, it will continue to do UNLESS acted on by an outside force

Action and Reaction = for every action, there is an equal and opposite reaction

Acceleration = if you know the force (push or pull) applied to an object and if you know the object's mass (product of the volume and density) then you can calculate the object's acceleration; the force acting on an object is equal to the object's mass multiplied by its acceleration


In summary, if you know the mass, speed and location of any object, you can predict its future position in time and space.