Newton's Three Laws of Motion and his Law of Gravity are probably the
most famous of all physics. They govern the motion of objects that we
see in our everyday lives.
Newton's First Law of Motion says
that an object will remain in its state of motion unless acted upon by
force. Essentially, this means that if an object is not moving, it will
remain still until a force (a push or pull) acts on it. Likewise, if an
object is moving, it will continue to move in the same direction with
the same speed in a straight line until a force acts on it. In either
case the motion is changed.
Newton's Second Law of Motion says
that force is equal to the mass of the object times its acceleration.
Remember, a force is a push or pull, and mass just means how much of
the object you have. Acceleration is how fast the velocity (speed with
direction) is changing. The Second Law essentially says that forces are
necessary to accelerate objects with mass, and it defines how big that
force is for the particular object.
Newton's Third Law of
Motion says that for every force acting on an object by another object
there is a force that acts upon the first object by the second one.
This second force is equal to the first and is in the opposite
direction. Whenever you have forces between two objects of about the
same size, you can see this. When a pool ball hits another one, you can
see that both change their velocities. The changes in their velocities
are in opposite directions. That's because the forces on them point
opposite ways. If you and a friend are on ice and one of you gives the
other a shove, you will move opposite directions. it doesn't matter who
decided to shove- the forces on the two of you were equal and opposite.
Of course when one of the objects is the Earth, it's hard to notice the
force on it, because its mass is so big it won't accelerate much if you
give it a shove.
Newton's Law of Gravity says that gravity is
a force that acts between any two objects with mass, and that force
increases if the mass increases and decreases if the distance between
the two objects increases. This force is always attractive, that is, it
always works to pull the objects closer together. This is why objects
fall to the ground and the moon doesn't fly away from the earth.
Newton's Laws have been indispensable in our world. They are certainly
not all of physics, but the use of them has accomplished much--not the
least of which was getting man to the moon!
(republished on 07/12/06)