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.
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.
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.