After covering some basic examples we should come to some conclusions about forces.

Lesson 1 - The net force is the key

Add up all the forces, as vectors, to find the net force acting on an object. That tells you what, if anything, will happen to the object's motion.

We represent the net force symbolically using: SF.

The Greek letter sigma symbolizes a sum, and here we're summing all the forces.

What we should have learned here is summarized in Newton's First Law. If no net force acts on an object its velocity is unchanged. If there is a net force the velocity changes.

Lesson 2 - Motion does not necessarily require a force to sustain it

The confusion here is that in many cases we do need to apply a force to sustain motion, but that's only when there are other forces, like friction, acting.

Lesson 3 - Force and velocity are not directly proportional

The net force is actually proportional to the rate of change of velocity, not to the velocity itself. Acceleration is our name for "rate of change of velocity". Newton's Second Law tells us that an object's acceleration is given by:

a

=

SF m

In other words, an object's velocity changes at a rate that is given by the net force acting on the object divided by the object's mass.