Newton's First Law of Motion is one that we are all somewhat familiar with.  An object in motion will stay in motion and an object at rest will stay at rest.  Unless acted upon by an unbalanced external force!

That last part is the kicker.  The external force must be unbalanced for the object to slow down, speed up, or change direction.  If there are external forces applied, and they do not add to zero (using a free-body diagram (FBD), or finding the vector sum algebraically), then the object must change its motion!

If the external forces DO add to zero (as in the case of the cliffhanger in section 9.2), then the object will remain as it was, whether at motion or at rest.

This helps us introduce the concept of inertia.  All objects want to resist change.  It just so happens that some objects resist change better than other objects.

Think About It:

Let's say we have two objects, one is children's shoebox full of feathers, the other is large iron bank safe.  Both lie on a completely frictionless surface.

If you could apply a force to each object, which one would you need to apply a larger force to (i.e. push harder on) to get it moving?

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