Confusion is a part of learning... Take a deep breath. ------------------------> Skip to Constant Velocity

Here we are in motion. Kinematics to be exact. Kinematics means "motion without thinking about why things are in motion." We don't really care about why cars gain speed every second, but we do look at how they change their speed or direction.

There is confusion as to why things travel the way they do. As we have talked in class, even Aristotle was wrong in his explanation of motion. And his ideas lasted for 1500 years.
You live in a world where everything stops. No matter what, no matter when, everything that is moving will eventually stop moving. (Except for the Energizer Bunny) You know the idea of perpetual motion is silly. You think that, because on Earth FRICTION IS KING. Friction is a force that comes in many forms (air resistance, rolling, sliding, static friction...) and it makes things stop moving. You've lived you life in a friction-filled environment... it's not your fault that you think that. (Well, it is your fault, but that's nothing to be ashamed of. We'll work on it.)

The problem, as Galileo and Isaac Newton eventually found, is that the universe is actually ruled by laws that claim that INERTIA IS KING. Inertia is the tendency of objects to remain in their current state of motion. To be clear, inertia is not a force keeping things moving. Inertia is something objects do. If they are not moving, they don't start moving unless they experience an unbalanced force. If objects are moving, they keep moving forever until they experience an unbalanced force.

So all objects want to keep moving forever?

Yes. Think of Earth. The Earth as we know it is (by modern dating standards) about 4.5 billion years old. That means that the Earth has been traveling around the Sun for 4.5 billion years without slowing down or stopping. Sounds like a perpetual motion to me...

So. If we apply that principal to everyday objects we can suggest that all objects will continue moving the way they are currently moving unless some unbalanced force is applied to them. When you stop pushing on a book, you cease to balance friction, and when friction becomes unbalanced, the object changes the way it moves (accelerates).

If we pretend that we can turn off friction, there will be no unbalanced force when we stop pushing, and the object will continue to move at the speed and in the direction that it was last traveling. THIS IS CONSTANT VELOCITY