Common misconceptions about force and motion (1)

Wrong Concept #1:

A force is needed to maintain a body's motion.

Explanation:

First of all, Newton's 1st Law of Motion states:

Every object will remain at rest or in uniform motion in a straight line unless acted upon by an external unbalanced force.

So it clearly says that an object can be in uniform motion in a straight line (i.e. moving at constant velocity) WITHOUT any force acting on it. Repeat: No force is needed to maintain a body's motion. (p.s. When someone says no force, you can assume he/she means no unbalanced force)

A lot of students feel uncomfortable with this concept because it violates their common sense. Everything will eventually stop if we are stop pushing it forward, therefore we need to push it(i.e. apply an external force) in order to maintain its motion. So what’s wrong?

The key is: There is always friction acting on moving bodies in our daily life. So if we are not pushing it, there is still unbalanced force acting on the object -- the friction slows it down. So it gives you an wrong impression that if you want to keep something moving, you will need to apply a force. We need to consider a situation that there is no friction to develop the correct sense.

Have you ever played air hockey before?

By U.S. Air Force photo by Airman 1st Class Alexxis Pons Abascal - http://www.cannon.af.mil/shared/media/photodb/photos/120213-F-YG475-032.JPG, Public Domain, Link

You should noticed that the air hockey puck can travel a very long distance before it stop, it is because the puck is pushed up by the air flowing out from the table surface so virtually no frictional force is acting on the puck. According to the Newton’s 1st Law, when there is no external force acting on the puck, it will travel in uniform motion in a straight line. (In reality, it will still eventually stop as it loses energy when colliding with the edge of the table, but if you have an infinite long table, it won’t stop.)