Objects that move with a constant velocity (same speed ... same direction) experience a balance of forces. Oppositely-directed forces are of the same strength. On the other hand, objects that accelerate experience an unbalanced force that is directed in the direction of the object's acceleration.

A disk at moving leftward at a constant speed across a 'no-friction' air table. The Force Diagram is shown. What will the Force Diagram look like if the air is turned off and this leftward-moving disk is in the state of slowing down? Select a force diagram from the choices provided on the right.


The Force Diagram provided for the disk shows a balance of forces: the up force and the down force are of equal strength. This will always be the case for an  object moving with a constant velocity. But you need to determine what the Force Diagram looks like if the air in the air table is turned off and the disk begins to slow down as it moves to the left. To do so, you should first determine the direction of the acceleration. If the acceleration is rightwards, then there must be more right force than left force. But if the acceleration is leftwards, then there would be more left force than right force. Read on ... 

The Direction of the Acceleration

The direction of an object's acceleration depends on two important features of the object's motion - the direction that the object is moving and whether the object is speeding up or slowing down. The general rule is: If an object is slowing down, then the direction of the object's acceleration is the opposite of the direction that the object is moving. On the other hand, if the object is speeding up, then the direction of the object's acceleration is the same as the direction that the object is moving.

Now apply this information to the disk and determine the direction of its acceleration.


Beware of Misconceptions

A common misconception that a Physics student may have is to believe that forces are associated with the direction that an object is moving. That is, students falsely believe that an object that is moving leftward must first of all have a leftward force and the strength of that leftward  force must be greater than any rightward force. But we doubt that you've been taught that way ... at least not by a card-carrying Physics teacher.  The fact is that forces are associated with accelerations or changes in the velocity. Forces don't cause objects to move in a given direction. Forces cause objects to change how they are moving in a given direction.  

You want to determine the Force Diagram for a disk that is moving leftwards and slowing down. You're probably paying attention to the moving leftwards part ... but don't miss the slowing down part. The slowing down describes the change that is occuring. Forces when unbalanced cause objects to change the way they are moving. The disk was moving with a constant speed, When it changes to a slowing down motion, you must make sure there is more force against its motion than there is force in the direction of its motion. And don't even presume that there even needs to be a leftward force.

What's Normal About Friction?

When the air table was turned on, the disk moved at a constant speed. The up force was an air force. The air blowing out of the holes on the air table levitate the disk above the table. The disk experiences an upward air force; it doesn't make any contact with the table. But when the air is turned off,  the disk is in contact with the table and the table would push upward on the disk with a normal force. And because there is a normal force, there would also be a force of sliding friction upon the disk as it slides across the table. Like any sliding friction force, it is directed opposite the motion of the disk.

Think Interactions ... Always!

Forces result from mutual interactions between two objects. To determine the forces that act upon the disk, ask what objects are the disk interacting with? The disk (as always) interacts with the Earth through gravitational forces between the disk the Earth. The disk pulls up on the Earth and the Earth pulls down on the disk. This is the gravity force (Fgrav). But the disk also interacts with the table. It presses down on the table and the table presses or pushes up on the disk. This is the normal force (Fnorm).  Finally, as the disk slides to the left across the table, it pulls the table to the left and the table pulls the disk to the right. This is the rightward friction force (Ffrict) upon the disk. Since the disk does not touch or interact with any other object, these are the only forces acting upon the disk.