## Wednesday, February 10, 2016

### No Net Force (no Push Force )Model

Tom has just been promoted and is pushing a file cabinet down the hall to his new office. He begins by looking at the file cabinet and considers how to best go about his task. Select Fx = 0 to model the zero force model. Note the motion of the block is no change in position and velocity is zero all the time.

### No Net Force ( 0<|Push| => Maximum Static Friction) Model

He then begins pushing on the file cabinet, which, at first, does not move at all. To simulate this case, select Push to be 3 N and observe what happens to the motion of the block. Note that the Push is cancel out by the Friction Force. Even at Push = 4.905 N is balanced out by Friction Force and the block does not move.

### Just Enough Push force (Push> Maximum Static Friction) to move Model

He pushes it slightly harder than the maximum static friction, and it is sliding . Thus, applying a Push force just larger than 4.905 N, the object begins to slide. But at t >0, the Friction becomes Kinetic Friction.
Eventually the file cabinet begins to slide across the floor, slowly moving towards his new office with acceleration.

### Big Idea

Thus, the evidences and model building process above suggests the condition of acceleration is the presence of non zero net force, which is F = ma, Newton's Second Law.

### Misconceptions

Students may think that it is possible to experience is pushing the file cabinet, and it is moving to the right with constant velocity.