The Student Should Be Able To

Learning Goals:

The student should be able to:

·  Recognize that the coefficient of static friction is larger than the coefficient of kinetic friction

·  Understand that static friction is always equal to the applied force

·  Understand that kinetic friction is independent of the applied force or the speed of the object

·  Use force analysis and free-body diagrams to determine frictional forces and frictional coefficients

·  Static and Kinetic Friction Simulation

Part I: Demonstration

1.  What happens to the friction force as soon as the crate begins to move?

2.  Based on your equation for Friction force, what does this say about the coefficient of kinetic friction compared to the coefficient of static friction?

3.  Before the object begins moving, how do the friction and applied forces compare?

4.  After the object is already moving, what happens to the friction force if the applied force is decreased? Why is this?

Part II: Before doing the lab:

Go to: http://www.colorado.edu/physics/phet, Click on “Simulations”, then “Motion” and select the “Forces in 1-D” option. Click on “Run Simulation”, then click on “Graph Applied Force” and “Graph Acceleration”

Un-click the “Show Ftotal” and “Show Fapplied” tabs on the left of the screen.

You will perform this lab twice – one time each for two different objects. You may use whatever two objects you like (not the refrigerator – it doesn’t move). They are all listed on the right-hand side of the screen.

Procedure:

1.  Hit “GO”, then click on the object and drag the mouse to the right. This should increase the applied force. Continue to increase the force until the object begins to move, then continue applying a constant force until the object has accelerated forward and run into the wall.

2.  Hit “Pause” as soon as your object has hit the wall.

3.  Now look at the force graph. Drag the gray bar over until the point JUST BEFORE the frictional force drops. This is the maximum applied force allowed before the object starts to move. Draw a FBD of your object at this point.

4.  Use this FBD to determine the frictional force and coefficient of static friction (μs ) for your object.

5.  Now drag the gray bar over to any point where the object is accelerating. Draw a FBD of your object at this point.

6.  Use this FBD to determine the friction force and coefficient of KINETIC friction (μk) for your object.

7.  Repeat this process with another object.