4. A Test of Kinetic Friction

Q: We expect the Kinetic Frictional Force to be different from the Maximum Static Frictional Force. Should it require more or less force overcome Kinetic Frictional Force?

4.1: Set up the system as you did in section 3. Then repeat 3.2, but for when the cart is moving. Not that the answers won’t be as simple, since we are considering an accelerating system.

Q: When the cart is moving, is the tension in the string equal to, less than, or greater than the weight of the hanger? Explain.

4.2: Drag the cart all the way to the right, with the breaks on. Based on your data from 3.3, set the mass of the hanger equal to a value where the cart will move (but not too quickly) when the breaks are released.

4.3: We will be using the data from the motion sensor for this procedure.

• To start taking data click on the motion sensor to the left of the track. In the simulation it will turn to face the track.

• Release the breaks so that the cart starts accelerating. You will want to do this quickly so you don’t have too much unnecessary data (from the cart sitting still).

• Once the cart hits the left side of the track, click on the motion sensor to stop taking data.

• In the plot provided by the program, your data should look something like the plot below: 

4.4: Now, export your data to Excel.

• To do this, click on the “Copy Data to Clipboard” button.

• Now open up a blank Excel document and create two column headings for “Time (s)” and “Position (m)” as shown to the right.

• Now, paste your data into Excel just below your headings.

4.5: To get the force of friction, you are going to fit the data you’ve just copied into Excel.

• First, make a scatter plot of some of your data. Make sure you only include the section of data between turning the breaks off and the cart hitting the end of the track, as circled below:

• Then insert a trendline to fit your data. What mathematical equation do you need to fit this segment? What do the fit parameters correspond to physically? Be sure to choose a physical model that lets you correctly get the acceleration from your fit parameters.

4.6: Use the acceleration you found in in 4.5 combined with the expression for the frictional force you found in 4.1 to calculate the coefficient of kinetic friction.