Friction Lab
Name______Period____
Purpose: The purpose of this lab is to investigate the force of friction, differentiate static and kinetic friction forces, and determine how friction changes for an inclined surface.
Procedure: 1. We will be using the electronic force probe and computer interface to measure the force of friction. Make sure the force sensor is plugged into a USB port. Start the Data Studio and select “Create Experiment”. A Force Vs time graph is displayed, delete it. Click on “Setup” and uncheck “Force push positive” and then check “Force pull positive”. Increase the sample rate to 50 Hz and close the window. Drag the “Graph” icon in the Displays window to Force pull positive in the Data window. Expand the graph by dragging the lower right corner.
2. Measure the mass of the cart and the friction block and record it below. Place the friction block WOOD SIDE DOWN on the track. Place the cart upside down on top of the block. Place the requested mass on top of the cart for each set of trials. Press the ZERO button on the force sensor while it is in a horizontal position. Put the force probe in the loop of string on the block and click Start. Pull on the friction block very slowly and horizontally with the force probe. Pull the block at a steady, slow speed for about 2 seconds. Click Stop. Click the first icon on the left at the top of the graph. This should rescale the graph around your data. The graph should show a spike in force at the beginning and then a relatively steady force to keep it moving. Note the spike in force required to get the block moving. This is the static friction force, record its maximum value in the data table. Note the average force required to keep the block moving. The computer can calculate this for you if you select this part of your graph by dragging the cursor and then clicking the ∑ icon. The average or "mean" is displayed. This is the kinetic friction force, record it in the data table. Repeat for all added mass trials.
3. Calculate the total weight of the block for each trial and record it in the data table. Since the track is level, the Normal force is equal to the weight. Quit Data Studio and start Graphical Analysis (in Application Folder on Dock) and graph Static Friction Force (y axis) Vs The Normal Force (x axis). Record the slope below and then repeat for Kinetic Friction Force Vs Normal Force and record the slope below.
Mass of Cart = ______kg Mass of Friction Block = ______kg
Trial / Added Mass / Total Weight / Static Friction Force / Kinetic Friction Force# / (kg) / (N) / (N) / (N)
1 / 0.5
2 / 1.0
3 / 1.5
4 / 2.0
5 / 2.5
6 / 3.0
Slope of Static Friction Graph = ______Slope of Kinetic Friction Graph = ______
4. Using the equation for a straight line and knowing that there is no friction if there is no Normal Force, write an equation for the Kinetic Friction Force as a function of the Normal Force. The slope of this equation is called the coefficient of kinetic friction, µK. The symbol µ is pronounced "mew". Show your work below.
5. Use your equation for Kinetic Friction to predict the Kinetic Friction Force for an added mass of 5.0 kg. Show your work below.
Q1: In general, how does µs compare to µk?
Q2: How does the Force of Kinetic Friction change as the weight of the cart increases? How does µK change as the weight increases?
Q3: Tables listing the coefficient of friction for different materials (such as in your physics book and in Lab 29) rarely list more than two decimal places. Based on your experience with this lab, why do they do this?
Q4: Why are there no units for µ?
6. Using the equation for a straight line and knowing that there is no friction if there is no Normal Force, Write an equation for the Static Friction Force as a function of the Normal Force. The slope of this equation is called the coefficient of static friction, µs. Show your work below.
7. Using your equation from #6 and your physics skills to predict the angle at which the block and cart (with no added mass) would just start to slide (see diagram on next page). Show all of your work on the next page. Hints: Draw a free body diagram and label all the forces on the block. Solve for the forces in the Y direction. The Y acceleration is zero. This will give you an expression for the Normal Force. Solve for the forces in the X direction. When the block is not sliding or just about to, the acceleration is zero. The friction force (Ff) is derived from this equation. The maximum friction force is equal to µs N. If you get stuck see Sample Problem 6-1 posted on the Newton’s Laws page of my website and/or Friction on a Ramp video.
8. Perform the experiment three times using the block and cart without any added mass and record the data below. q can be determined by measuring the opposite side (height of bottom of track end) and the hypotenuse (length of track) when the block first starts to slide while slowly lifting the track.
Theta Slide 1 = _____ Theta Slide 2 = ______Theta Slide 3 =______
Average Theta Slide = ______
9. Take the average of your Theta Slide data and write it above. Compare it to your predicted value and calculate the percent error.
Q5: Why did we use µS instead of µk to calculate the friction force?
Q6: Is µs dependent on the angle of the surface? Justify your answer using your data.
Q7: What does the Y component of the weight equal?
Conclusion: Describe the relationship between the Normal Force and the Force of Kinetic Friction.