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Physics 20: eLab Activity

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Coefficient of Friction

Background:

When surfaces rub together, a certain amount of friction is generated. The amount of friction is dependent upon the nature of the surfaces being rubbed together. The amount of friction between surfaces can be expressed with a coefficient - the greater the amount of friction, the higher the coefficient value.

In this lab, the coefficient of friction will be determined for a block sliding on an incline plane.

The Question:

What is the coefficient of friction for an object on an incline plane?

Variables:

This experiment involves these variables: surface type, ramp angle, block mass, velocity and coefficient of friction. Identify and state the manipulated, responding, and controlled variables in this investigation.

Materials:

  • motion sensor
  • rectangular block with felt glued to two sides
  • USB link
  • lab stand and support rod (books can also be used to raise the incline)
  • smooth metal ramp
  • ramp angle indicator
  • computer and DataStudio software

Procedure:

Step 1:

Connect the USB link (or equivalent) to your computer and then plug the motion sensor into the USB link. Make sure the motion sensor is set to "cart". The motion sensor should then be attached to the top of an inclined ramp. Incline the ramp using a lab stand and support rod, or just raise it by piling up books.

Step 2:

Record the mass of the rectangular block, then place it with the wide, wooden side of the block down on the incline.

Step 3:

Using trial and error, adjust the incline of the ramp so that the block just slides down the ramp at a constant velocity. It will be necessary to give the block a slight push in order to start it sliding down the ramp. If it does not slide, increase the angle. If it accelerates down the ramp, decrease the angle.

Step 4:

Once you think the block is sliding down with a constant velocity, confirm it by using the motion sensor. Configure the DataStudio software (or equivalent) to display a position-time graph. Press the START button in DataStudio and make the block begin sliding down the ramp. Click STOP in DataStudio just before the block reaches the end of the ramp. If the block produces a straight line in the position-time graph, the velocity is constant. If the line curves, the cart is accelerating, and the ramp should be adjusted accordingly to negate the acceleration. Use the motion sensor as many times as necessary to confirm constant velocity.

Step 5.

Once constant velocity has been confirmed, record the ramp angle in an appropriate data table.

Trial Angle (degrees)
Wooden side, wide
Wooden side, narrow
Felt side, wide
Felt side, narrow

Analyzing and Interpreting:

1. Draw four force diagrams representing the block as it slid down the incline for each of the four block orientations.
2. Calculate the coefficient of friction for each of the four block orientations.

Forming Conclusions:

3. Use your hands to feel the wood on the block on the wide and narrow sides. Can you detect a difference? Do your calculations show a significant difference between the coefficients of friction for the wide and narrow bare wood sides? Explain.
4. Use your hands to feel the felt on the block on the wide and narrow sides. Can you detect a difference? Do your calculations show a significant difference between the coefficients of friction for the wide and narrow felt-covered sides? Explain.
5. Does the coefficient of friction depend on the area of contact surface? Explain.

Applying and Connecting:

6. Precious little surface area connects a vehicle with the road, only the bottom of the tires touch and provide the friction that keeps the vehicle from sliding off the road.

What do you think is more important to consider when attempting to maximize the lateral friction that keeps the vehicle on the road - the width of the tire, or the design of the tread? Explain your answer.
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