Talk About a Drag
Purpose: The student will determine the coefficients of static and kinetic friction for four
different situations.
Materials:
Spring scale (5N, Green)
Spring scale (20N, Red)
Dynamics Cart
Wood block
Weight bar
Procedure:
1. Make a data table that has 8 columns and 12 rows. The columns will be labeled in pairs
as follows: Wood, Felt, Metal/Plastic, and Weighted Metal/Plastic. Under each
surface, the two columns should be labeled S, for Static, and K, for Kinetic.
2. The rows will be labeled as follows: Trial 1, Trial 2 and so on, up to Trial 10, Average,
Mass, Weight, Friction Force, and Coefficient of Friction.
3. Use the balance to find the mass of the cart, calculate its weight, and record this in the
chart. Also find the weight of the friction blocks, and the weight of the black metal
weight, and record these in the chart.
4. Invert the cart so that the wheels are up. Use the spring scale to pull the empty cart
across the table. Keep the spring scale horizontal as you pull. Record the static and
kinetic friction for 10 trials. This is the value for Metal/Plastic.
5. Repeat step 4, except place the black weight on top of the cart before you pull it.
6. Place the friction block in the cart so that the wood side of the block is directly on the
table, and the inverted cart is on top of the block. Use the spring scale to pull the cart
and block along the table. Keep the spring scale horizontal as you pull. Record the
static and kinetic friction for 10 trials.
7. Turn the friction block so that the black felt side is against the table. Repeat step 6.
8. Clean up all materials, making sure that everything is returned to its proper place in the
lab.
Calculations:
1. Find the average for the 10 trials in each column.
2. Draw a free-body diagram and calculate the coefficient of friction (µ) for both Static and
Kinetic friction for each surface. Remember that the weights will change once the
blocks are added to the cart.
Questions:
1. What differences did adding the weight make to the pulling force required to move the
cart?
2. What difference did adding the weight make to the coefficients of static and kinetic
friction for the cart? Explain your answer.
3. Why is the coefficient of static friction greater than the coefficient of kinetic friction?
4. As long as you pull at a constant velocity, does the velocity with which you pull effect
the coefficient of friction? Explain your answer.