EXPERIMENT: FRICTION INVESTIGATION
Here are your goals for this lesson:

Calculate coefficient of friction for the matchbox
Answer questions and summarize results

Friction is a force that resists motion. It occurs when a surface in motion rubs against another surface. Friction occurs parallel to the surfaces moving against one another and in the opposite direction of the motion.

Friction is caused by the interference of the atoms or molecules of each object sliding over each other. A rough surface will produce more friction than a smooth surface. No matter how smooth surfaces may appear, they are rough on a atomic level, and when they rub, friction always produces some amount of heat which is lost as unusable energy.

The ratio between the force of friction and the perpendicular force is called the coefficient of friction.

The coefficient of friction (the Greek letter mu) is:

µ K = FK ÷ Fn

The coefficient of kinetic friction permits a comparison of the kinetic friction of various materials. In this investigation, you will determine the coefficient of friction for the same mass under two different conditions.

Materials needed: small match box, pebbles, coins (pennies are best), string (about 24 inches long), small plastic bag, 8 or 10 round sticks (pencils, Tinker Toys, dowels), small scale or balance

Follow these steps.

1. Fill the match box with pebbles. Weigh the matchbox with the pebbles inside. Record that weight.

2. Tie the string to the box. Allow the string to hang over the edge of the table.

3. Tie the other end of the string to a corner of the plastic bag, leaving an opening to put in coins.

4. Add coins one by one until the box is pulled off the table.

5. Count and record the number of coins and the weight of the bag with the coins in it.

6. Lay the round sticks on the table about 1 inch apart and about 2 inches from the edge of the table.

7. Put the match box on the rollers farthest from the edge of the table.

8. Now add coins one by one to the bag until the box is pulled off the table.

9. Count and record the number of coins and the weight of the bag with the coins in it.

10. Repeat the experiment. Determine your margin of error if your results varied. For accuracy, repeat the experiment again if desired.

11. Using the equation for coefficient of friction in the text above, determine the coefficient of friction for the matchbox in each experiment. Include this data in your summary.

Summarize your findings in a short report of 150 words. Include your hypothesis, observations, data, and conclusions. Be sure to answer the questions below as well as including your own findings and thoughts in your summary.

How many coins did it take to pull the bag off the table without the rollers (pencils)? With the rollers?

Why didn't the box slide off the table before you put enough coins in the bag?

What happened differently when you put the rollers underneath the matchbox? Why?

What factors caused a margin for human error in this investigation? What was your margin of error when you repeated this experiment?

What was the coefficient of friction for the matchbox? Would the coefficient vary if you tied the string to a similar object of different mass? What about a different object with the same mass? Explain.

What did you learn from this investigation? Be thoughtful in your answer.