EXPERIMENT: FRICTION INVESTIGATION OBJECTIVES
Calculate coefficient of friction for the matchbox.
Answer questions and summarize results.
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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 an atomic level, and when they rub, friction always produces some amount of heat which is lost as unusable energy.

In a previous lesson, you learned there are two main types of friction, static and kinetic. Static friction (Fs) stops an object from moving, while kinetic friction (Fk) acts to slow down an object while moving. For an object to begin moving the applied force must be greater than the object's force of static friction.

Another factor that comes into play when discussing friction is the coefficient of friction. The coefficient of friction is a ratio between friction force and normal force. The coefficient is dependent on the materials of the object versus the material of the surface. Interestingly, the coefficient of static friction for an object will be larger than the coefficient of its kinetic friction. Therefore, it takes more force to start something to move, than to keep it moving. The coefficient of friction (represented by the Greek letter mu), can be calculated by dividing the force of friction by the force of normal (Fn):

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In this investigation, you will determine the coefficient of static friction for the same mass under two different surface conditions, one smooth surface (the table surface) and a rough surface (placemat or towel). You will use the weight of the bag of coins to provide enough tension on the string to begin moving the matchbox, thus overcoming the static friction. Before you begin the experiment, hypothesize whether the smooth or rough surface will have a larger coefficient of static friction.

Materials needed:

a matchbox (or a small box)
pebbles
coins (pennies are best)
string (about 24 inches long)
small plastic bag
textured placemat or a towel
small scale or balance
img/s808_05.gifFollow these steps.

Fill the matchbox with pebbles. Weigh the matchbox with the pebbles inside. Record that weight in the data table. You will only have to do this once.
Tie the string to the matchbox. Allow the string to hang over the edge of the table. Place the matchbox at least 6 inches from the edge of the table.
Tie the other end of the string to a corner of the plastic bag, leaving an opening to put in coins.
Add coins one by one, until the box begins to move on the table.
Count and record the number of coins and the weight of the bag with the coins in it. Place the data in the table for smooth surface.
Empty the bag of coins, and repeat the experiment two more times to produce a total of three trials. Place all data in the table for smooth surface.
For the rough surface, use either a textured placemat or a towel on the table. You may need to use tape or a heavy object to keep the material from moving on the table. Once again place the matchbox at least 6 inches from the table's edge.
Now add coins one by one to the bag until the matchbox begins to move.
Count and record the number of coins and the weight of the bag with the coins in it. Place the data in the table for rough surface.
Empty the bag of coins and repeat the experiment two more times to produce a total of three trials.
Average the weight of coins using your data from the smooth surface. Divide this average by the weight of the matchbox. The answer is the coefficient of static friction for your matchbox on the smooth surface. Repeat this calculation for the rough surface.