College Physics Lab
PH 141

Force of Friction

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Purpose:
To gain a better understanding of friction by exploring how friction is affected by other physical variables.
Although we are all familiar with the concept of friction, we may know very little about it. For example, can we predict how big friction will be in any given situation?  In order to be able to predict the effect of friction we need to have a scientific model. A scientific model is a set of rules that apply to a system which enables us to predict the behavior of that system under various circumstances. Some scientific models attempt to describe the behavior of systems in terms of our understanding of the nature of matter. Other scientific models just express observed patterns in the properties of systems.
In this laboratory we will determine the properties which affect friction.

Discussion:
    Consider an block sitting on a table (see diagram below). Since you observe that the block is not accelerating, you can conclude that it is in static equilibrium. In order to explain this condition of static equilibrium, you must assert that the sum of the forces acting on the block add up to zero (application of Newton's Second Law).

    Now suppose you attach a string to the side of the block and attempt to pull the block along the surface of the table by slowly increasing the strength with which you pull the string. What happens? At first there is not motion. But as you continue to increase your pull, the block finally slips and slides in the direction of your force.  How do you describe this situation in physics terms?  Initially, when you apply your sideways force to the block, nothing happens. That is, you observe that the block is still in static equilibrium (a = 0). But how can that be? You know that there is a force on the block in your direction, yet the block doesn't accelerate toward you.  Since you do not observe any acceleration toward you, the total force on the block toward you must be zero.  But to accomplish that there must be another force on the block, in a direction opposing your force and equal in size to your force.  That force (that phenomenon) is what we mean by friction.
    What are the properties of the situation which affect the force of friction?

Questions:
    What is the relationship between the maximum static friction force and the normal force?
    What is the influence of the type of material on the maximum static friction force?

Procedure:

1. Place the wooden sled on the surface material of your choice (varnished wood, aluminum, rubber, glass, Plexiglas).
2. Place the string from the sled over the pulley.  Adjust the pulley so that the string is level with the table.
3. For several different loads (M) on the sled determine the mass (m) that must be hanging from the other end of the string in order to just get the sled sliding.  Record this information.  [For purposes of analysis it may be useful to determine the mass (mmax) which always makes the sled (with a specified load) slide and the mass (mmin) which never makes the sled slide. These masses represent the upper and lower limits for the mass (m) that you are trying to determine.  The mass that you are looking for lies in the range mmin - mmax.]
4. Compute the normal force and the maximum static friction force from your measurements.
5. Plot maximum static friction force vs normal force.
6. Determine a "best fit" line for your data.  What is the slope of this line?  The slope should correspond to the coefficient of friction for those surfaces.
7. Repeat this procedure for two other surfaces.
8. How do the coefficients of friction compare for the different surface combinations that you investigated?

Question:
What is the relationship between the maximum static friction force and the area of contact between surfaces?

Procedure:
1. Measure the area of the bottom of the sled and of the side of the sled.
2. Place the wooden sled on the table.
3. Place the string from the sled over the pulley.  Adjust the pulley so that the string is level with the table.
4. For three different loads (M) on the sled determine the mass (m) that must be hanging from the other end of the string in order to just get the sled sliding.  Record this information. [For purposes of analysis it may be useful to determine the mass (mmax) which always makes the sled (with a specified load) slide and the mass (mmin) which never makes the sled slide. These masses represent the upper and lower limits for the mass (m) that you are trying to determine. The mass that you are looking for lies in the range mmin - mmax.]
5. Compute the normal force and the maximum static friction force from your measurements.
6. Plot maximum static friction force vs normal force.
7. Place the sled on its side. Repeat the above procedure.

Question:
What is the influence on the maximum static friction force due to placing wheels under the object?

Procedure:
1. Place 3 to 4 plastic rollers on the surface of your choice (varnished wood, aluminum, rubber, glass, Plexiglas).
2. Place the wooden sled on the rollers.
3. Place the string from the sled over the pulley.  Adjust the pulley so that the string is level with the table.
4. For several different loads (M) on the sled determine the mass (m) that must be hanging from the other end of the string in order to just get the sled sliding.  Record this information.  [For purposes of analysis it may be useful to determine the mass (mmax) which always makes the sled (with a specified load) slide and the mass (mmin) which never makes the sled slide.  These masses represent the upper and lower limits for the mass (m) that you are trying to determine.  The mass that you are looking for lies in the range mmin - mmax.]
5. Compute the normal force and the maximum static friction force from your measurements.
6. Plot maximum static friction force vs normal force.
7. Determine a "best fit" line for your data.  What is the slope of this line?  The slope should correspond to the coefficient of rolling friction for those surfaces.
7. How does the coefficient of rolling friction compare to the coefficient of sliding friction for the same surfaces?