Friction - What a Drag

John J. Miller                 Maine East High School
1743 E. Forest Ave             2601 W. Dempster
Des Plaines IL 60018           Park Ridge IL 60068
(708) 297-6368                 (708) 825-4484

Objectives:

At the end of this lab students will be able to:
        1) recognize that weight and surface type affect friction.
        2) recognize that surface area does NOT affect the friction.
        3) control variables
        4) Recognize that some things are hard to measure like friction 
           because the spring scale needle vibrates.

Materials Needed:

4 wood blocks for each lab group (approximately 2 inches x 3 inches x 6 inches 
is a good size because then the ratio of areas of the different sides is simple- 
6 sq. inches by 12 sq. inches by 24 sq. inches). 

Small screw hooks that can be screwed into the blocks to hook the blocks 
together.  

1 spring scale for each group (if spring scales are not available you may 
substitute a rubber band and note the amount the rubber band stretches). 

Different surfaces like a table, carpet, glass, etc.

Strategy:

Ask the students what factors they think affect the size of the frictional 
force.  Give the students the equipment and let them try various combinations.  
At this point DON'T TELL THE STUDENTS WHAT COMBINATIONS TO TRY.  Let them 
explore combinations such as a different sides, different surfaces areas, a 
train (one hooked after the other), stacking on top, or combinations thereof.  
Regroup the students together as a whole class after approximately 15 minutes of 
experimentation to discuss preliminary results.  At this point you can remind 
students to control variables, remind them that they should not pull the spring 
scale at an angle and that the different sides of the block might have a 
different grain which can affect results.  Let the students go back into their 
groups so that they can fine tune their results.  Have one representative from 
each group make a brief, final presentation of their results. 

Performance Assessment:

Each group can be assessed informally by the lab work they do.  I award extra 
points for each clever idea I see.  The presentation can also be used to 
determine that they have the correct concepts.  Also good questions to ask are: 

1. What happens if I double the weight by stacking one block on top of the 
other?  Answer: The frictional force doubles. 

2. What happens if I keep the weight the same but double the surface area?  
Answer: The frictional force stays the same. 

3. What happens if I double the surface area and double the weight?  Answer: The 
frictional force is doubled (the increase in weight doubles the force and the 
surface area has no effect). 

4. How does the surface type affect the frictional force?  Answer: The answers 
will vary.  Typically the smoother the surface is the less friction.  However, 
sometimes glass which is very smooth will produce a large frictional force, 
specifically if it is very clean.  FYI: There is a weak vacuum that is formed 
that pulls the blocks together when there is little or no air between the 
surfaces. 

Conclusions:

The frictional force is affected by the surface type and weight.  It is not 
affected by the surface area. 

Evaluation:

See performance assessment.

References:

1. Almost any general physics book will give you information.

2. Me- Give me a call.