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22 May 2013

The friction force is NOT the "force of friction on the object."

Given today on a review quiz:  

A 0.5 kg puck sliding on a horizontal shuffleboard court is slowed to rest by a friction force of 1.2 N.  

(a) On the dot below, draw a free body diagram of the puck.

(b) For each force, indicate the objects applying and experiencing the force.

(c) Determine the amount of the normal force on the puck.

I've answered part (a) in the picture.  Only three of my forty students got this wrong: one forgot the friction force, two forgot the normal force.  To their immense credit, not a soul put some bull honkey such as "force of motion."

Also to their immense credit, not a soul misidentified the object applying the force marked "weight."  Every one of the class listed weight as the "force of earth on the puck."  And, everyone got the normal force right: it's the force of the shuffleboard court (or of the "surface") on the puck.  So I'm proud of my class for avoiding two of the more common problems introductory students would have with this question.

So why did at least a third of the class say "force of friction on the puck?"  No, no, no, "friction" isn't an object!  Only objects can apply forces!  The correct statement is that Ff is the "force of the shuffleboard court on the puck."  

I expect this mistake (and many, many others) in the winter when we first cover forces and free-body diagrams.  Anyone know why I managed to teach about weight and normal force successfully, but not friction?  :-)


  1. Possibly just a problem with prepositions. The phrase "force due to friction" would be correct, though not highlighting the object applying the force the way you want.

  2. I have never in the past had students write a "key" for free-body diagrams, as you describe, but I'm going to start it next school year with my students. To avoid the problem you described above, I'm going to have the students write each one in the form "____ force of the ____ on the ____." The first blank will be used to describe the type of force, e.g. gravitational, frictional, normal, etc., and the remaining two blanks will be used as you described. Hopefully when a student writes "frictional" in the first blank, they won't be inclined to use "friction" in the second blank.