Find the coefficent of kinetic friction

  • #1

Homework Statement



This is for a lab, I need help finding the coefficient of kinetic friction since when I solve I get a negative answer. In the lab we have a block of wood that has a downward weight of 2.12 N. It is on a ramp set at 20 degrees and we pulled it UP THE RAMP. Using the laptop and a sensor it recorded the applied force at 1.2 newtons.



Homework Equations



coefficient of friction= force of friction/ normal force



The Attempt at a Solution



So i set the weight into components and found the one going parallel to the ramp to be .73 Newtons and the normal force to be 2. To find the force of friction I did .73- 1.2 = -.47. Then I divided that by the normal force and got -.235. Obviously that does not make sense. Usually im used to problems in which the applied force is down the ramp. So now it might be different since its going up ( and i dont know how to solve it that way). Please help, thanks
 

Answers and Replies

  • #2
A negative sign seems to make sense to me. It depends on your choice of a coordinate system.

In this case, you have chosen the positive direction to be up (+y). Since friction opposes motion, it is going in the opposite direction (-y).
 
  • #3
oh wow, that makes sense. Since the applied force is in the positive direction I can do Positive 1.2 minus friction which is .73 (since its in the opposite direction) and get positive .47. I can divide that by 2 and get a positive answer

Or i can do -.73 +1.2 and get the same answer
 

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