1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Average Coefficient of Kinetic Friction between Ice and Puck

  1. Jan 20, 2017 #1
    1. The problem: A hockey puck is hit on a frozen lake and starts moving with a speed of 11.6 m/s. Five seconds later, its speed is 7.4 m/s. What is its average acceleration? The acceleration of gravity is 9.8 m/s. Answer in units of m/s. What is the average value of the coefficient of kinetic friction between puck and ice?

    2. Relevant equations: Sum of the forces' x-components equals mass times acceleration. Sum of the forces' y-components equals zero. Kinetic friction equals the coefficient of kinetic friction times the normal force.

    3. The attempt at a solution: I solved for the acceleration earlier and got -0.84 m/s squared. I did this by taking the difference in velocities and dividing by the time (7.4 - 11.6 = - 4.2. - 4.2 / 5 = - 0.84). I then applied what I knew about the normal force: FN=mg. I also knew that FK=μkFN. Therefore, FK=μk(mg). While ∑Fx=ma, μk(mg)=ma. The masses cancel out. Then I put in my numbers: μk(9.8 m/s squared)= - 0.84 m/s squared. From there I divided my acceleration by 9.8 and got μk= - 0.0857. I checked it and the system on the website I was using for homework (UT) deemed my answer incorrect. Can anyone help?
     
  2. jcsd
  3. Jan 20, 2017 #2
    You can't have a negative coefficient of friction.

    P.S. Welcome to Physics Forums.
     
  4. Jan 21, 2017 #3
    Your work is correct but you have forgotten to take into account directions. Part (i) has the correct answer, as for part (ii), as @TomHart mentioned, you can't have a negative coefficient of friction. What do you think your mistake is then, and why?
     
  5. Jan 22, 2017 #4
    I had realized that shortly after posting the thread. Thank you for the help though! I really appreciate it!! :biggrin::doh:
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted