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!

Homework Help: Power problem

  1. Dec 15, 2009 #1
    1. The problem statement, all variables and given/known data

    A box of mass M= 1 kg rests at the bottom ofa frictionless plane inclined at an angel theta= 20 degrees. The box is attached to a string that pulls with a constant tension T=5 N.

    What is he power in Watts being delivered by the tension T as the block reaches x=50 cm

    2. Relevant equations

    P= dW/dt

    3. The attempt at a solution

    w= (5N)(.5m)=2.5 J

    Then to find the change in time I used kinematics, and to get acceleration I used newton's second law.

    Net force= T-mgsin(theta)= 5N-(1kg)(9.81 m/s^2)sin(20)=1.644782394 N




    then I set the position equation equal to .5m to get t


    sqrt(1/a)=t =.7797327501 seconds

    the from here I just put W/t


    The correct answer is 6.41

    Could someone please help me get going in the right direction.

    Thank you.
  2. jcsd
  3. Dec 16, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Note that the distance traveled by the box, is not 0.5 m.
    The box is pulled along the inclined plane, so a horizontal distance of 0.5 m corresponds to a greater distance along the plane.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook