1. Not finding help here? Sign up for a free 30min 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!

General Physics 1

  1. Mar 1, 2005 #1
    General Physics 1....

    A crate of mass 30.0kg rests on level surface. The coefficient of kinetic friction between the crate and surface is .400 . What force applied at an angle of 30.0 degrees below the horizontal (i.e. pushing down) is required to keep the mass moving at constant speed?......................on this problem i came up with the equation Fn= Uk[mgcos(30)]......with Uk=.400......Fn=102N......as the answer......iam i doing this right.....need help......please.
     
  2. jcsd
  3. Mar 1, 2005 #2
    You made some erroneous assumtions. The normal force of the crate is not just [itex]mgcos\theta[/itex], if the angle is that of an applied force. This method assumes that the box is on an incline (in which case that would be true). This is a level surface, so the normal force is caused by the box's full weight (mg) and the force you apply ([itex]F_y=F\cos\theta[/itex]) by pushing or pulling.

    Force of friction: [itex]f=\mu N[/itex] with [itex]N=mg+F\sin30[/itex] and so [itex]f=\mu\left(mg + F\sin30\right)[/itex]

    To move at a constant speed, [itex]F\cos30[/itex] must equal the force of the friction trying to keep it from moving:
    [tex]F_{x net} = F\cos30 - F_{friction} = ma =0[/tex]

    That should help.

    PS, as an answer, I got close to [itex]180N[/itex] - I'll hold off on the exact answer I got until you've done the problem. :)
     
    Last edited: Mar 1, 2005
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: General Physics 1
  1. General physics (Replies: 1)

  2. General Physics 1 (Replies: 4)

  3. General physics (Replies: 2)

  4. General physics (Replies: 3)

  5. Physics 1 (Replies: 1)

Loading...