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: Drats, my brain hurts!

  1. Oct 21, 2005 #1
    I need some direction with two homework problems.

    1) A 215kg crate rests on a surface that is inclined above the horizontal at an angle of 26.1 degrees. A horizontal force, 528 N, is required to start the create moving down the incline. What is the coefficient of static friction between the crate and incline?

    I know that static friction = Frictional Force / Normal Force

    Is the normal force for the crate = 215 * 9.8 * sin 26.1

    Is the force that is required to move it down the ramp = 528/cos 26.1

    I'm getting .63429 for the static friction, but the answer is supposed to be 0.884

    So what am I missing?

    2) An arrow, starting from rest, leaves the bow with a speed of 26.4 m/s. If the average force exerted on the arrow by the bow was increased 3 times and the arrow was accelerated over the same distance, then with what speed would the arrow leave the bow?

    I'm thinking that I would use two equations that solve for x and set them equal to each other, but I don't think I have enough information.

    Thanks,

    Serena
     
  2. jcsd
  3. Oct 21, 2005 #2

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    Serena,

    For the first problem, the normal force will be reduced since the applied horizontal force has a component normal to the incline and must be accounted for.

    In the second problem, recognize that work = force X distance and that is what imparts energy (or speed) to the arrow.
     
  4. Oct 22, 2005 #3
    Thanks, it works now.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook