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: Torque and Lever Arm

  1. Jul 25, 2005 #1
    A lunch tray is being held in one hand. The mass of the tray itself is 0.201 kg, and its center of gravity is located at its geometrical center. On the tray is a 1.33 kg plate of food and a 0.285 kg cup of coffee. Assume lever arm of the thumb is 0.0620 m, lever arm of the finger is 0.113 m, lever arm of the plate 0.231 m, lever arm of the coffee cup is 0.361 m and lever arm of the tray is 0.383 m. Obtain the force T exerted by the thumb. This force acts perpendicular to the tray, which is being held parallel to the ground.

    The picture that I have (but could not upload for some reason) had the thumb on top of the tray have a force T exerted in the -y direction when drawn with a force diagram and the finger had a +y force.

    I plugged in to a force table and then calculated a torque table, and I substituted with the two equations and got an answer of -133 Newtons, but I checked it and it was wrong.

    Any ideas on what I did wrong?
  2. jcsd
  3. Jul 25, 2005 #2
    torque is T=F * D

    just take the moment of all forces at one point and then use statics equations to find the rections
  4. Jul 25, 2005 #3


    User Avatar
    Homework Helper

    From which axis of rotation the arms are measured. Check axis of rotation you have taken and the directions of the moments of force
  5. Jul 28, 2005 #4
    thanks guys i appreciate it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook