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!

Static problem

  1. Nov 8, 2006 #1
    Imagine that a ladder leans against the house at an angle of 20o with respect to the vertical. A person with a weight of 650 N stands on the ladder at a point 1.0 m from its bottom. What are the magnitude and direction of the torque that this person exerts on the ladder around the point where the ladder touches the ground?

    Where do I start?
     
  2. jcsd
  3. Nov 8, 2006 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Start by drawing a sketch, showing the angle the ladder makes with the wall, then look up the definition of torque. What do you get?
     
  4. Nov 8, 2006 #3
    Well torque will be T=rFsinx

    x = 20 degrees
    r = 1m
    F=650

    so T=1(650)sin(20) = 222.3?
     
  5. Nov 8, 2006 #4
    Is the direction towards me then? If I am looking at the house, the ladder is on the right of the house.
     
  6. Nov 8, 2006 #5
    From using the right-hand rule, I determined that the torque will be counter-clockwise towards me...but I usually mess up the right hand rule somehow...
     
  7. Nov 8, 2006 #6

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Magnitude is correct, don't forget units!(joules).
    Direction is also correct, using right hand rule, curl your fingers of your right hand in the direction of the force, with your thumb in the direction of the axis about which the torque is taken. Your thumb will point in the direction of the torque, which in this case is out of the plane towrd you along the z axis. The torque is counterclockwise. For practice, try the same problem, but with the ladder on the left side of the house. What do you get for a magnitude and direction of the torque in this case?
     
  8. Nov 8, 2006 #7
    well, the torque will be the same but shouldn't it be negative so -222.3J and the direction would be towards me again but clockwise?
     
  9. Nov 8, 2006 #8

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What are you afraid of straining your wrist? The direction is clockwise, but AWAY from you this time. You must get that thumb passing through the point about which the torque is taken. That negative sign indicates that the direction is opposite the direction in the first case when the ladder was on the other side. Usually clockwise moments are considered positive, and counter clockwise moments are negative, but it doesn't generally matter too much as long as you are consistent. Try it again, and strain your wrist and you'll get it!
     
  10. Nov 8, 2006 #9
    haha people are looking at me like i'm trying to hitch hike in my room...but i see now why it is pointing away from me.
     
  11. Nov 8, 2006 #10

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Ya, thanks a lot, now i need an Ace bandage.
     
  12. Nov 8, 2006 #11
    im sorrrry!! :(
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Static problem
  1. Statics Problem (Replies: 13)

  2. Statics Problem (Replies: 1)

  3. Statics problem (Replies: 15)

  4. Statics table problem (Replies: 5)

Loading...