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Question of Force - why it differs in reality?

  1. Dec 31, 2014 #1
    1. The problem statement, all variables and given/known data
    pic.png

    2. Relevant equations


    3. The attempt at a solution
    could you give me the reason for part i. Can't figure the reason
    for the second part. it says ans is 1080N to left. shouldn't it be 900N. and why is it to the left?
     
  2. jcsd
  3. Dec 31, 2014 #2

    CWatters

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    What's the difference between an ideal winch and a real winch?
     
  4. Dec 31, 2014 #3

    CWatters

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    The tension T is 900N but that's not the only force on the drum/axle.

    The tension and one other force act to the right. If the drum/axle doesn't accelerate to the right there must be another force acting to the left so that the sum of all forces on the drum/axle is zero.
     
  5. Dec 31, 2014 #4
    is work done as friction. what exactly would be the wasted energy here?
    if it's the reaction of 180N, won't it be on the axle itself, but how does the trailer exert this force. the axle is in the air?
     
  6. Dec 31, 2014 #5
    another thing. by calculation, i went as follows, Tx0.08 = 180x0.4 giving T = 900N
    why it don't understand why it's this way. isn't there actually a resultant moment there. so, why should we equate the moments. if it was force, then I understand from newton's 3rd law, its a reaction. but why is it this way here??? i only got the numerical value without understanding what's really happening and why? could you explain plz.
     
  7. Dec 31, 2014 #6

    NascentOxygen

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    The vertical supports holding the axle are part of the trailer. If the axle is to remain in place, there must be a horizontal force to the left balancing the sum of horizontal forces to the right.
     
  8. Dec 31, 2014 #7

    NascentOxygen

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    That's the tension in the rope.
    The winch's handle length vs. drum radius offer a mechanical advantage. A narrower drum means the handle will be easier for you to turn, but you'll have to turn it more times to draw in the same length of rope.
     
  9. Dec 31, 2014 #8
    yeah, but why equate the moment. shoudl there be a resultant moment, for the drum to rotate
     
  10. Dec 31, 2014 #9

    CWatters

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    Nothing to do with energy... The axle is subject to three torques that sum to zero if it's rotating at constant (angular) velocity. The input torque, the output torque and the torque due to friction.

    No. If you assume the drum is revolving at constant (angular) velocity then it's not accelerating and the torques must sum to zero.

    Via the bearing.

    If it's not accelerating the linear forces sum to zero.
     
    Last edited: Dec 31, 2014
  11. Dec 31, 2014 #10

    NascentOxygen

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    If the boat is being drawn in at a steady speed, you can consider that an equilibrium, so sum of forces is zero.
     
  12. Dec 31, 2014 #11

    CWatters

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    Might help if you can see the similarity between...

    Force = mass * acceleration
    and
    Torque = moment of inertia * angular acceleration
     
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