CGP Physics workbook moments answer seems incorrect to me
The question:
Two men, one at each end, hold a 0.8m long metal pole weighing 130N so that it is in a horizontal position. One man accidentally lets go of his end.
What is the moment on the pole due to its weight an instant afte he lets go?
When I was doing I thought it seemed really easy, but anyway, my reasoning was the pivot would be the man at the other end (because no one is holding pole in middle) and so calculation would be:
moment = w1 x d1 = 13 x 0.8 = 104Nm
But answer says:
The weight of the pole acts at its centre of mass. The centre of mass of the pole is 0.4m from each end. So the weight of 130N acts 0.4 m from the pivot.
m = F x d = 130 x 0.4 = 53 Nm.
This does not intuitively seem logical. How is the pivot the centre of the pole in this case?
Two men, one at each end, hold a 0.8m long metal pole weighing 130N so that it is in a horizontal position. One man accidentally lets go of his end.
What is the moment on the pole due to its weight an instant afte he lets go?
When I was doing I thought it seemed really easy, but anyway, my reasoning was the pivot would be the man at the other end (because no one is holding pole in middle) and so calculation would be:
moment = w1 x d1 = 13 x 0.8 = 104Nm
But answer says:
The weight of the pole acts at its centre of mass. The centre of mass of the pole is 0.4m from each end. So the weight of 130N acts 0.4 m from the pivot.
m = F x d = 130 x 0.4 = 53 Nm.
This does not intuitively seem logical. How is the pivot the centre of the pole in this case?
The pivot isn't the centre of mass. The pivot is the man still holding it. The weight acts from the centre of mass, which, assuming the pole is uniform, lies in the middle.. which is 0.4m from the pivot.
Edit: Re-reading what you've posted, this is pretty much what they are telling you. I'm sorry, I'm not sure how to make it any clearer!
Edit2: Actually, draw a diagram if you haven't already.
Edit: Re-reading what you've posted, this is pretty much what they are telling you. I'm sorry, I'm not sure how to make it any clearer!
Edit2: Actually, draw a diagram if you haven't already.
(edited 10 years ago)
Original post by NJam
The pivot isn't the centre of mass. The pivot is the man still holding it. The weight acts from the centre of mass, which, assuming the pole is uniform, lies in the middle.. which is 0.4m from the pivot.
Edit: Re-reading what you've posted, this is pretty much what they are telling you. I'm sorry, I'm not sure how to make it any clearer!
Edit2: Actually, draw a diagram if you haven't already.
Edit: Re-reading what you've posted, this is pretty much what they are telling you. I'm sorry, I'm not sure how to make it any clearer!
Edit2: Actually, draw a diagram if you haven't already.
OK I think I have got it now. Thanks. I see what you mean.
Original post by acomber
OK I think I have got it now. Thanks. I see what you mean.
np, glad to have helped!
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