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Mechanics 2 problem

Hi, I'm revising mechanics 2, OCR, I'm on the June 07 paper question 8. It looks like this:


I'm on part ii) it says use moments about O. I thought you just did force x distance (parallel) for moments. The mark scheme uses the distances 0.8 and 1.2 (in red), but I thought you would use the vertical disdances (in green).

Can anyone clarify? ;(
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dknt
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Original post by jamie092
Hi, I'm revising mechanics 2, OCR, I'm on the June 07 paper question 8. It looks like this:


I'm on part ii) it says use moments about O. I thought you just did force x distance (parallel) for moments. The mark scheme uses the distances 0.8 and 1.2 (in red), but I thought you would use the vertical disdances (in green).

Can anyone clarify? ;(


Think about what's happening. You only have 2 forces that concern you. The tension and the friction. The weight isn't needed as that acts through the CofM and the Reaction acts through the CofM (tangent to the circle). Now because point O is essentially a point, not a face or side, both the friction and the tension are perpendicular forces to the point O and so you just equate their moments. The tension force acts 1.2m away from O, and since the friction acts at a tangent to the hemispherical surface, the friction acts a distance 0.8m from point O.
Reply 2
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jamie092
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Original post by dknt
Think about what's happening. You only have 2 forces that concern you. The tension and the friction. The weight isn't needed as that acts through the CofM and the Reaction acts through the CofM (tangent to the circle). Now because point O is essentially a point, not a face or side, both the friction and the tension are perpendicular forces to the point O and so you just equate their moments. The tension force acts 1.2m away from O, and since the friction acts at a tangent to the hemispherical surface, the friction acts a distance 0.8m from point O.


Well in that case doesn't the tension act at the corner? or does it act everywhere along that top line? and then does the friction only act at the singular point where the hemisphere meets the surface? =P
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jamie092
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Oh I see, thanks a lot :wink:
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dknt
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Original post by jamie092
Well in that case doesn't the tension act at the corner? or does it act everywhere along that top line? and then does the friction only act at the singular point where the hemisphere meets the surface? =P


We can essentially extend forces along their line of action, since they act along that line.
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jamie092
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Original post by dknt
We can essentially extend forces along their line of action, since they act along that line.


That sounds logical I suppose ;P

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