Finding the Force Exerted on a Log Using Virtual Work Calculations

AI Thread Summary
The discussion revolves around calculating the force exerted on a log using virtual work principles, specifically in the context of a fireplace tong. Participants express confusion about how the equations from the virtual work chapter relate to the problem, particularly the application of moments and angles involved in the force calculations. The conversation highlights the need to consider the moments induced by forces at various points and the relationship between applied forces and their moment arms. One participant arrives at a numerical solution but still struggles to fully grasp the underlying concepts. Overall, the thread emphasizes the complexity of applying virtual work to practical scenarios involving levers and moments.
balogun
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Virtual work (fireplace thong)

Homework Statement


http://s2.photobucket.com/albums/y31/bambidurojay/?action=view&current=fireplacetong.jpg

I have been given a fireplace thong and need to determine the force exerted on a log.


Homework Equations


It is in the virtual work chapter of my textbook but I cannot see how the equations they have given me relate to the question.


The Attempt at a Solution


see above
 
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Think of it as a series of levers ( like individual see-saw or teeter-totter )

ps I think you mean 'tong', fireplace thong sounds very painful.
 
Still can't see how the equation U=intg[F costeta ds] or U=int[M dteta] apply to situation.
help appreciated please.
 
One has to work with moments (applied force and moment arm). F is applied about A and induces a moment (force) at C. C then applies a moment about D at N.

There are angles involved, since F is applied at some angle with respect to the moment arm (FA). And the force at C (from the moment with respect to AC) acts at an angle with respect to the moment arm CD.

Statically the moments induced by FB must be equal and opposite FA.
 
Didnt really understand your explanation but i was just playing around with the numbers and seem to have got the anwser

24/17 x17/17 x 17/6=N=4F

thats the right anwser but stiil don't understand it.
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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