Calculating the Magnitude of Charge Q (in μC) for an Object

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To calculate the magnitude of charge Q for a 0.7 kg object balanced by an equal but opposite charge, the weight is determined to be 6.86 N using gravity. The force between the two charges is described by Coulomb's law, where the force is set equal to 6.86 N to solve for Q. The initial calculation yielded approximately 0.36443 μC, but the poster later realized a mistake in their approach. Guidance was sought from the forum, which was noted to be the wrong section for homework questions, prompting a reminder to use the appropriate forum. Ultimately, the poster found their own error and confirmed the correct method for solving the problem.
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The weight of a 0.7 kg object of charge Q is just balanced by another object of equal but opposite charge fixed to a support 132 cm above it. What is the magnitude of the charge Q (in μC)? [Note that 1 μC=1x10-6C.]

Ok I know how to do this but it says I'm wrong... am I really wrong or is the computer wrong?

.7kg times gravity = 6.86N

The force between these two objects would be coulombs law.

F= Ke (QQ/r^2) set force = to 6.86 and solve for Q

It gave me .36443 micro C.

What do you guys get?
 
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bump.. please just give me some guidance, I don't mean to impose I am just in a crunch
 
1. It's not realistic to expect a response in less than a half hour.

2. You still posted this in the wrong forum. :frown: We have a forum specifically for homework questions. This thread is going to be moved there, so don't worry about it, just remember to use that forum in the future for questions like this, OK? :smile:
 
Last edited:
Anybody can help me solve this one. I was using the same method as post 1 but don't know if it is correct.
Thanks in advance
 
never mind found my own mistake and btw this is the correct way to solve this problem
 
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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