- #26

gneill

Mentor

- 20,875

- 2,838

Okay, good so far!v_{1}= 4 * 10^{-4}m s^{-1}.

F_{1}= m g = 10^{-14}* 10 = 10^{-13}N.

v_{2}= 8 * 10^{-5}m s^{-1}.

F_{2}= ?

F_{1}= k v_{1}, where k = F_{1}/ v_{1}= 10^{-13}/ 4 * 10^{-4}= 2.5 * 10^{-10}.

So F_{2}= k v_{2}= 2.5 * 10^{-10}* 8 * 10^{-5}= 2 * 10^{-14}N.

Friction always works against the direction of motion. You've added the friction force to the "Upthrust" and electric force, which is not correct. You might find it helpful to write out the force balance so that it sums to zero. Since this mysterious "Upthrust" is always zero, just drop it from your equation:Weight = Upthrust + EF + AR_{Rising or F2}

10^{-13}N = zero + Q E + 2 * 10^{-14}N

Q E = 8 * 10^{-14}N

Q = 8 * 10^{-14}/ 125 000 = 6.4 * 10^{-19}Q.

Then 6.4 * 10^{-19}/ 1.6 * 10^{-19}= 4, not 6 as in the answer. What's wrong? I recalculated everything and checked the book answer, it's 6. So, don't think that it's the calculation.

##F_E - Weight - F_2 = 0##

Where ##F_E## is the electric force.