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kaspis245
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Homework Statement
Small mass ##m## ball has a negative charge ##q## and is hanging on an inelastic string which has a length of ##l##. What is the smallest velocity that one need to impart on this ball for it to make one revolution? There is also a uniform magnetic field ##B## as shown in the drawing.
Homework Equations
##F=Bqv##
##F_c=\frac{mv^2}{r}##
The Attempt at a Solution
We need to find ##v_o##
Conservation of energy:
(1) ##\frac{mv_o^2}{2}=2mgl+\frac{mv^2}{2}##
The force ##F=Bqv## always points into the center of the circle. When the ball reaches the top of the circle, it will be affected by two forces ##F=Bqv## and ##mg##. Both point downwards, hence the sum of those forces must be the centripetal force.
(2) ##Bqv+mg=\frac{mv^2}{l}##
Now I can express v from this equation and place it into (1). Is that correct?