Question

1. A. An oil drop has a mass m. The drop is falling at a terminal speed vfall. Draw a free body diagram for the drop. Write the Newton’s Second law equation for the drop. Find the expression for the value of k in terms of the mass of the drop (m), the terminal speed (vfall), and g.

B. The oil drop from C has a charge +q. It is now in a uniform electric field that points upward with strength E. The electric force is strong enough that the drop slowly moves upward. Because of air drag, the drop moves with a constant speed (vup). (Remember, the drag force points opposite to the direction of the velocity.) Because the air and the oil drop are the same as in C, the value of the air drag constant (k) is the same. Draw a free body diagram for the oil drop when it is moving up at its new terminal speed. Write a Newton’s Second Law equation for this situation. Substitute your previous expression for k into this equation and solve for the charge (q) on the drop. The expression should include m, vfall, vup, E (the electric field strength), and g.

Answer 1

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