Prof. Lippert wants to levitate a frog of mass 0. 1 kg. The frog is sitting on a

Question

Prof. Lippert wants to levitate a frog of mass 0. 1 kg. The frog is sitting on a flat, horizontal table, and Prof. Lippert arranges a positively charged plate 0. 5 m directly above the frog, parallel to the table, which generates a constant vertical electric field of 20 N/C pointing straight downward. The frog is then given an electrical charge. (a) What is the charge needed in order for the frog to begin rising up off the table? (b) For each of the three forces acting on the frog, the Coulomb force from the charged plate, the gravitational force from the earth, and the normal force from the table, describe the reaction force which is a consequence of Newton's Third Law. In each case, explain on what object and in which direction the reaction force acts. (c) Suppose the frog is now 0. 3 m above the table and has a charge of-0. 07 C. What is the frog's acceleration? Does it rise or fall?

Explanation / Answer

a) Coulomb force = qE

Force needed to rise the frog

qE = mg

q = mg/E = 0.1*9.8 / 20 = - 0.049 C

Negative sign used since plate is positively charged

b) Coulomb force acting upward

Gravitational force in downward direction

Normal force is acting upward

c) Coulomb force = qE = 0.07*20 = 1.4 N in downward direction

Weight = 0.1*9.8 = 0.98 N in downward direction

Hence total force = 2.38 N i downward direction

Hence frog will fall

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