# Physics 30 electricity problem

Ok so im on the last 2 questions of my electricity and electric fields assignment and im totally stumped.
heres the first one
12. Object A with a mass of 1.4 X 10-3 kg has a charge of 3.8 X 10-10 C. This object is at rest a distance of 48 cm from a fixed object B of charge 4.2 X 10-5 C. The two objects will repel each other. With what speed will object A be moving at when it is 96 cm from object B?
(4 marks)
Ok so far this i figured that first i ahve to find the total energy at rest at 48cm and ET=EP because Ek=0 cuz its at rest. however i cant think of a formula for EP because for EP=mgh i dont have any hieght and for EP=1/2kx^2 i dont have k or x. then i know i find the EP at 96 cm and solve for v, but i cant figure out how to get there.

THe next question is sort of the same:
13. Two charged objects A and B have masses of 1.3 X 10-2 kg and 2.6 X 10-2 kg respectively. Their charges are -1.7 X 10-4 C and -3.8 X 10-4 C respectively. They are released from rest when they are 3.6 m apart. What will their speeds be when they are a "large'' distance apart? (6 marks)
For this one im totally confused because i dont know how to start. I mean i wrote down the variables but im not sure how to approach this question.

andrevdh
Homework Helper
Try the work done by the electrostatic force as a mass is moved form
$$48\ cm\rightarrow\ 96\ cm$$
is equal to the change in it's kinetic energy.

andrevdh
Homework Helper
In the second case all their potential energy gets converted into their kinetic energy.

try using the potential at the two points then finding out the work done and hence the kinetic energy
then find the velocity