Physics Question & Answer Key

A 5.00 x 10^ -2 kg mass with a charge of .75μC is hung by a thin insulated thread. A charge of -0.9μC is held .15m directly to the right so that the thread makes an angle Θ with the vertical. What is the angle Θ and the tension in the string?

Force between charges:

F = [(9e9)(0.75e-6)(0.9e-6)] / (0.15²) 

   = 0.27 N

Gravitational force:

mg = 5e-2 x 9.8 

      = 0.49 N

From a force vector diagram:

0.49N vertically down & 0.27N horizontal - -  combining to form θ from vertical) 

tan θ = 0.27 / 0.49 

θ = 28.9º

sin θ = 0.27 / T 

T =0.27 / (sin28.9) 

   = 0.56 N