Kinetic energy of particles and objects

Question

Calculate the rest energy and the kinetic energy of:

a) A 100g ball travelling with the speed of 100m/s
b) An electron travelling at a speed of 0.999c.

Which one has greater rest energy? Which one has greater Kinetic energy? Don’t forget you are dealing with relativistic effects in at least one case.

Answer 1

KE = 1/2 * m * v²

 

a) m = 100 g= .1 kg

    v = 100 m/s

 

KE = 1/2 (.1kg)(100 m/s)²

      =500 J

 

b)

electron m = 9.1 × 10^-31 kg

v = .999c = .999*3.0 x 10⁸ m/s =2.997 x 10⁸

 

KE = 1/2 (9.1 × 10^-31 kg)(2.997 x 10⁸)

      = 4.09 x 10^-14 J

 

Rest energy is determined by

E=mc²

 

completely dependent on mass so the ball.