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2. The electron and positron each have a rest mass of 9.11 x 10^-31 kg. In a certain experiment, an electron and positron collide and vanish, leaving only electromagnetic radiation after the interaction. Each particle is moving at a speed of 0.20c relative to the laboratory before the collision. Determine the energy of the electromagnetic radiation.

First of all do i have to solve this question like a collision problem, if i do there is no after situation.

Anyways i think i have to solve for Ek.

So Et=Ek + Erest

Et=mc^2/sqrt(1-v^2/c^2)

Et=9.11 x 10^-31 x (3.00 x 10^8)^2/sqrt(0.96)

Et=8.37 x 10^-14

Erest = 8.2 x 10^-14

Ek= 8.37 x 10^-14 - 8.2 x 10^-14

Ek= 1.7 x 10^-15

But this answer isnt right. The books has 0.615 Mev

2. The Big Bang, which is a theory predicting the origin of the universe, is estimated to have released 1.00 x 10^68 J of energy. How many stars could half this energy create, assuming the average star's mass is 4.00 x 10^30 kg.

I did this but again my answer is different from the back of the book.

E=mc^2

m=1.00 x 10^68/(3.00 x 10^8)^2

=1.11 x 10^51

=1.11 x 10^51/4.00 x 10^30

=2.78 x 10^20 kg

The book's answer is 1.39 x 10^20 stars. What did i do wrong.

3. A supernova explosion of a star with a rest mass of 1.97 x 10^31 kg, produces 1.02 x 10^44 J of kinetic energy and radiation.

a) How many kilograms of mass are converted to energy in the explosion?

This one i have no idea how to do.

Please i really need help with all these questions i really have to get these done soon.