Can someone explain the concept of Mass-Energy Equivalence in simple terms?

[skoks]

Homework Statement

My textbook is only confusing me further and I need to understand this for a presentation in front of the class! The chapter is entitled Mass-Energy Equivalence, with sub titles Relativistic Momentum and Relativistic Energy. I don't understand relativity, I'm reading the first part of the unit to understand it now.

Homework Equations

The Attempt at a Solution

 

[Eloc Jcg]

Mass-energy equivalence is the formula stating the relationship between energy & mass. Also known as E = mc^2. It states that the difference in Energy is equal to the difference in mass time time the speed of light squared. Wikipedia has a page on it if you search E=mc2.

 

[skoks]

I understand this but I don't understand how momentum can be relative... What is dilated mass?How can energy be relative aswell?

 

[HallsofIvy]

I understand this but I don't understand how momentum can be relative... What is dilated mass?How can energy be relative aswell?

momentum is "mass times speed". Since both mass and speed are "relative", how could momentum NOT be? Likewise, energy is "1/2 mass times speed squared" so it also is relative to the viewer.