# physics

posted by
**Damon** on
.

i need some help wih this, thanks!!

Blocks A and B are moving toward each other along the x axis. A has a mass of 2.0 kg and a velocity of 50 m/s ( in the positive x direction ), while B has a mass of 4.0 kg and a velocity of -25 m/s ( in the negative x direction). They suffer an elastic collision and move off along the x axis. After the collision the velocities of A and B, respectively are:

a -50 and 25 m/s

b 50 and -25 m/s

c -25 and 50m/s

d 25 and -50 m/s

e -25 and -50 m/s

Momentum is conserved:

2*50+4*(-25)=0 so the final momentum has to be zero. Which also means they are going in opposite directions, which means that A has a neg v eloctiy, and B a positive velocity. Answers A, c fit that. Now, a is zero momentum. And c is zero momentum.

Conservation of energy:

Initial energy: 1/2 2*50^2 + 1/2 4*25^2 equals something. You can see that answer a is the only answer possible.

thankyou