Homework Help: Really Physics

Posted by Bob on Sunday, March 13, 2011 at 6:45pm.

A 2.4 Kg object moving to the right at 0.8c collides with a 3.2 kg object moving to left at 0.6c. They stick
together. Find the mass and velocity of the resulting combined object by;
a) assuming that you have no knowledge of relativistic physics,
b) using the special theory of relativity.
c) Compute the chance in the kinetic energy, what does it correspond to?

Really Physics - Ryan, Saturday, April 9, 2011 at 2:11pm
Well, we knoew that M1 = 2.4kg, and m2 = 3.2kg. We can then use the convservation of momentum law to find the resulting velocity and momentum after the collision. Since they stick together, we can add their masses.

m1v1 + m2v2 = (m1+m2)v
so
(2.4)(.8) + (3.2)(.6) = (2.4+3.2)v
1.92 + (-1.92) = 5.6v
0 = 5.6v
v = 0
Therefore, the carts stick together and stop.

CHANGE IN KINETIC ENERGY:
EK2 - EK1
1/2mv^2 - 1/2mv^2
1/2(2.4)(.8)^2 - 1/2(3.2)(.6)^2
.768 - .576
= .192J

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