- #1

Notaphysicsmajor

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## Homework Statement

A 10kg block moving at 1 m/s hits another 10kg stationary block. Those two blocks (now stuck together) collide with another 10kg stationary block. What percent of the original energy is lost after the two collisions?

## Homework Equations

m1v1 + m2v2 = (m1+m2)v3

Kinetic energy = .5(m)(v)^2

## The Attempt at a Solution

Hello,

I'm not sure if I understand this problem correct so I hope someone here can help me, but here is how I solved it.

First collision:

(10)(1) + (10)(0) = (20)v

Velocity = .5 m/s

Second collision:

(20)(.5) + (10)(0) = (30)v

Velocity = .3

Initial energy (before any collision):

.5(10)(1)^2 = 5 J

Energy after first collision:

.5(20)(.5)^2 = 2.5 J

Energy after second collision:

.5(30)(.3)^2 = 1.6 J

1.6 J is amount of kinetic energy after both collisions, but problem asks what percent of the original energy was lost after first two collisions.

So 5 - 1.6 = 3.4 J energy lost after first two collisions.

3.4 / 5 = .68 or 68%. 68% of original energy was lost after first two collisions.

Is this correct?