# Homework Help: Kinetic Energy

1. Nov 17, 2009

### xshezsciencex

A 1kg block is at rest on a horizontal frictionless surface touching the end of an unstretched spring of spring constant 200N/m. Tjhe other end of the spring is fixed to a wall. A 2kg block traveling at a speed v(initial)=4m/s collides with the 1kg block, and the two blocks stick together.

I am not sure if momentum or kinetic energy is conserved in this collision, if it is please explain.
When the blocks momentarily stop, by what amount is the spring compressed?
I understand that when the spring expands it will accelerate the coupled blocks from rest so what is the speed of the blocks when they are 0.20m to the left of the spring's end?

2. Nov 17, 2009

### tiny-tim

Hi xshezsciencex!

Momentum is always conserved in collisions.

Energy is never conserved unless the question says so!

In this case, the question says that the collision is perfectly inelastic (whcih is just about the opposite of conservation of energy), in that the two final velocities are the same.

So use conservation of momentum, plus vf1 = vf2.

(After the collision, when the spring is being compressed, energy is conserved, and of course momentum isn't)

3. Nov 17, 2009

### Staff: Mentor

Perhaps putting "kinetic" at the beginning will make this phrase easier to swallow

4. Nov 18, 2009

### tiny-tim

Hi Borek!

"mechanical" energy, actually!

5. Nov 18, 2009

### Staff: Mentor

Good point. At least we agree that the statement was a little bit bold :tongue: