# A bullet going trough a disk

1. Jul 19, 2012

### EBBAzores

1. The problem statement, all variables and given/known data

A bullet with 5 g of mass and a velocity of 100m/s goes through a disk that was rested initially the disk is solid and as a radius r=20cm. It's 2cm thick and as a 2kg mass that can spin with no friction in an axis that goes through the center. The bullet goes trough the disk perpendiculary to the axis and in between its thickness at a distance d=10cm of its center with a final speed of 80m/s.

Calculate the kinetic energy of rotation of the disk

2. Jul 20, 2012

### Andrew Mason

What have you done so far? Hint: what can you say about the angular momentum (ie of the bullet before the collision and the bullet+disk after) relative to the disk axis?

AM

3. Jul 20, 2012

### EBBAzores

The angular momentum will variate it self so by saying that, there will be a torque due to the variation and with the torque we can calculate de work of the bullet and by that the kinetic energy of rotation

4. Jul 20, 2012

### HallsofIvy

Staff Emeritus
Why should the disk rotate at all? It appears that the only force on the is parallel to the axis.