Inelastic collision kinetic energy lost?

[girl52]

inelastic collision...kinetic energy lost?

Homework Statement

a bullet with a mass of 6 g is fired through a 1.25kg block of wood on a frictionless surface. initial speed of the bullet is 896 m/s and speed of bullet after it passes through it is 435 m/s. what's the final velocity? how much KE is lost?

Homework Equations

ke lost=ke initial-ke final
ke lost=1/2mvf-1/2mvi

i got 11.2 m/s for the final velocity of the wood

The Attempt at a Solution

KEi=1/2(.006)(896^2) + 0
KEf=1/2(.006)(435^2) + 1/2(1.25)(11.225^2)

i got 2408.228 - 488.925

thats not negative..where did i go wrong??

 

[PhanthomJay]

Don't forget to square the velocity when calculating KE. But first try using conservation of momentum, and show your work, please.

 

[girl52]

edited

 

[PhanthomJay]

Homework Statement

a bullet with a mass of 6 g is fired through a 1.25kg block of wood on a frictionless surface. initial speed of the bullet is 896 m/s and speed of bullet after it passes through it is 435 m/s. what's the final velocity? how much KE is lost?

Homework Equations

ke lost=ke initial-ke final
ke lost=1/2mvf-1/2mvi

i got 11.2 m/s for the final velocity of the wood

how did you arrive at this number?

The Attempt at a Solution

KEi=1/2(.006)(896^2) + 0
KEf=1/2(.006)(435^2) + 1/2(1.25)(11.225^2)

i got 2408.228 - 488.925

thats not negative..where did i go wrong??

The change in energy (final minus initial) is negative...if you use (initial minus final), you get your signs reversed. Don't do it that way, the minus signs are bad enough without compounding them.

 

[rwisz]

I would like to clarify that inelastic collisions involve the transfer of kinetic energy from one object to another, resulting in a change in their velocities. In this case, the bullet's kinetic energy is transferred to the wood, causing it to move with a final velocity of 11.2 m/s. Therefore, the kinetic energy lost in this collision is equal to the initial kinetic energy of the bullet, which is 2408.228 J, minus the final kinetic energy of the bullet, which is 488.925 J. This results in a loss of 1919.303 J of kinetic energy.

There are a few potential reasons for your calculation not resulting in a negative value. One possibility is that you may have made a mistake in your calculation, such as using the wrong mass or velocity values. Another possibility is that your calculation is correct, but the negative sign may have been lost due to rounding or other errors. It is important to double check your calculations and make sure you are using the correct values.