# Inelastic collision kinetic energy lost?

• girl52
In summary, a bullet with a mass of 6 g is fired through a 1.25kg block of wood on a frictionless surface. The initial speed of the bullet is 896 m/s and the speed of the bullet after it passes through is 435 m/s. The final velocity of the wood is 11.2 m/s and the kinetic energy lost is 1919.3 J. However, using conservation of momentum, the final velocity of the wood can be calculated to be -11.2 m/s and the kinetic energy lost is -1919.3 J. The negative sign indicates that the wood is moving in the opposite direction of the bullet and that kinetic energy is lost in the collision.
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??

Last edited:

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

edited

girl52 said:

## 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.

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.

## 1. What is an inelastic collision?

An inelastic collision is a type of collision where kinetic energy is not conserved. This means that some of the kinetic energy of the colliding objects is lost and converted into other forms of energy, such as heat or sound.

## 2. How is kinetic energy lost in an inelastic collision?

In an inelastic collision, kinetic energy is lost due to the objects deforming or sticking together upon impact. This deformation or bonding causes a transfer of energy from kinetic to other forms of energy.

## 3. What factors influence the amount of kinetic energy lost in an inelastic collision?

The amount of kinetic energy lost in an inelastic collision depends on the elasticity of the objects involved and the amount of force applied during the collision. Objects with higher elasticity will experience less kinetic energy loss compared to objects with lower elasticity.

## 4. Can kinetic energy be gained in an inelastic collision?

No, kinetic energy cannot be gained in an inelastic collision. Kinetic energy can only be transferred or lost during a collision.

## 5. How is the loss of kinetic energy calculated in an inelastic collision?

The loss of kinetic energy in an inelastic collision can be calculated by subtracting the final kinetic energy of the objects from the initial kinetic energy. This will give the amount of kinetic energy that was lost during the collision.

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