# Inelastic collision problem: Bullet striking a wood block

• naji0044
In summary, the conversation discusses the final part of an assignment involving finding the total work done by a block and a bullet during a collision. The mass, velocity, and kinetic energy of the bullet and block are considered, with the final speed and distance calculated. The formulas used for calculating work done on the bullet and block are also mentioned, with a reminder that work is not the same as the change in kinetic energy.
naji0044
Hey everyone

I'm struggling on the last part of this assignment. I need to find the total work done by the block and the bullet, when the collision happens. The informations is:

mblock=0.3 kg
mbullet=0.01 kg
vg=700 m/s
Height=0.72m

The final speed after the collision is vf=22.6 m/s and the distance X=8.66 which i have calculated.
My thought on the work done on the bullet must be kinetic energy that is impressed as K=1/2*m*vg^2
The work done on the block is probably using (k-k0)+(U-U0)=0, where we have potentiel energy. Picture of the situation is showed in the picture down below. Would appreciate if you could tell me which kind of formula you are using and why

[Moderator's note: Moved from a technical forum and thus no template.]

Last edited by a moderator:
The wording may be killing you. Unfortunately I cannot read German. Also, I did not check your calculation of the final speed.

Consider the bullet. You have got a mass,a velocity before, and a velocity after. The kinetic energy therefore changes by an amount you can calculate. The work done on the bullet is just the change in kinetic energy.

Also consider the same things for the block. You have a change in kinetic energy you can calculate. And you can then get the work done on the block.

After that you can look at the total kinetic energy before and the total after. For an inelastic collision, the after value is less. The difference is the energy lost to things such as deforming the bullet or block, heating the two objects, sound, etc. From a mechanics point of view this is not work. Work is "f dot D" that is, force times distance it acts. It's not the thermodynamics idea of work.

DEvens said:
From a mechanics point of view this is not work. Work is "f dot D" that is, force times distance it acts.
Your calculations of work done on an object based on the change in bulk kinetic energy of the object indicate that you are talking about "center of mass" work. Center of mass work is not force times distance it acts. Center of mass work is force times distance the center of mass moves while it acts.

## What is an inelastic collision?

An inelastic collision is a type of collision in which kinetic energy is not conserved. This means that the total amount of kinetic energy before the collision is not equal to the total amount of kinetic energy after the collision.

## What happens during an inelastic collision?

During an inelastic collision, the objects involved stick together and move as one mass. This is in contrast to an elastic collision where the objects bounce off each other and move separately.

## How is momentum conserved in an inelastic collision?

Momentum is conserved in an inelastic collision because the total momentum of the system remains constant before and after the collision. This means that the total mass and velocity of the objects involved must be taken into account in order to calculate the momentum before and after the collision.

## What factors affect the outcome of an inelastic collision?

The outcome of an inelastic collision can be affected by factors such as the mass and velocity of the objects involved, the angle and direction of the collision, and the type of material the objects are made of.

## How is the coefficient of restitution related to inelastic collisions?

The coefficient of restitution is a measure of the elasticity of a collision. In an inelastic collision, the coefficient of restitution is less than 1, indicating that the objects involved do not bounce off each other. Instead, they stick together and move as one mass.

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