Inelastic collision problem: Bullet striking a wood block

[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.]

 

[DEvens]

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.

 

[jbriggs444]

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.