Ok, you now have the velocity of both, so figure KE of both combined. Now compare that to initial KE of the bullet, and the difference is work in the board.
the depth is due to bullet speed being great than board speed, when they are equal, it stops penetration.
Massbullet*timepenetrating=mbullet*changevelocity+massboard*changevelocityboard.
now figure time penetrating. Now figure average velocity of the board/bullet combination (vf/2).
depth=time/avgvelocity
A bullet of mass 20gm penetrates a board of thickness 10cm and mass 1kg at rest. If the board is free to move,find the thickness to which the bullet will penetrate.
I used conservation of momentum to find the combined velocity of bullet and the board in terms of the initial velocity of the bullet but can't figure out what to do next.
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