Ballistic Pendulum initial velocity of bullet

AI Thread Summary
The discussion revolves around calculating the initial velocity of a bullet using a ballistic pendulum setup. A 20.0g block swings to a height of 5.00cm after a 0.50g bullet embeds itself in it. The potential energy of the pendulum is calculated using U = mgh, resulting in 9.8 Joules. The kinetic energy equation is applied to find the bullet's speed, yielding a final velocity of approximately 62.6 m/s. Concerns are raised about the assumption of work conservation in this scenario, indicating a potential error in the calculations.
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Homework Statement


A ballistic pendulum is used to measure the speed of a bullet fired from a gun. The pendulum consists of a 20.0g block hanging from a string. A bullet, with mass 0.50gg, imbeds itself in the block causing the block to swing up to a height of 5.00cm. Find the speed of the bullet as it leave the gun.


Homework Equations


W_nc = ΔK + ΔU
K = 1/2 mv^2
U = mgh
W = Fdcos∅
p = mv



The Attempt at a Solution


I wanted someone to double check my work, and if I used correct units or not.

pendulum
U = mgh
U = (20.0g)(9.8)(0.05cm)
U = 9.8 Joules (this is not gravity it is what mgh came out to)

bullet
K = 1/2 mv^2
9.8(U of pendulum) = 1/2(0.005)v^2
v^2 = 9.8/0.0025
v^2 = 3930
v = 62.6099
v = 62.6 m/s
 
Physics news on Phys.org
The bullet EMBEDS itself in the block.
 
In case you don't understand the significance of voko's comment, you have assumed work is conserved. That is unjustified (and quite wrong here).
 
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