Ballistic Pendulum initial velocity of bullet

MathHelpa

1. 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.

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

3. 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

Related Introductory Physics Homework Help News on Phys.org

voko

The bullet EMBEDS itself in the block.

haruspex

Homework Helper
Gold Member
2018 Award
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).

"Ballistic Pendulum initial velocity of bullet"

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