# Bullet and pendulum bob initial speed help! momentum.

bullet and pendulum bob....initial speed...help! momentum.

## Homework Statement

An 7.0 g bullet is fired into a 3.5 kg pendulum bob initially at rest and becomes embedded in it. If the pendulum rises a vertical distance of 8.0 cm, calculate the initial speed of the bullet.
________m/s

## Homework Equations

well there's many ways to do it, but heres one way:
v=sqrt((1+M/m)*2gh)

i tried it another way too, and got the same answer that i did with the above equation

## The Attempt at a Solution

so first, 7g bullet = .007kg bullet and height of 8cm = .08m

ok so then v=sqrt((1+3.5/.007)*2*9.8*.08) = ~28.03 m/s

now, i tried this TWO WAYS and got the same answer.
but webassign says it's wrong.

why?

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Kurdt
Staff Emeritus
Gold Member
The velocity of the pendulum and the bullet is given by:

$$v=\sqrt{2gh}$$

The mass shouldn't enter into that part. Once v is known one can use the conservation of momentum to find the bullets initial speed.

EDIT: I see what you've done to include the mass. The problem is the mass part should be outside the square root.

The velocity of the pendulum and the bullet is given by:

$$v=\sqrt{2gh}$$

The mass shouldn't enter into that part. Once v is known one can use the conservation of momentum to find the bullets initial speed.

EDIT: I see what you've done to include the mass. The problem is the mass part should be outside the square root.
which mass are you talking about?

Kurdt
Staff Emeritus