# Bullet and pendulum bob initial speed help! momentum.

#### lettertwelve

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

1. The problem statement, all variables and given/known data

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

2. Relevant 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

3. 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
Science Advisor
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.

#### lettertwelve

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
Science Advisor
Gold Member
The (1+M/m) term.

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