Bullet and pendulum bob initial speed help! momentum.

  • #1
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?
 

Answers and Replies

  • #2
Kurdt
Staff Emeritus
Science Advisor
Gold Member
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6
The velocity of the pendulum and the bullet is given by:

[tex] v=\sqrt{2gh} [/tex]

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.
 
  • #3
The velocity of the pendulum and the bullet is given by:

[tex] v=\sqrt{2gh} [/tex]

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?
 
  • #4
Kurdt
Staff Emeritus
Science Advisor
Gold Member
4,812
6
The (1+M/m) term.
 

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