1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Bullet is fired into block at at offset, conversation of kinetic energy?

Tags:
  1. Oct 25, 2014 #1
    1. The problem statement, all variables and given/known data
    Hello, first year engineering student here :)https://fbcdn-sphotos-h-a.akamaihd.net/hphotos-ak-xpf1/v/t35.0-12/10744040_871781156179440_107104578_o.jpg?oh=73c4261941e7ef6b7c682558c0925a63&oe=544F5CA4&__gda__=1414428985_0415c28b8b2e194bf0b57f065267fe49


    The question is

    in Case B, what percentage of the bullet's initial KE is conserved as KE in the impact?
    2. Relevant equations

    KE=(1/2)*m*v^2 + (1/2)*I*w^2 where I=mass moment of inertia, w=angular velocity
    (angular momentum) H=m*v*r*sin(theta) (maybe? Not sure if this is revelant...)
    (angular momentum) H=I*w (also not sure if relevant)
    3. The attempt at a solution
    I found the mass moment of the block using I=(1/12)*m*(a^2+b^2) and the bullet I=m*r^2,
    then I used H=m*v*r*sin(theta) to find H, then use w=H/I to find angular velocity, where I is I(bullet)+I(block)

    then I plugged in all the values into the first KE equation above, but got the wrong answer. (BTW, I found final v of the block+bullet by Conservation of Momentum)
     
    Last edited: Oct 25, 2014
  2. jcsd
  3. Oct 25, 2014 #2
    I've had enough conversations of kinetic energy for one night :)
     
  4. Oct 25, 2014 #3

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    About what point will bullet+block rotate? What is the moment of inertia about that point?
     
  5. Oct 25, 2014 #4
    I think the bullet+block will rotate about its centre of mass? If that's the case then the moment of inertia can be found using I=1/2*m*(a^2+b^2). But the question hints that we should include the bullet's effect on the moment of inertia (the bullet's moment of inertia would be I=m*r^2, yeah?).
     
  6. Oct 26, 2014 #5

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    The centre of mass of what, precisely?
    Where r is what?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Bullet is fired into block at at offset, conversation of kinetic energy?
Loading...