1. Not finding help here? Sign up for a free 30min 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!

Kinematics problem - Bullet in block of wood

  1. Feb 15, 2014 #1
    1. The problem statement, all variables and given/known data
    A 3.50kg block of wood is at rest on a 1.75-m high fencepost. When a 12.0g bullet is fired horizontally into the block, the block topples off the post and lands 1.25m away. What was the speed of the bullet immediately before the collision?


    The attempt at a solution
    d=1/2at^2
    t=*sqrt(2d-vertical-)/a
    t=*sqrt[(2)(1.75m)/(9.81m/s^2)
    t= 0.597309633s

    d= vt
    vf = d(horizontal)/t
    vf= 1.25m/0.597309633m/s^2
    vf= 2.09216961m/s

    Conservation of momentum (P1+P2)before = (Psystem) after
    (m of bullet * vi of bullet) + (m of block * vi of block) = (m of system * vf of system)
    (0.012kg*vi) + (0) = (3.512kg*2.09216961m/s)
    (0.012kg)vi = 7.349621967kg*m/s
    vi = 7.349621967kg/.012kg
    vi = 612.4684m/s

    The initial velocity seems a bit high and I'm not sure how else to approach the question. Help please!! :)
     
  2. jcsd
  3. Feb 15, 2014 #2

    PhanthomJay

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    looks good! Bullets travel mighty fast.
     
  4. Feb 15, 2014 #3
    Apart from too many significant figures in the final answer, I do not see anything wrong in your solution. And 612 m/s is not too high for a bullet.
     
  5. Feb 15, 2014 #4

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    One cause for concern: it says "topples". That implies a rolling movement, so some of the energy has gone into rotation.
     
  6. Feb 15, 2014 #5

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Why are you multiplying by 3.512kg?

    AM
     
  7. Feb 15, 2014 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Mass of block+bullet?
     
  8. Feb 16, 2014 #7

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    The problem does not say that the bullet stays in the block but perhaps it is implied. I cannot imagine how a 3.5kg block of wood is going to stop a 12g rifle bullet at 2000 feet per second.

    AM
     
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: Kinematics problem - Bullet in block of wood
Loading...