Physics help

Discussion in 'Community Discussion' started by Jasonbot, Sep 11, 2007.

  1. Jasonbot macrumors 68020

    Jasonbot

    Joined:
    Aug 15, 2006
    Location:
    The Rainbow Nation RSA
    #1
    Hi again, got more assistance requirements again. BTW, last time MR helped me with maths I got 96% for the exam- the highest in the grade.

    So...

    A toy cannon (0.4kg) is at rest. The cannon shots a ball (0.025kg). Immediately after firing the shot the cannon rolls back at 0.2m.s-1, then comes to rest.

    1. Calculate teh magnitude of the velocity of the ball. (I got 3.2m.s-1)
    2. If the acceleration of the cannon is 0.05m.s-2 calculate the magnitude of the frictional force of the ground on teh cannon.

    so far I have got
    F=m.a=Fball - Ffriction
    =0.4 * 0.05 =Fb-Ff

    At this point I cant get Fb since no time is given.
     
  2. squeeks macrumors 68040

    squeeks

    Joined:
    Jun 19, 2007
    Location:
    Florida
    #2
    i failed my physics final...still got a C in the class though, but that was from association with some of my teachers favorite students being my best friends
     
  3. elfin buddy macrumors 6502a

    elfin buddy

    Joined:
    Sep 16, 2001
    Location:
    Tuttlingen, Germany
  4. Dandaman macrumors 6502

    Joined:
    Jan 31, 2004
    Location:
    soCal
    #4
    someone correct me if i'm wrong, but I dont think you have to take the force of the ball into consideration. The frictional force will equal that of the force from the cannon. so F = ma = Ffriction.

    again, I could be wrong. It's been awhile since i've done a physics problem.

    daniel
     
  5. Jasonbot thread starter macrumors 68020

    Jasonbot

    Joined:
    Aug 15, 2006
    Location:
    The Rainbow Nation RSA
    #5
    to restate; Fresultant=Fball exerts on cannon - Ffriction

    If you look @ the vectors:

    Fres= --->
    Fball= ---->
    Ffriction= <-

    so: Fres=Fball-Ffric
    or: ---> = ----> - <-
     
  6. Dandaman macrumors 6502

    Joined:
    Jan 31, 2004
    Location:
    soCal
    #6
    hmmm, not sure if your force vectors are 100% correct, but I could be wrong. the Fres should be opposite of the direction of the ball.

    if you need time, you can use (vfinal = vinitial - at) where vfinal = 0 since the cannon comes to rest.

    daniel

    EDIT: hmm, i just realized that you're looking for the time for the ball. I don't think you can find this because you are not given acceleration for the ball.
     
  7. Jasonbot thread starter macrumors 68020

    Jasonbot

    Joined:
    Aug 15, 2006
    Location:
    The Rainbow Nation RSA
    #7
    I was meaning force of ball on cannon as it is equal but opposite :p

    And as for the time I need the time of the flight of the ball or something to calculate the Fres of the ball as it affected the cannon.
     

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