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!

Homework Help: How to find initial velocity

  1. Apr 3, 2013 #1
    A circus clown in a cannon is shot vertically upward with an initial speed of 18.0 m/s. after ascending 4.5 m ,she collides with and grabs a performer sitting still on a trapeze. They acend together and then fall. what is their speed when they reached the original launch height. The clown and trapee have the same mass.

    I tried using m1v1=m2v2
    where 1*18=x*1 which is not right.
    I do not know what to do with the height.
  2. jcsd
  3. Apr 3, 2013 #2
    The equation you used is for momentum, which shouldn't have to come into play here.

    Try thinking about it in terms of energy
  4. Apr 3, 2013 #3
  5. Apr 3, 2013 #4
    Sort of on the right track. Break it down into steps:

    1) out of the cannon
    2)grabbing the other performer
    3) at the height of the launch

    Also, what are the two energy equations you should be using?
  6. Apr 3, 2013 #5
    should the second equation be m1v1+m2v2=m1v'1+m2v'1+m2v'2?
  7. Apr 3, 2013 #6
    That is still momentum.

    What is the equation for the kinetic energy of an object?
    What about potential energy?
  8. Apr 3, 2013 #7
  9. Apr 3, 2013 #8
    almost. PE = mgh

    try now
  10. Apr 3, 2013 #9
    I would assume mass is 1 since they have the same mass so
  11. Apr 3, 2013 #10
    That's a terrible idea!

    use m for mass and see where you get
  12. Apr 3, 2013 #11
  13. Apr 3, 2013 #12
    Side note: The units of energy are Joules, the units of distance m. Don't just guess!

    OK. Try first finding the height at which the pair of the start to fall.

    To start off:

    The kinetic energy as the clown leaves the cannon: .5*m*v^2
    The potential energy of the pair before they drop: 2m*g*h

    note it is 2m, not just m, because he grabbed the performer on the way up.

    Also recall that the energy is conserved. How would you get h, the height before they drop?
  14. Apr 3, 2013 #13
    so can I set PE=KE to find mass(m)?
  15. Apr 3, 2013 #14
    You can set PE = KE, but look at what happens to the masses
  16. Apr 3, 2013 #15
    the masses cancel out,
  17. Apr 3, 2013 #16
    Exactly. Now you can find the height they are at when they start to fall down, and in turn the velocity
  18. Apr 3, 2013 #17

    Doc Al

    User Avatar

    Staff: Mentor

    Careful. When the clown grabs the other performer, energy is not conserved. (Treat that as an inelastic collision.)
  19. Apr 3, 2013 #18
    I did not understand what you said in the last part .@dinospamoni
  20. Apr 3, 2013 #19
    Right. I completely forgot about that Doc Al.

    Out of the cannon:


    He grabs the person:
    PE = mgh_1
    KE = .5 (2m)*(v_2)^2
    Solve for v_2
    This is the speed of just the clown before he grabs the person

    Now you can use m1v1=m2v2, Where the left side is the clown and the right side is the clown+person

    They now have KE .5 (2m)*(v_3)^2
    The new KE gives a PE (2m)g*h_2

    Now you can solve for height h_2
  21. Apr 3, 2013 #20

  22. Apr 3, 2013 #21
    Almost. When he picks up the other person he has both KE and PE

    I have this:

    .5*m*(v_1)^2 = m*g*h_1+.5*m*(v_2)^2

    You know everything except for v_2
    Last edited: Apr 3, 2013
  23. Apr 3, 2013 #22
    So we know what the mass is?
  24. Apr 3, 2013 #23
    Whoops forgot to include mass in a term on that post. Fixed now.

    They still cancel though
  25. Apr 3, 2013 #24
    If the masses cancel then v2= 0.46
  26. Apr 3, 2013 #25
    That's not what I'm getting. Don't forget to subtract the g*h_1 from both sides
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted