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: Ball Launched from spring gun (max height)

  1. Apr 24, 2010 #1
    1. The problem statement, all variables and given/known data

    A ball of mass m is launched at an angle theta from a spring gun of length D. The lower end of the gun, where the spring is attatched, is at ground level. The spring has spring constant k and an unstretched length D. Before launch, the spring is compressed to a length d.

    Determine the maximum height h above the ground the ball reaches.

    2. Relevant equations


    ?? x = mgcos(theta)

    3. The attempt at a solution

    Plugging in "x" I get: h=(k(mgcos(theta))^2)/2mg

    Is this correct? If not please explain in detail and provide the steps and solution. TIA.
  2. jcsd
  3. Apr 24, 2010 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Here x is a velocity. You can use conservation of energy to find the additional height reached after the ball leaves the gun, if x is the vertical component of the speed at that point. Note that h will not be the height above the ground.

    You can't set a velocity (x) equal to a force.

    No. Note that you haven't even used the spring constant or the amount the spring was compressed.

    Try to solve it in two steps: Find the speed of the ball as it leaves the gun. Then find the height that it reaches after it becomes a projectile.
  4. Apr 24, 2010 #3
    Well then I guess I have no idea what i'm doing then...A little more help please

    K_i + U_i + S_i = K_f + U_f + S_f

    K = .5mv^2

    U = mgh

    S = .5kx^2

    What is the muzzle velocity?

    How to find the height that it reaches?
  5. Apr 25, 2010 #4

    Doc Al

    User Avatar

    Staff: Mentor

    To find the muzzle velocity, use conservation of energy as you outlined above. Hint: What's the change in height of the ball as it travels through the gun?

    Once the ball leaves the gun it's a projectile. Solve that part like any other projectile motion problem.
  6. Apr 25, 2010 #5
    Could someone please solve the problem for me.....BTW this is NOT a homework problem....It is a final test review problem....I learn differently than most.....By looking at the steps involved in solving the problem along with the answer I can use deductive reasoning as to why those methods were used....TIA
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook