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: Projectile at unknown angle

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data

    A projectile is launched at 50 m/s at an unknown angle. Calculate its speed at a height of 80m and a height of 100 m.

    i dont know how to even begin this one, any input would be greatly appreciated
  2. jcsd
  3. Dec 16, 2008 #2
    This question is trying to trick you, in a sense.

    The thing about projectile problems is understanding the components. This problem is actually giving you the heights, in which all you have to do is find the velocity at those given heights with respect to gravity.

    All projectiles launched at an angle have an x-component, and a y-component. This problem gives you the y-components, so you do not have to worry about the angle.
  4. Dec 16, 2008 #3
    yes but how do i go about calculating the speed or the projectile at the given heights, wat formula would i use?
  5. Dec 16, 2008 #4
    Try using energy
  6. Dec 16, 2008 #5
    can u give me a hint as to wat formula to use?
  7. Dec 16, 2008 #6
    again energy is conserved.

    [tex] mgh_{1}+\frac{1}{2}mv_{1}^2=mgh_{2}+\frac{1}{2}mv_{2}^2 [/tex]

    Again use set the lowest point as a potential energy of zero. so h1 is zero. you have v1 and h2 the v2 is unknown. Note: the masses are canceled out.

    Physics is not about formulas, you have to understant the concepts first.
  8. Dec 16, 2008 #7
    you are absolutely right, physics is about the concepts, thank you very much for ur help
  9. Dec 16, 2008 #8


    User Avatar
    Homework Helper

    In projectile motion total energy in conserved at every point. And the total energy = PE + KE. Initially the projectile has a total energy is KE = 1/2*M*v*^2. Find the height it reaches if it is projected vertically. All KE is converted in to PE. When it is at 80m and 100 m, see how much PE is lost.Find the remaining KE, and find the velocities.
  10. Dec 16, 2008 #9
    thanks for the info
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook