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What was the speed of the projectile when it left the cannon?

  1. Apr 7, 2008 #1
    1. The problem statement, all variables and given/known data

    A projectile is shot from the surface of te Earth by means of a very powerful cannon. If the projectile reaches a height of 35,000 m above Earth's surface, what was the speed of the projectile when it left the cannon?

    a. 355 m/s
    b. 827 m/s
    c. 710 m/s
    d. 906 m/s

    2. Relevant equations

    v= square root of GM/r

    3. The attempt at a solution

    theres no mass, so i wasnt sure how to solve it exactly
     
  2. jcsd
  3. Apr 7, 2008 #2
    What does capital M denote?
     
  4. Apr 7, 2008 #3
    mass of the Earth
     
  5. Apr 7, 2008 #4
    Well, that's not an unknown mass.
     
  6. Apr 7, 2008 #5
    yea so i plugged it in but didnt get the right answer.

    the answer is suppose to be B.
     
  7. Apr 7, 2008 #6
    Well, use conservation of energy:

    [tex]U_0 + K_0 = U_f + K_f[/tex]

    You know that [tex]U = -\frac{GmM}{r}[/tex]
     
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