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Homework Help: 3 problems I need help starting

  1. Oct 1, 2004 #1
    HELP! I have 3 algebra based physics problems I need help starting. I dont want answers, just help picking formulas to use and such.

    1) a fighter plane has an air speed of 200 m/s. A projectile is launched at 50 m/s in a forward direction, with a 30 degree angle relative to the plane's motion. What is the speed of the projectile with respect to a stationar observer on the ground?

    2) An anti-tank weapon fires a 3kg rocket which aquires a speed of 50m/s after traveling 90cm down a launching tube. What force is acted on it?

    3) A 10 kg block rests on a table. The coefficient of static friction os 0.50. If someone pushes down on the block with a force of 48N what is the maximum horizontal pull that will NOT move the block?

    Any help would be greatly appreciated! Thanks.
  2. jcsd
  3. Oct 1, 2004 #2

    You haven't mentioned your specific difficulties with these problems. Please post your reasoning/approach before you ask for help here. Anyway, I assume that you just cannot start thinking about these problems so I'll give you some hints:

    For Question1: you seem to be unfamiliar with the kinematics of projectile motion. Perhaps a diagram illustrating the initial direction of motion of the projectile. Do you know what the absolute initial velocity of the projectile is? Can you resolve it in two mutually perpendicular directions?

    For Question 2: Do you know the mathematical formulation of Newton's Second Law, "The rate of change of linear momentum of a body equals the force acting on it"? Can you use it somewhere in your reasoning? What is given and what is to be found?

    For Question 3: Can you write the equation of "motion" (read further) for this block? If you can do so using F = ma you should be able to adapt it for the specific case in question, that is, of equilibrium of the block under the forces acting on it. Which direction does the friction act? Will it aid motion or oppose it in this case? Can you resolve the forces in two mutually perpendicular directions?

    Hope that helps...

  4. Oct 3, 2004 #3
    Okay, I got #1, and #3. #2 is still dancing it's evil little dance on my grave. Am I doing this right?
    a= 1389m/s^2

    F= 4167Newtons
  5. Oct 3, 2004 #4
    Didn't check the actual calculations but your approach is right.
  6. Oct 4, 2004 #5
    thank you!
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