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Average Acceleration

  1. Oct 5, 2006 #1
    A rocket blasts off and moves straight upward from the launch pad with constant acceleration. After 2.6 s the rocket is at a height of 80.0 m.
    (a) What is the acceleration of the rocket?
    ______m/s2
    (b) What is its speed at this time?
    ______m/s

    Hello, I have been stumped on this question all day. I have no clue on how to do this problem, I have tried 80/2.6/2.6 to get the average acceleration squared, but that was not the correct answer. Can anybody please explain how to do this?
     
  2. jcsd
  3. Oct 5, 2006 #2

    Doc Al

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    Staff: Mentor

    Actually, you're closer than you realize. What kinematic relationships are you familiar with? Especially one that connects acceleration with time and distance. You might find this useful: Basic Equations of 1-D Kinematics
     
  4. Oct 5, 2006 #3
    When it comes to physics, I am completely lost in general. According to the equations I followed.. The answer should be 11.834 m/s^2 because I plugged the velocity I found into the acceleration equation.
     
  5. Oct 5, 2006 #4

    Doc Al

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    Show what you did. What velocity did you find and how did you find it?
     
  6. Oct 5, 2006 #5
    What I did,

    I took 80 m/s which is supposedly the final velocity, and divided it by 2.6, which gave me 30.76923077. I divided it again by 2.6 because the final answer is in m/s^2. which gave me 11.83431953. 11.83431953*2.6^2=80. I still dont see where I am going wrong here
     
  7. Oct 5, 2006 #6
    maybe this will help?

    Here's something a little different to try- make a table with
    initial velocity, final velocity, average velocity, displacement, acceleration, and time.

    For your problem, start filling in what's known; here's an example using different (but similar in magnitude) numbers: "After 3.0 seconds, the rocket is at an altitude of 90 meters"

    initial velocity = 0 m/s
    final velocity = ?
    average velocity = ?
    displacement = 90 m
    acceleration = ?
    time = 3.0 seconds

    Looking at these, the average velocity must be 90m/3.0s = 30 m/s
    Fill it in...
    initial velocity = 0 m/s
    final velocity = ?
    average velocity = 30m/s
    displacement = 90 m
    acceleration = ?
    time = 3.0 seconds

    Now, since you have an average velocity and initial velocity, you can find the final velocity, since (vi + vf)/2 = v.avg
    Final velocity must be 60m/s. Fill it in.

    initial velocity = 0 m/s
    final velocity = 60m/s
    average velocity = 30m/s
    displacement = 90 m
    acceleration = ?
    time = 3.0 seconds

    Now, since the rocket accelerated from 0m/s to 60m/s in 3.0 seconds, the velocity changed by 60 m/s in 3.0 seconds, or it changed at a rate of 20m/s per s. Fill it in, done. :)

    I recognize the usefulness of using x = x0 + vot + 1/2at^2, but until you understand what's going on, sometimes it might help you to learn the concepts by using the more basic equations. The only time the vvvdat table will fail you is if you need to use v^2=vo^2 + 2ad
     
  8. Oct 5, 2006 #7

    Doc Al

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    Staff: Mentor

    Why do you think 80 m/s is the final velocity? Study drpizza's post and you'll see how to find the average and final velocity.
     
  9. Oct 5, 2006 #8
    Wow, I got it correct! And thank you sooo much for the tip on using more basic equations like that, it is starting to make ALOT more sense!
     
  10. Oct 5, 2006 #9
    One last question, it asks me to find the speed at this time. The speed equation Speed = Distance / Time, but isnt the distance 80m? I tried 80/2.6 = 30.769 but that did not work out, where am I going wrong?
     
  11. Oct 5, 2006 #10
    ****bump****
     
  12. Oct 5, 2006 #11

    Doc Al

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    That equation tells you the average speed. You need the final speed. (Reread drpizza's post to see the difference.)
     
  13. Oct 5, 2006 #12
    You might want to closely read the question again. See if that helps you at all.
     
  14. Oct 5, 2006 #13
    Ah! So Velocity = speed, I guess you can tell by now how lost I am in physics. I got the answer correct, thank you.
     
  15. Oct 5, 2006 #14
    Velocity does not equal speed. Velocity has direction, while speed does not. A ball thrown up in the air will return to your hand with the same speed, but the opposite velocity. Speed is a velocity's magninude.
     
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