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Homework Help: Need hw help and basic concept please

  1. Aug 31, 2010 #1
    1. The problem statement, all variables and given/known data
    A rocket moves upward, starting from rest with an acceleration of +30.0 m/s2 for 5.00 s. It runs out of fuel at the end of this 5.00 s and continues to move upward for a while. How high does it rise above its original starting point?


    2. Relevant equations

    Hmm this is one of my hw questions i have like 5 more of these problems, can someone help solve this one showing a step by step.

    3. The attempt at a solution
    What i did was multiplied 30 x 5 to so 150m I dont understand what formula i should use to calculate the part 2 of this problem
     
  2. jcsd
  3. Aug 31, 2010 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    I've moved your question from the Advanced Physics forum to the Intro Physics forum. The Advanced Physics forum is for upper-division and graduate-level questions.

    Also, per the PF rules (see the link at the top of the page), we don't do your schoolwork questions for you. We can provide hints, ask questions, check your work, etc., but you must do the bulk of the work.

    So having said that, what can you do with each part of this problem? Break it down into the various parts (start, middle, end, etc.), and see what equations you can write for each part. Like, it has constant accelration for some amount of time at the start, so what can you say about it's speed and KE and PE at the point it runs out of fuel?
     
  4. Aug 31, 2010 #3
    A tennis ball with a speed of 8.0 m/s is thrown perpendicularly at a wall. After striking the wall, the ball rebounds in the opposite direction with a speed of 6.0 m/s. If the ball is in contact with the wall for 0.019 s, what is the average acceleration of the ball while it is in contact with the wall? Take the initial direction of motion to be the positive direction. (Indicate the direction of the acceleration by the sign of your answer.)

    I actaully sovled the one above.. can someone help me with this one
     
  5. Aug 31, 2010 #4
    Man.... Im taking ap physics..... LOL
     
  6. Aug 31, 2010 #5

    berkeman

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    As I said already, you need to show some work. What is the definition of acceleration in terms of velocities and times?
     
  7. Aug 31, 2010 #6
    Alright if its work you need... im coming with it right now
     
  8. Aug 31, 2010 #7
    8m/s would be its velocity, so accel equals Final v - iniital v /time = acceleration
     
  9. Aug 31, 2010 #8
    Hmm but what i dont understand is... how will i be able to calculate final velocity iniital is 8 m/s
     
  10. Aug 31, 2010 #9

    berkeman

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    The problem statement says that its velocity is 8m/s one way before hitting the wall, and 6m/s the other way after hitting the wall. What's the total delta-v?
     
  11. Aug 31, 2010 #10
    okay i understand thanks so the total v

    -8+-6 SO -14 now we have the equation for accel which is a = total v/ Time so its -14/.019 so its 736.84 Nice
     
  12. Aug 31, 2010 #11
    A person takes a trip, driving with a constant speed of 94.5 km/h except for a 21.0 min rest stop. If the person's average speed is 65.6 km/h, how much time is spent on the trip and how far does the person travel?

    LMAO i have no clue about this hw.... .... what type of work am i suppose to show on this.. can someone give me a hint
     
  13. Aug 31, 2010 #12
    so far what i found was 65.6 = Distance (unknown) / x(time unknown) + 21 mins
     
  14. Aug 31, 2010 #13

    berkeman

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    Staff: Mentor

    First of all, please start a new thread when you have a new question. It's too hard for folks trying to help if you append new questions on the end of an existing thread.

    Second, the type of work you show is your try at figuring out the problem. Spoon feeding and hand holding you won't help you learn how to learn, and that's a big part of what we do at the PF.

    So how should you approach a problem like this? Remember what I said a ways back in this thread about breaking a problem up into its parts. Break this up into parts, start labelling variables and things (like times t1, t2, etc.), and see how the whole thing looks to you.

    Post your work and thoughts.
     
  15. Aug 31, 2010 #14

    berkeman

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    And he was going 94.5km/hr for the time x that you list.... (maybe use t1 instead of x)
     
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