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Homework Help: Two acceleration questions

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    1) A rocket traveling at 155 m/s is accelerated at a rate of -31.0 m/s^2.
    a. how long will it take before the instantaneous speed is 0 m/s.
    b. how far will it travel during this time?
    c. what will be it's velocity after 8.00 s?

    2. Relevant equations
    i have all of the following formulas:
    vf = vi+at
    d = (vf+vi)/2 x t
    d = (vi)(t) + 1/2(a)(t)^2
    vf^2 = vi^2 + 2(a)(d)

    i don't think any of these equations will work for the problem. i really need help deciding which one of these to use, if any of these are even usable for the problem. do i make the initial velocity 0 m/s and the final velocity 115 m/s? and then solve for time? i really have no idea. i'm just assuming that's what i'd do.

    one more...

    1. The problem statement, all variables and given/known data
    2) one such gun can give a small object a velocity of 3.5 km/s moving it through only 2.0 cm.
    a. what acceleration does the gun give this object?
    b. over what time interval does the acceleration take place?

    2. Relevant equations
    do i use 2.0 cm as the displacement, and make initial velocity 0 km/s and final velocity 3.5 km/s? or am i completely wrong?

    thank you so much for any help!
  2. jcsd
  3. Oct 14, 2008 #2


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    Science Advisor

    Why are you guessing? You are told (1) the rocket starts (initially) moving at 115 m/s and finally stops at 0 m/s. So it should be clear to you what the "initial" and "final" speeds are. You are also told the acceleration and exactly ONE of the equations you wrote above has "initial velocity", "final velocity", "acceleration", and "time". Since you know three of those, you should be able to solve that equation for the fourth.

    Yes, that is exactly right. Here, you are NOT given the acceleration but are asked for two things: the acceleration and the time. You need to use two of the equations you have above, ones that involve the things you are given, and then solve the two equations for a and t.
  4. Oct 14, 2008 #3
    i got them. thanks a lot! my "teacher" doesn't go over problems with us, just gives us formulas, so every bit of help goes far. :)
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