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Homework Help: Express g in terms of s?

  1. Sep 23, 2014 #1
    1. The problem statement, all variables and given/known data

    In a physics laboratory the value of g, the gravitational acceleration in the vicinity of earth, has been measured accurately by projecting a ball up an evacuated tube and electronically timing the passage of the ball in its upward and downward flight through two light beams, an accurately known distance s apart. If the successive times of passage through the beams are: t0, t1, t2, and t3, express g in terms of s, and the times of the passage of the ball

    2. Relevant equations

    I really do not know how to begin this problem.

    3. The attempt at a solution

    I want to multiply g to t0, t1, t2... etc and then set it equal to s. But I dont think that is the correct way
  2. jcsd
  3. Sep 23, 2014 #2


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    Almost every problem of this type starts with a look at what Newton's second law says about it. Use it to find a function that gives you the ball's position at each time. (The formula will contain some variables whose values are unknown). Then find a way to use that function.

    If you just multiply g with one of those times, you get something with units of length/time, i.e. a velocity, so it can't be equal to s.
  4. Sep 24, 2014 #3
    Hi, this is my first semester of physics and I want to get as much practice as possible so I decided to give this a whirl and would love if someone could confirm if i'm correct or not.

    Basically by using newtons second law I started off with F=ma, well in this case force could also be considered weight or mg so now I got mg = ma.

    I changed "a" to equal distance (s) over time squared so mg = m(s/t^2) and cancelled out the mass to get g = s/t^2. Thanks to anyone who might respond with some feedback
  5. Sep 24, 2014 #4


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    That's not the correct answer, and if it had been, your post would have been in violation of the rules of the homework forum. We don't give out solutions here. We just give hints, and point out mistakes. So I'm afraid I can't help you (Godliath) much in this thread. If you and I discuss this problem here, we would be giving away too much information to monkeylord714.

    I will elaborate just a little bit on what I said before. If we denote the distance from the floor by x, we can write Newton's 2nd in the form mx''(t)=F. What is F? Figure it out, and then integrate twice to find x.
  6. Sep 24, 2014 #5
    Hi monkeylord 714. Welcome to physics forums!!!

    You didn't write down any equations in item 2 of the template, but you need a place to start. Did you learn any equations in your course for relating distance, velocity, acceleration, and time for systems in which the acceleration is constant? These will have to be your starting point. Please write down the associated equations you have had in your course.

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