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Kinetic and gravitational energy

  1. Jan 28, 2006 #1
    Q: An object at a height of 15 m above the ground is travelling at 25m/s. Find its

    a) kinetic energy
    b) gravitational energy

    If I find out how to do (a), I am sure I will figure out B. The problem is I don't know how to figure out (a)

    I know that Kinetic Energy = 1/2 mass (speed)^2

    I also know that work = (change of kinetic energy)

    Both these formulas involve masses. How do you solve?

    I do know that masses cancel out when you are dealing with the law of conservation of energy, but I am only determining the energy at this specific point. Help.
  2. jcsd
  3. Jan 28, 2006 #2
    In my humble opinion this exercise is not well elaborated. For what I know and it's logic, an object of 100 kg at a height of 15 m above the ground travelling at 25m/s has more kinetic energy than other of 3 kg at the same situation (the kinetic energy formula shows that very clear). In order to calculate his kinetic energy you must know is mass. In any case, I think you must have to admit that the mass of the object is 1 kg. Check the solutions.

    PS- As i said this is a modest opinion, wait for the experts.
  4. Jan 28, 2006 #3

    thanks for your opinion

    i still think its solvable though. I will wait for the experts.
    Last edited: Jan 28, 2006
  5. Jan 29, 2006 #4
    I really don't understand. I have an exam tommorrow.

    My recent thought processes include

    ET= ek + eg
    = 1/2mv^2 + mgh

    but how is that possible.. you need a mass?
  6. Jan 29, 2006 #5
    In physics courses, "solving a problem" does not always (in fact rarely) means getting a number.

    Even when you're given all the necessary numbers to get a numeric result, you'll generally want to first solve it as generically as possible. Only then do you plug in the numbers.

    Just use the information you were given to solve it as much as much as you can. Often times, the information you weren't given turns out to be irrelevant (mathematically, terms might cancel).

    With the information you gave and assuming ideal situations (in a vacuum, etc), we can say the only force is that of gravity, and the energy is conserved.
    So we can look at the system at two different times:
    1) When the object is at h=15m with v=25m/s, and
    2) When the object hits the ground.

    This is helpful because when the object is at ground-level, we know its potential (gravitational) energy is zero, and the total energy is equal to the kinetic energy.
    Solve for its kinetic energy when it hits the ground, and you have an expression for total energy that contains a mass. Now this will cancel out the m's in the original kinetic and potential energy expressions.

    However, you need to know the direction of the velocity in its initial state. If the object is traveling up or down at 25m/s initially, it's a 1 dimensional problem. If it's traveling in any other direction, it's a 2 dimensional problem.

    Hope this helps. I've been awake for far too long, so if anything doesn't make sense.. please ask. ;)

    Edit: made things a little clearer.
    Last edited: Jan 29, 2006
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