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Homework Help: Conservation of energy of two thrown stones

  1. Nov 23, 2008 #1
    1. The problem statement, all variables and given/known data

    Two stones are thrown with the same initial speed at the same instant from the roof of a building. One stone is thrown at an angle of 30 degree above the horizontal; the other is thrown horizontally.(Neglect air resistance.), which of the following is true:

    (a)The stones strike the ground at the same time and with equal speeds.
    (b)The stones strike the ground at the same time with different speeds.
    (c)The stones strike the ground at different times with equal speeds.
    (d)The stones strike the ground at different times with different speeds.

    2. Relevant equations


    3. The attempt at a solution

    So I guess the answer is c but I can't explain it. Can anyone help me ?
  2. jcsd
  3. Nov 23, 2008 #2
    The answer is C, by reasoning. I didn't do any math, and I haven't taken intro-physics yet, but my reasoning is this:

    The speed will stay constant, unless altered by another force, which neglecting air resitance, speed is constant.

    So when the rock hits the ground, it will be a the same speed as the other rock. So now it can only be A, or C

    However, the rocks have a longer arc-distance to travel, and since one rock won't speed up, the rock that is travelling 30 degrees will hit the ground first, followed by the second rock that was thrown horizantal.

    So, we know that the rocks can't hit the ground at the same time, if they are traveling at the same speed,


  4. Nov 24, 2008 #3
    Energy is conserved since we are ignoring friction.They also start off with the same amount of kinetic and potential energy. Since they both come to rest at ground level then their potential energies are equal at the end. Their kinetic energies at the end are equal so their velocities are equal (since masses didn't change or anything like that).

    Energy conservation is a clean way to look at this, but I don't know if you are familiar with this yet though so let me know if what I said was just confusing.
  5. Nov 24, 2008 #4
    I get it. How about the part of different time ?
  6. Nov 24, 2008 #5
    one stone starts off going against gravity (has some velocity going up), it takes some time for the gravitational force to overcome this upward momentum and make the rock start to fall, while the other rock can doesn't have an upward velocity and can start falling vertically right away.

    Beyond this explanation we'd have to use calculus and some equations, which I believe were invented because it gets way to hard and ambiguous to describe the stuff in English.
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