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Thread: accelaration

  1. #1 accelaration 
    Forum Professor leohopkins's Avatar
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    Hi Guys,

    If I know the mass of something and know that i want to to attain a certain speed (lets say 3 metres per second) when it hits the ground (assuming acceleration due to gravity as being 9.81 ms^-2

    How would I go about finding out at what height the object should be dropped?

    (ignoring air resistence)


    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  3. #2  
    Administrator KALSTER's Avatar
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    Don't know if I am missing something?



    s = the distance between initial and final positions (displacement) (sometimes denoted R or x)
    u = the initial velocity (speed in a given direction)
    v = the final velocity
    a = the constant acceleration
    t = the time taken to move from the initial state to the final state

    Replace with . The equations will work with any mass falling to earth.


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  4. #3  
    Forum Professor leohopkins's Avatar
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    Hi, thanks,

    Trouble is, I dont know from what height the object is to be dropped, nor do i know the length of time it is allowed to drop. :s

    All I know is the acceleration due to gravity is 9.81 ms^2

    And the said object is 3.1kg

    and it needs to attain a speed (once dropped from "x" height) of 5.42 ms^-1.



    Quote Originally Posted by KALSTER
    Don't know if I am missing something?



    s = the distance between initial and final positions (displacement) (sometimes denoted R or x)
    u = the initial velocity (speed in a given direction)
    v = the final velocity
    a = the constant acceleration
    t = the time taken to move from the initial state to the final state

    Replace with . The equations will work with any mass falling to earth.
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  5. #4  
    Administrator KALSTER's Avatar
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    Use the forth one. You have all the variables except for displacement.

    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
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    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  6. #5  
    Forum Professor leohopkins's Avatar
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    Quote Originally Posted by KALSTER
    Use the forth one. You have all the variables except for displacement.

    aha, makes sense thanks,

    Another quickie....... I know that F = m x g

    But how would i calculate the force of something travelling at say 20 ms^-1 if the mass is say 10 kg.

    Because if i were to just use the old F = m x g then I would calculate.....

    F = 10 x 9.81

    Force = 98.10 Newtons. But im sure im supposed to use 20 ms^-1 somewhere but not too sure where?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  7. #6  
    Administrator KALSTER's Avatar
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    Is this homework?

    Anyway, in which direction is that ?

    If it is in the downwards direction, then just add it to the final velocity. If it is perpendicular to the force of gravity, then it would not affect the equation. The object would still hit the earth at the same time as it would have if it was just falling straight.

    What is it specifically that you want to work out?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  8. #7  
    Forum Professor leohopkins's Avatar
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    Quote Originally Posted by KALSTER
    Is this homework?

    Anyway, in which direction is that ?

    If it is in the downwards direction, then just add it to the final velocity. If it is perpendicular to the force of gravity, then it would not affect the equation. The object would still hit the earth at the same time as it would have if it was just falling straight.

    What is it specifically that you want to work out?
    I am trying to work out the force that 3.1kg will experience when it hits the ground at a speed of 5.42 ms^-1

    but F = m x g doesnt seem to take into account that the object would be travelling at 5.42 ms^-1

    Common sense would dictate that i could use the formula F = m x v. but something tells me this isnt quite right ?
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  9. #8  
    Forum Professor leohopkins's Avatar
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    Its okay, i knew there was a variable missing somewhere !! :-)

    Got it
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  10. #9  
    Administrator KALSTER's Avatar
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    What did you figure out? I was thinking of impulse. :?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  11. #10  
    Forum Professor leohopkins's Avatar
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    Quote Originally Posted by KALSTER
    What did you figure out? I was thinking of impulse. :?
    I failed to mention (and take into account) the rate of deceleration which is 250g

    therfore i CAN use F = m x g

    (but i dont use 9.81 as "g")

    I use 250 * 9.81 as "g"

    Thats where i kept going wrong with it, but i got it now XD

    Thanks
    The hand of time rested on the half-hour mark, and all along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness, and the presence of death, and having done with all pleasant things, advanced across No Man's Land to begin the Battle of the Somme. - Poet John Masefield.

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  12. #11  
    Administrator KALSTER's Avatar
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    Ah, ok. would be then in this context.

    So, was it homework or something?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

    "Gullibility kills" - Carl Sagan
    "All people know the same truth. Our lives consist of how we chose to distort it." - Harry Block
    "It is the mark of an educated mind to be able to entertain a thought without accepting it." - Aristotle
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  13. #12  
    Forum Masters Degree Numsgil's Avatar
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    As KALSTER mentioned, force doesn't make sense in this frame (the force in a collision is HUGE, but only for a split second). What you want is impulse.

    The equation is basically mass * change in velocity. If the collision is inelastic (your mass "sticks" into the ground without bouncing) then impulse is simply mass * velocity at impact.

    If you want to know how much energy is produced from the collision, you would just take the kinetic energy of the mass before collision (1/2 * mass * velocity ^ 2). If the collision is elastic and bouncy, it's a little more complicated. Check out the coefficient of restitution.
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