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Question about gravity and falling

  1. Sep 11, 2007 #1
    I haveve taken into to physics in high school and my teacher was a really great teacher but he was an @ss of a person. In my entire life i have never met someone so mean / ignorant to someone else so i never really asked him anything or tried talking to him so I am trying here.

    Well here it is. I have seen the little experiment they do to show that no matter how big an object is, if dropped with a smaller / lighter object they will hit the ground at the same time if dropped from the same height. So i think if I'm wrong correct me that they are both falling at 9.8 m/s2. So if gravity is constant how come if i fall from a swing for example and hit the concrete floor i need stitches and if i jump off a building i splatter against the floor? Wouldn't the speed be constant so whats the difference? Thanks a lot i thought of this earlier and just couldn't find anyone with an answer so i thought i would try online.
  2. jcsd
  3. Sep 11, 2007 #2


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    In freefall, the speed is not constant [in time] but the acceleration [the time-rate of change of velocity] 9.8m/s^2 is constant, independent of the mass of the object...

    In the little experiment, the speed at each time will be the same for objects of different masses dropped from the same height... that's why they hit the ground at the same time.

    In your swing and building example, the same mass is being dropped from different heights. Upon reaching the ground, each will have a different impact velocity. To change that impact velocity to zero [say, in a fixed time of contact], requires an acceleration (and therefore a net force). A large impact velocity requires a large force to come to rest.
  4. Sep 11, 2007 #3
    ah thanks for the response finally got that off my chest. I was confused and lost all day. Thanks again
  5. Sep 11, 2007 #4

    Doc Al

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    Realize that what's constant is the acceleration due to gravity, not the speed. 9.8 m/s^2 tells how the speed of an object changes as it falls. As you well know, the speed at which you land depends on how far you fall. Ignoring air resistance, the longer you fall, the faster you end up falling: After falling for 1 second you are moving at 9.8 m/s; after falling for 2 seconds, you are moving at 19.6 m/s. Each second of falling adds another 9.8 m/s to your speed.

    If you drop two objects from the same height, then their final speeds will be the same.

    (robphy beat me to it!)
    Last edited: Sep 11, 2007
  6. Sep 11, 2007 #5
    well doesnt matter if he did you both helped alot it was just bugging me very much lol
  7. Sep 11, 2007 #6
    If you know the measure of distance (meters), velocity is simply the change of distance over time (meters/second), and acceleration is furthermore the change of velocity over time (meters/(second)2). Acceleration, a, is important because it represents the motion from a force, F, acting on a mass, m; that is, F=ma, Newton's second law. Gravity may contribute to a force, as may air viscosity, the resistance of a spring, the pull or push of a magnet, the attraction or repulsion of charges, or the elasticity of a rubber band.
  8. Sep 11, 2007 #7
    see i want to take physics so badly but my college is telling me they highly recommend i don't because most people who study those fields are usually doing pre med or something around those lines. I want to study physics because I think its extremely interesting. This sucks.
  9. Sep 11, 2007 #8


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    You could do engineering....
  10. Sep 11, 2007 #9
    would that help teach me basic physics?
  11. Sep 12, 2007 #10
    Engineering *is* applied physics, so, yes.

    But you should have some basic physics under your belt before you take a first-year college physics class. I suggest that since you seem to enjoy the subject and have questions you'd like to ask in a classroom environment, then you should ask an administrator at your school (perhaps a Vice Principle) to transfer you to another physics teacher who you'd enjoy learning from more than your current teacher.

    Never lose a chance to explore the subjects which fascinate you, and your mind will never be bored ;)
  12. Sep 12, 2007 #11
    Thanks i will try and do that. Well i do have some level of knowledge due to my high school teacher. As i said he had taught me alot just he was a d***head and was ignorant and thought he was better then anyone else and he would fail your whole test if you forgot to put the ^2 in something like m/s^2. I know thats important but to fail someones whole test over 1 mistake is ridiculous. Anyways I'm going to see what i can do.
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