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How would you explain this?

  1. Oct 18, 2015 #1
    An object falling from a short distance (e.g. 5cm) has less of an impact than the same object falling from a height of 5m.
     
  2. jcsd
  3. Oct 18, 2015 #2
    The object which is dropped from a greater height will be travelling faster when it hits the ground.
    It carries a greater amount of energy as momentum.
    When it impacts the ground and stops there is more energy which has to be released.
    Some of the energy will be released as heat, some will be released as a shock wave in the ground material, which could cause destructive/distorting effects in the material.
     
    Last edited: Oct 18, 2015
  4. Oct 20, 2015 #3
    But why is it traveling faster?
     
  5. Oct 20, 2015 #4
  6. Oct 20, 2015 #5
    No, I am in a physics course for social science students. They keep it very simple.
     
  7. Oct 21, 2015 #6

    Nugatory

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    It takes longer to fall from a greater height because there's more distance to cover before the falling object hits the ground. That gives gravity more time to accelerate the falling object.

    (no matter how simple they're keeping things, I hope they're giving you the relationship between time, speed, and acceleration: ##v=at##, which says speed equals acceleration times time).
     
  8. Oct 21, 2015 #7

    CWatters

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    The longer you accelerate in your car the faster you go.
     
  9. Oct 21, 2015 #8
    Gravity on Earth will accelerate all objects at 9.807 meters per second, for every second that the object falls, until that object reaches terminal velocity (the air friction balances with the gravitational acceleration).

    So, for instance, a penny dropped from 9.807 meters with a starting velocity of 0 m/s will fall for exactly one second and will be traveling at 9.807 m/s (21.9 MPH) when it hits the ground. A penny dropped from 19.613 meters with a starting velocity of 0 m/s will fall for exactly two seconds, and will be traveling at 19.613 m/s (43.8 MPH) when it hits the ground. A penny dropped from 29.419 meters with a starting velocity of 0 m/s will fall for exactly three seconds and will be traveling 29.419 m/s (65.8 MPH) when it hits the ground.

    Thus, an object falling from a greater height will have more time to accelerate (unless it's moving faster to begin with... for instance, a penny dropped from 10 meters with a starting velocity of 0 m/s vs. a penny dropped from 11 meters with a starting velocity of of 22 m/s... the second penny would have less time to accelerate in that case).

    Because the object falling from a greater height has more time to accelerate, it's moving faster than if it were dropped from a lesser height, so it's got more kinetic energy.

    Basically, for every second an object falls, you can add ~22 MPH to its speed until it reaches terminal velocity.
     
    Last edited: Oct 21, 2015
  10. Oct 21, 2015 #9

    ZapperZ

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    But I'm sure even without having to study it, you must have some idea what an "acceleration" means! It is not as if this is a new or an unusual concept, is it?

    Zz.
     
  11. Oct 21, 2015 #10
    I am wondering why physics is even relevant to social science, other than recognising a smile if you you are lucky enough to see one.
     
  12. Oct 22, 2015 #11
    If the object falls for t seconds, the velocity will be gt when it hits the ground, with g = 9.807 m/s^2, but the vertical distance covered is (1/2)gt^2. (the falling time multiplied by the average velocity).
    So to make the object fall for 1.2 and 3 seconds, the initial heights should be 4.903, 19.614 and 44.131 metres.
     
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