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Homework Help: Falling mass onto flat plate pressure exerted

  1. May 14, 2012 #1
    1. The problem statement, all variables and given/known data

    hi i'm trying to design a small experiment based on soil pressure
    i've not had much use of some of the equations and principles for a few years so i'm a little rusty

    basicly i'm trying to exert pressures onto grass seeds (lolium perenne) in a rough field experiment in order to see how it affects germination rates

    effective material
    small flat plate 0.25m*0.25m for this purpose it is effectively massless
    me approx mass 100kg

    i can exert approx 15-16kPa by standing on the plate
    but i'm trying to calculate how high i need to jump to exert around 30kPa

    looking for a max pressure
    i'd assume soil compression to be about 2 cm from surface height
    i don't need high precision but a rough and ready approximation

    ignore any mitigation by the body i.e knees and ankles

    any help gratefully received
    2. Relevant equations
    mass m = 100kg
    grav = g = 9.8m/s^2
    area = a = 0.0625m^2

    3. The attempt at a solution
  2. jcsd
  3. May 14, 2012 #2


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    welcome to pf!

    ih llamedos! welcome to pf! :smile:
    jumping will only exert a very temporary pressure, probably totally useless for your purposes

    easier would be to erect a frame with a bar going over your head, so that you can push up against it to press down on the plate continuously …

    and you can put ordinary bathroom scales on the plate to show you the force

    btw, i assume you'll have a control area with an identical plate with no pressure on it? :wink:
  4. May 14, 2012 #3
    its a form of simulated rolling so effective time period is about 0.1 seconds and lower
    which is similar to the exertion period of jumping on it

    yes there is a control i don't do anything to it

    but i could get lost in the complexities of soil matrix dynamics under compressive forces and incident periods so i don't want to.

    also when i say field i literally mean field
    cutting a plate with half the area and using it twice could work but then there would be the challenge of compensating for more edge effects

    on another note i joined back in 2007 when i was doing my a-levels been lurking ever since but never really needed to ask for assistance
  5. May 14, 2012 #4


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    (if you want to simulate rolling, why not use an actual roller? :confused:)

    let's see … if a roller with mass M passes over the area of a plate in time t,

    then that's an impulse (force times time) of Mgt​

    if your own mass is m, and your centre of mass rises by a height h (from crouch position to maximum position), then the impulse is the initial change in momentum, = mvi, = m√(2gh)

    (if you both jump from the plate and land on the plate, then that's multiplied by 2)

    so for the same effect you need h to be (M/m)2t2g/2
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