Ok....let me state this before anyone of those of you who read my post decides to chide me for asking such elementary questions.... Without completely justifying my ignorance to math and science, let me simply state it was not my strongest skill while in school.
And as I am reminded now, to my detriment.
I need help.....probably with stuff that seems basic to most of you, or if not basic than at least something that you all can tackle.
I am an artist and I need help with the science behind a design of mine so that it can work optimally.

I have a mass with a weight of 200 lbs (forgive me if my description is not technically accurate), which i plan to drop from a height of 8ft. What is the "weight" of this falling object once it hits the ground. The pressure, or force, on the ground is what i need to calculate.

I need to provide cushion to prevent this mass from "breaking" and survive the fall, but i first need to determine the "weight" at impact. What formula can you provide me to plug in different weights prior to falls of differing heights to end up with force at impact.

Also a quick definition or clarification of terms would be appreciated.

Thank so much in advance for considering responding to this.

Brian

Pounds are actually a measurement of force, so you already have your answer: the object should hit the ground with a force of 200 lb. Using google's calculator, that is equivalent to 889.644323 newtons. I'm a beginner at physics, though, so somebody please correct me if I am wrong.

Also, don't worry about asking "elementary questions" - everybody here's pretty friendly and willing to help.

ok....well thats not exactly what I am looking for....and maybe its me thats not clear...

for instance, a man on a trampoline does not go up simply by standing still, with lets say 200lbs of force downward.....but when he jumps, he subsequently bounces back up, because his weight is coming down with a greater force.

or maybe another example of a man standing on a box....without movement it supports him, but if he jumps 3 ft into the air and comes back down on it it will crush.

how is that force calculated, the formula....and what are the resulting units called

Newtons second law: ΣF = ma

Where ΣF is the resultant force (a vector), m is the mass of the object and a (a vector) is the acceleration of the object.

In you case, a is g, therefore

G = mg

Hey everybody, what he wants is obviously to find how hard the object will hit the ground.

This is the "normal force" between the ground and the object during the impact. Without going into the theory, I'll tell you that the main thing you need to know is the ratio between the falling distance and the stopping distance. If the object falls for 8 feet but then hits a stiff foam pad with a thickness of 1 foot (say), then it must stop 8 times as fast as it started, and so the total force must be 8 times its weight. However, this total force is net result of the normal force and gravity. Therefore, the normal force will actually be 9 times the weight, because it needs to balance against the original weight and still have 8 times the weight left over.

So the rule is, impact force = weight X (falling/stopping + 1). This tells you how thick to make the padding if you know how much force the object can withstand. You also have to remember to make the padding appropriately stiff. If the padding is too fluffy, it will not seriously begin to slow the motion of the falling object until after it is compressed into a thin layer. This means that most of the slowing occurs in the last fraction of an inch of travel, and so the stopping distance is actually much shorter than the thickness of the pad. The stopping force could then be very large and possibly damage the object.

You want the pad to be both the right thickness and the right stiffnes. If you have a sample of the padding available, cut a small square of it a few inches across and set a flate plate or board on top of it, and place it on a bathroom scale. Now place weight on the plate with your hand or by placing heavy objects on it, and watch for what the weight is when the pad is compressed to half its original thickness. Take that number of pounds and divide by the area of the pad sample in square inches. This tells you how stiff your pad is in pounds per square inch.

Now look at the size of the bottom side of the object you plan to drop. How many square inches of are will come into contact with the pad? Multiply this number by the pad stiffness from the previous step to find the total number of pounds of impact force the pad will exert on your object when it hits. If this amount of force is "safe" (i.e. will not break your object) then your pad stiffness is OK. If the force is too high, you need a softer pad.

Once you have got the right pad stiffness, you need to know how thick the pad must be. This thickness must be at least as great as the stopping distance of the object to avoid the object "bottoming out" the pad. You can find the stopping distance by rearranging the formula I gave back at the beginning. You get stopping = falling/[(impact force)/weight - 1]. Calculate the stopping distance, and make your pad at least this thick, by placing several layers if necessary.

If you have any further questions, please let me know.

Hope this helps!

--Stuart Anderson

Mr homm is correct, the solution is in slowing the deceleration of the object, but another question to ask would be how far can you drop the 200 lb object without breaking it with no cushion?

Protecting an egg is one thing protecting the turtle that laid it is another.

Arthur

I have a simular question which may go along with the first question that was ask on how to messure the impact force.

If you take a baseball bat and hit a piece of glass what impact force it place on the glass at impact and how do you messure that.

I have seen on mythbuster a device that looked like a scale that they use to simular test with but I can not seem to find or know the correct name to search the internet.

I know according to ANSI Z97.1 impact stadard test that a 8lb steel ball that is drop from 4 feet has apprx impact force of 400 lb how do I show this.

Please feel free to email me at mwgaston@hotmail.com

The above comments about stopping distance are correct, however, there is a slightly different formula that works as well.

Pounds are indeed a unit of force, however, if you are trying to find out the pressure that an object falling from a certain height or travelling at a certain speed one must first determine the amount of kinetic energy the object has.

Therefore the weight of the object should be divided by the acceleration due to gravity to get its mass. Then one puts the final velocity (at impact) into the momentum equation (mv) and divides the resulting momentum by the time over which the momentum is dissipated to get the force.

For an 8lb weight dropped from a height of 4ft. the momentum at impact is 4.0 slug*feet/second thus if the weight is stopped in about .01 seconds (by the floor I suppose) the force of impact is equal to that of a stationary 400lb weight.

I have a question. For one, I am just a 7th grader doing my science project, otherwise I might know the answer to my question. My question is this, what is the fromula to figure the pressure an object on the ground recieves if you know the height at which an object is dropped on the object on the grond and how much the object being dropped weighs?
Thank you, andybody who can answer this question!!

Hi, im trying to do a basic physics independent assignment at school but i've hit a snag, What i want to measure is the force at which a 50g object hits the ground at varying heights. i was working on the theory F=ma where m=50g and a= v-u/t where v=d/t, u=0 so my ultimate theory would be F=(d/t)/t. But i did some pelimanary test and my theory was wrong why?

i had another thought that maybe i had to include gravitational potential energy in the equation? possibly F=ma where m=50g and A= 9.8m/s^2(accelaration due to gravity)+GPE(gravitational potential energy i.e. weight x height)

does this even make any sense? can some one please please help me??? im tieing myself in knots i have no idea where to go from here and my teachers not being any help!!

cheers, mik

Stopping 100kg falling from 2,4m:

Yes, this doesn't convert directly to a force, but to a force multiplied by the stopping distance (and the force will vary over the distance), plus the weight that still exists. The stopping distance and the force are related through the compliance of the ground (and of the falling object), which is essentially unknown. So making forecasts is difficult if you don't choose a well-known ground.

Since stopping 100kg from 2,4m is already a small challenge, I suggest that you choose a stopping material accordingly. If not, only experiments will give you sensible information. Polyurethane may be a good solution, protecting your floor and surviving many shocks and being silent and giving little rebound, but not really easy to predict.

Another possibility is a material used once, like sand, mud... put back in form each time, or steel sheet replaced each time.

A nice solution (very appreciated in my last job, designing crash-test facilities) are hydraulic shock absorbers. They are designed for this task, can absorb a huge energy as many times as you want, and give a perfectly known deceleration force. Not completely for free... Google the above words, for instance Taylor Devices Inc delivers them.

I guess the choice depends heavily on the artistic effect you want. If you want a big noise, then only the difficult way, with less known deceleration curve, is possible. In such a case, maybe I would choose a ground made locally of steel just as the mass. Mind your ears!

So... How many times shall it fall? How many tries do you allow? How hard is the falling object: full steel, hollow plastic? Want it noisy or silent? Shall it go splash in the public by falling into ketchup?

Hey j14crab, are you too disappointed?
Or will you tell us a bit more about your materials and intentions?

Mr. homm,nice to see you back again in this forum.

OK So i have a problem is someone is still reading these, I am no engineer and all these are dutch t me, I have a 160kg object I want to drop 6 meters on steel bands with a rating of 250KG it will not hit the floor what effective weight or strain will that place on the steel bands? please email me andrewdavi@aol.com

My vote is for a trampoline-type setup where you use a piece of strong fabric, of the desired color and texture, and put in eyelit grommets (or whatever those metal-reinforced holes they put in tarps and shower curtains are called). Then get lots of bungee rope and manipulate the length and number of strands per hole until you've reached the desired springiness.

You will need to tie the bungees to something to hang it on. A trampoline frame would work, but you would probably want to change the legs so it is higher off the floor. If you don't want to buy a trampoline and take it apart, you could probably build something out of wood or galvanized steel pipe used for water lines. If you have welding capabilities, or know someone who does, you could make something really neat out of recycled materials and or the steel rods used to reinforce concrete structures.