Force of a dropped object on impact?

actuallylike

Hi,
I've seen a few questions related to my problem but not quite like it.

I'm currently trying to run an analysis on solidworks of a ball of a certain mass falling and impacting a stationary wire gauze made out of a certain material. I want to get the deflection of the gauze. Unfortunately, the only simulation package I have only allows me to pick the static force in Newtons acting on the gauze.

Is there any way of measuring what force would act on the gauze from the ball dropped from a certain height? I've gone through all the Equations of Momentum and have the velocity it would strike the gauze, the time it would take to reach it and all that but still drawing a blank.

Obviously F=ma (or mg in this case), but surely a ball dropped from a higher height would increase its force? The equations I found require the distance traveled through the material to get the Newton value but seeing as though this is what I am looking for in the first place, I cannot give that.

Basically, I'm trying to say that I think that a ball dropped from 60mm as opposed to 30mm would have a greater force on impact or am I wrong about that? Probably missing something quite straightforward but it's doing my head in. Any help would be appreciated.

FISMA

You are correct that an object dropped from a greater height would exert a greater Force than the same object at a lower height.

I call this the scale problem. The Force on the ground may be measured by a scale.

If you weigh 150lbs and stand on a scale, it reads 150lbs.

If you jump on the scale from a height, it may read 160lbs. The higher you go the greater the reading.

The scale needs to do two things: support your weight AND change your momentum.

As the object falls, each piece of mass, m, loses potential energy in the amount of ΔPE = mgh. Neglecting air resistance and assuming the object to be represented as a point particle, mechanical energy is conserved, and the object gains as much kinetic energy as the lost potential energy.

So the

ΔPE = ΔKE
mgh = 1/2mv²

simplfying

v = √2gh

where everything on the RHS is under the √ sign.

.... be back soon, have to go, sorry! I will finish this later, in a nutshell, use impulse - momentum to solve for F,

Ft = mv

F = vm/t

but don't forget about the weight...

Morbert

actuallylike wrote: »

Hi,
I've seen a few questions related to my problem but not quite like it.

I'm currently trying to run an analysis on solidworks of a ball of a certain mass falling and impacting a stationary wire gauze made out of a certain material. I want to get the deflection of the gauze. Unfortunately, the only simulation package I have only allows me to pick the static force in Newtons acting on the gauze.

Is there any way of measuring what force would act on the gauze from the ball dropped from a certain height? I've gone through all the Equations of Momentum and have the velocity it would strike the gauze, the time it would take to reach it and all that but still drawing a blank.

Obviously F=ma (or mg in this case), but surely a ball dropped from a higher height would increase its force? The equations I found require the distance traveled through the material to get the Newton value but seeing as though this is what I am looking for in the first place, I cannot give that.

Basically, I'm trying to say that I think that a ball dropped from 60mm as opposed to 30mm would have a greater force on impact or am I wrong about that? Probably missing something quite straightforward but it's doing my head in. Any help would be appreciated.

The force exerted will depend on the gauze. You won't be able to derive the force solely from the properties of the falling object. Do you have any properties of the gauze?

actuallylike

Morbert wrote: »

The force exerted will depend on the gauze. You won't be able to derive the force solely from the properties of the falling object. Do you have any properties of the gauze?

Yeah, I have the Young's Modulus and strength and all that of the material it's made of but I want the program to determine the displacement for me so it's annoying the way it says I need that value.

I might just put in the maximum allowable displacement and see what happens. I also have the option of using pressure in a certain direction as opposed to force, would that change things?

Morbert

actuallylike wrote: »

Yeah, I have the Young's Modulus and strength and all that of the material it's made of but I want the program to determine the displacement for me so it's annoying the way it says I need that value.

I might just put in the maximum allowable displacement and see what happens. I also have the option of using pressure in a certain direction as opposed to force, would that change things?

Unfortunately no. The force is not constant with time, so it will be hard to approximate it with a static force or pressure.

Though it might be possible to work it out analytically. I will have a think and get back to you.

ZRelation

Is this for a school project or research/work? If its the latter you may be better off getting your hands on a proper dynamics package like LsDyna or Abaqus.

If its a school thing you might be able to get away with making a few approximations.

Professor_Fink

Morbert wrote: »

Though it might be possible to work it out analytically. I will have a think and get back to you.

I don't see how. The force will depend how long the ball and mesh are in contact, which will depend on what they are made of. A rubber ball will deform more than a more rigid ball of the same mass and so will apply less force but over a longer period of time.

Does the application not allow for impulses?

ZRelation

Professor_Fink wrote: »

Does the application not allow for impulses?

It depends which version of the software the OP has. Some newer, higher end versions have dynamic modelling ability. Should be fairly obvious by the user's manual if the version is capable of dynamic modelling. If it isn't I'd get one that is rather than messing around.

actuallylike

Cheers for the replies lads, I'll try and explain a bit more here. I'm using solidworks 2010. Doing a drop test on it. I have a metallic mesh shape falling and coming into contact with an anvil. Solidworks allows for drop tests using gravity but alas my design is pretty intricate and the memory of the computer won't allow me to test this way so I'm left with the basic simulationexpress option which only allows me to choose fixed points and forces or pressures on other points.

I've fixed the points where the anvil will come in contact and now have to apply either a force or a pressure to the whole mettalic mesh as if it has already come down a fixed amount due to gravity. I just found out there that I know the energy in which it is supposed to hit in Joules so will have a look to see if I can express that in N or Pa using the other factors I have (velocity, mass, etc.).

The problem arises when people start talking about displacements after impact. The mesh will of course bend, it will stop dead at one point when it hits the anvil and I am looking for the displacement around it (ie how much the mesh will deform).

Morbert

Professor_Fink wrote: »

I don't see how. The force will depend how long the ball and mesh are in contact, which will depend on what they are made of. A rubber ball will deform more than a more rigid ball of the same mass and so will apply less force but over a longer period of time.

Does the application not allow for impulses?

I have assumed that the ball is rigid, and that the mesh can be approximated by a spring with a specified spring-constant. Though it's not terribly informative so I didn't post any of it.

Professor_Fink

Morbert wrote: »

I have assumed that the ball is rigid, and that the mesh can be approximated by a spring with a specified spring-constant. Though it's not terribly informative so I didn't post any of it.

That might work, and you could compute the spring constant using a static known force and measuring the deflection. You could check if it was a good model by incrementing the force and looking at a loglog plot of deflection.