Calc force from mass striking an object
Calc force from mass striking an object
(OP)
Hi all.
This one's simple for you dynamic types.
I have a mass, and its distance to a fixed object. This mass is subjected to an acceleration that varies in time, from 0 to a peak back to 0, triangular pulse.
How do I calculate the force exerted on the object?
I do not have any dynamics texts here at home.
Any help would be appreciated, this is for evaluation of a SS bracket on a train, not a school assignment...
Thanks in advance.
tg
This one's simple for you dynamic types.
I have a mass, and its distance to a fixed object. This mass is subjected to an acceleration that varies in time, from 0 to a peak back to 0, triangular pulse.
How do I calculate the force exerted on the object?
I do not have any dynamics texts here at home.
Any help would be appreciated, this is for evaluation of a SS bracket on a train, not a school assignment...
Thanks in advance.
tg





RE: Calc force from mass striking an object
In the latter case, there is no closed-form solution, since the force is a function of how long or how far it takes to complete the collision.
TTFN
RE: Calc force from mass striking an object
RE: Calc force from mass striking an object
RE: Calc force from mass striking an object
Much depends on the shape and material properties of the objects. Questions that can't be answered by kinematic equations:
http://www.EsoxRepublic.com
RE: Calc force from mass striking an object
TTFN
RE: Calc force from mass striking an object
It sounds like you might be describing a pile driver. If so then you are looking for an "average force of blow" Which =(W*S)/d where W is the weight of the pile driver; S is (distance that the weight falls plus the distance the pile is driven) and d is the distance the pile is driven. The impact force is not accurately known, thus "average force of blow" is the solvable term. This, of course neglects losses due to dissipated heat and strain energy.
RE: Calc force from mass striking an object
Your equation results in units of mass per time squared. Doesn't seem to have any physically possible interpretation.
Trainguy,
Not nearly enough information to solve the problem.
A mass subjected to a time-variant acceleration implies a time-variant force as an external input, as noted by IRstuff.
Differentiate the acceleration vs time curve twice wrt time to arrive at position vs time. Does the mass close the distance to the fixed object before or after the input force drops back to zero?
In either case, the force on the fixed object is not constant wrt time. It is depentant upon the elastiscity of the mass and fixed object. How long does it take to absorb the energy? This will define the deceleration rate of the collision. If the input pulse falls to zero before the start of the collision, then the average force on the object is simply F=ma. If the input pulse is still driving durring the collision, you need to add that force in as well.
As noted by The Tic, it seems as if this problem could be better solved by looking at energy.
RE: Calc force from mass striking an object
The only thing that is certain is that the impulse (i.e., the area under the force time curve is equal to the change of momentum by Newton's second law, viz.
mv=area under ma-time curve= impulse
Dividing by m the both sides of the equation , the equation becomes
v=area under the acc-time curve. If the assumption that the pulse is triangular and you call the peak acc A,and the impulse time,T, then the area under the Acc-time curve is
AT/2=v.
The impulse time, T, is a function of the wall/mass material and is not easily determined.
RE: Calc force from mass striking an object
I admit that my pile driver analogy doesn't exactly describe his problem as stated but as for not having any "physically possible interpretation", maybe you should have directed that criticism toward Machinery Handbook so that they can remove their discussion and formulas dealing with pile drivers from their book.
RE: Calc force from mass striking an object
I am wondering where in Machinery's Handbook they discuss pile drivers...I thought I knew that book pretty well. There are so many things in it it's easy to miss something.
RE: Calc force from mass striking an object
Re. Machinerys Handbook.
The discussion of pile drivers can be found in the MECHANICS section under the subtitle "Force of a Blow".
RE: Calc force from mass striking an object
By the way, I was talking about a railway passenger impacting a fixed table on a railway car during a crash. The objective is to determine whether or not a bolted table bracket will fail or not for a certain crash pulse.
Cheers, and happy holidays to all.
PS:
Fred Garvin,
Are you THE Fred Garvin, from SNL? Welcome back... If not, please disregard this.
tg
RE: Calc force from mass striking an object
You stated that the acceleration was a triangular pulse, so the force would be, for no energy absorbed by anything, a similar triangular pulse. The maximum acceleration would provide a maximum force. The integral of the either would be equivalent to the kinetic energy of course. Now, the problem is whether or not there was energy conversion due to deformation etc. As you seem to be interested in a shear problem, then solve for the load as you don't need an exact answer but one with a sufficient safety factor. If it deals with litigation, and you need to know if you designed with a sufficient safety factor, you will need to do real world testing to demonstrate safe design. As there would be energy absorbed by materials in the impact, the ideal values would yield a higher value than in the real world.