Some boffin work required

[etch]

Okay this is an unusual one.

We have a casting that i want to see what happens when i impact it. Now my plan isnt complicated (just as well), but i plan to use an steel 'H' beam and drop it from a height. I want to measure simply (no need to include air resistance, wind drift and the like );

a) the velocity at impact
B) Force on impact

Now my caluclations are

Mass m = steel beam = 25kgs
Gravity g = 10 m/s-1
height h = 5 m

therefore Potential energy stored would be

Pe= m.g.h
=1250 Joules

Kenetic Energy at impact is 1/2mv2
and assuming that all Pe = Ke then.

1250 = 1/2(25)v2
= 10ms

this i think is the velocity upon impact. Any idea what that is in real money i.e Miles per hour.

Now the force upon impact i thought would just be

F=MA,

but the more i think about it, it surely depends on the dent it leaves in the casting. or at very least the height that it is dropped from.

I know i ask the strange one, but hey it keeps it intresting

 

[tomwalz]

Also what impacts the casting. Does the I beam hit flat, a corner of the I beam.

It depends on what the casting is sittng on when it is hit.

We built a drop weight impact tester to test carbide tipped brazed tools. 20# weight in a tube. Held up by a rope. Ruler on the side to measure how far it falls. The tube kept it falling straight.

We were looking a flat impact so we a UHMW replacable botton as the impact object.

Tom

 

[Hush]

Your 10 m/s is right, about 22 mph.

F=MA is the right formula but the A you need is the deceleration from 10 m/s to 0. I.E. it depends on the height dropped, the depth of the dents, plus how many milliseconds it took to create the dents. I recall a thread in here somewhere that provides better insight.

 

[etch]

thanks hush i knew i was on the right track. It just a simple test to see if a aluminium casting will stand up to this type of impact. The only problem i got was converting 10m/s into mph, i got a really stoopid figure.

so i take it then it would be near impossible to calculate impact if you could not accurately determin length of impact.


no matter 22 mph will do fine.

 

[Hush]

I'm sure there's some way to do it with strain energy or spring constants or how high the beam bounces. I just can't think of how to approach it off the top of my head.

 

[bilge]

Etch,

The problem is not that simple. What you have is a question of impact. In such, the force varies with the time, going from zero at the moment of contact, through a maximum, and finally back to zero. What's termed the "impulse" of the impact is the area under that curve. This is equal to the change in momentum of the falling body.

A conversion mnemonic: The nuber of keys on a piano is 88. 88 feet per second is 60 miles per hour.