Physics 101

[TransformX]

Something I once knew and forgot:

1. You hold out your hand and I put a 1kg package of flour on it, you resist a weight of 1kg.
2. You hold out your hand and I drop a 1kg package of flour on it from 1m height, you resist a weight of...?

What is the weight multiplication by final speed?

Also, since we're already there - Bill "Superfoot" Wallace could kick at an astounding speed of 60mph, what amount of force would one's face be dealing with?

[UtwigMU]

F=m*a

Any object falling from 1m would have a velocity of SQRT(2*s*g) = 4,43m/s

From the time it takes for coming flour to come to a halt or from the way the hand travels while lowering from the impact of the flower you can calculate the decceleration. Let's say hand lowers 3cm, thus a=-327,08m/s2.

Thus force needed to decelerate 1kg of flour falling from 1m in 3cm to a standstill would be 327,08 Newtons which would equal the force of weight of 33,34kg. This is assuming no force is used up in deformation of flour bag or hand. Because it is hard to make assumptions on time needed to decelerate objects and it would be even harder to calculate this from dynamic deformations, it's not easy to calculate.

Just yesterday I was fixing my notebook and service manual said several 1000 g can destroy accelerometer and that dropping motherboard from 6 inches on a hard surface can result in 6000g. This calculation assumes that motherboard would decelerate from velocity of 1,73m/s to a halt in 0.0254mm. Force of 6000g = force of weight of 6000kg.

[Brian Ellis]

If the bag of flour doesn't burst open, in which case you would have an unholy mess and your wife would give you hell!

[Dr Mordrid]

Not to mention the fuel-air explosion if someone's smoking or if there's an open flame nearby

[UtwigMU]

I actually found some article where they studied physics of karate strikes.

http://books.google.si/books?id=9-8EkIhxeK0C&lpg=PA10&ots=GY-I7YUO0m&dq=mae%20geri%20force%20measured&pg=PA8#v=onepage&q&f=false

[TransformX]

Thanks Utwig!
Are you sure about the numbers you wrote above? 1kg after 1m of free fall creates a pressure equivalent to that of over 30 static kg?

[UtwigMU]

It does if you assume it goes to a standstill in 3cm or in time of 135,44ms. If you change distance and time of decceleration force is bigger or smaller. It is not constant and if you check the article on physics of karate punches you also have to account for deformations and accoustics of materials. Furthermore hand is not made of same material as skin, muscles and bones have different properties.

In the article they analyzed board breaking and came to velocity of hand 7-10m/s, time of impact 5ms and deceleration of hand of 3500-4000m/s2

You can calculate roughly this stuff by Force = mass * acceleration (F=m*a). So the shorter time of impact or the shorter deformation or distance travelled means higher acceleration and thus higher force.

You can calculate acceleration on impact from distance of deformation (it's a simplification that doesn't count work done on deforming materials, acoustics...). velocity of impact squared = 2 * acceleration * distance of path (v^2=2as).