Force on impact calculation.

[jackw72]

Hi,

Anyone got any idea what calculation I would use to calculate the force a human being would have whilst sprinting 100m.

I.e. how hard would a man running 8 meters per second hit another object.

I have tried the kinetic energy formula but am wondering if anybody knows what calculation would be used for this?

Thanks,

Jack
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[at106]

Are you trying to find out how hard he would hit another object if he ran into it?

Do you have any other details like his mass? Or the the time taken?

You should be able to use a suvat equation to find the acceleration. And then f=ma to find the force.

[UnknownStuntm...]

Let me guess, you're a television media executive, been tasked with working on the advertising placement project at the Olympics, and you're exploring your creative opportunity options for the finish to the main event?

[Stevey]

For Usain Bolt, when he set the world record, it would be 205 N (3s.f.)

[D O G]

What you are trying to work out is the decellerative force the object would exert on the man.

All you need to decide is over what distance the man would be brought to rest. There is always some 'squish' when a body (of anything) hits another body. In this case it would be the human flesh and potentially skeleton compressing, and some flex in the object.

When you have determined this distance, you could extrapolate the time taken to achieve the stop by taking an average speed for the distance at 4m/s (assuming a constant deceleration).

Then you will know the deceleration in ms^2, and, together with your assumption of the individual's mass you will be able to calculate the average force exerted.

That's how I woud do it.

Problem is the 1st part - I'm sure there will be research kicking about.

[jackw72]

Thanks all for your replies (even stuntman which confused me more than anything).

Mass I am looking at 90kg, as previously said a speed of 8.6 meters per second.

Yes I am looking at how much force a man would impact on something by running at 8.6 mps. Time take 11.6 seconds over 100m. I made the calculation and i was running at 8.6 meters per second.

If I find the acceleration (using suvat) and the force (using f=ma) what do I do from there?

Thanks again,

Jack
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[smegballs]

[jackw72]

Whatever is easier to calculate I suppose stopping dead would be easiest.

Though if I just want to find the potential energy that a man could impact with does it matter about deceleration?

I guess potential energy is what I want. To be able to say that a 90kg mass moving at 8.6 meters per second hits with XX force.

Thanks,

Jack
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[D O G]

He cannot stop dead - that would require infinite force.

If you want to know the kinetic energy a 90kg man moving at 8.6m/s has, that is something different.

This site tells you that it is 3.3kJ.

[jackw72]

Ah that is the one that I want.

So if I wanted to relate that amount of kilojoules to an actual measurement of weight as if to say subject 1 would hit with the force of 10 tonnes or a small car doing 20 mph. How would I convert the kj to tonnes or something along those lines?

Thanks
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[D O G]

Just use that site I linked, you can change the variables.

Now you know that the energy of the running man is 3.3kJ, fix that value and alter the mass.

In your case of a 10 tonne mass, from that site, if it were travelling at .8m/s it would have the same energy as the running man.

[smegballs]

To damage something you need to apply work. Work is another word for energy. In an impact scenario surely the impact energy is more important than the force? You don't measure bullets by the magnitude of the force that accelerates them, you look at their muzzle energy.

Work done = Force x distance.

If know the energy of the running man, and how far he moves the thing he collides with, then you can get the force?

I'm trying to work out whether "hit with the force of" actually means force (as in newtons) or its a semantic issue that means something else.

[ThoughtContro...]

What's he hitting and is it static?

Running at 3 m/s into a 50 tonne train travelling towards you at 50 m/s would involve more Newtons force than running into a static wall
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[jackw72]

Ok guys let me put a scenario out but it is kinda complex.

I have just started playing American football and did a test 100m at said speed and weight. I wondered what sort of impact me running into someone stationary at that speed would be equal to.

The faster a mass moves the more force it gets (is there a formula for working out how much the mass increases as it gets faster?)

So lets say I am facing a man who weighs 80kg who isn't moving running towards him at 8.6 meters per second at 90k with a 3.3 kj energy.

I think working out the newtons force would be best considering there is no way to tell how far a man @ 80kg would move in this equation without going into masses of detail.
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[D O G]

[Frost]

How much force gets imparted depends on contact time, which bits of the mass contact etc etc. But assuming a full contact that imparts all of the energy, and assuming that you both weigh the same amount then your energy will accelerate him to the same speed that you were once going and cause you to come to a complete halt.
So the faster you go the further and faster you will push him. If you weigh more than him you will impart the same amount of energy to him, but retain some of your own momentum.
The actual numbers involved would depends on how hard he and you are, which will determine the amount of squish and therefore acceleration speeds. e.g strap a pillow to your front and you'll put basically the same energy into him but more slowly so the G forces will be lower even though the energy is the same.

So basically when you ask about how much energy you'd hit him with that's ambiguous as to wether you mean impact force (G) or your energy (Momentum)

[jackw72]

[ThoughtContro...]

[el_oso]

it would probably be easier and more useful to calculate momentum.
in most/all sports force is not important. what is important is power or the application of force.
momentum, force, power, work and energy are all related by reasonably simple relationships.
momentum = mass x velocity. which is probably what you are thinking of, not mass increasing with velocity. mass only increases with velocity near the speed of light 3x10^8m/s and i'm pretty sure you can't run that fast. the relationship is one of the most know equations - E = mc^2
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[mickfulton]

If a runner stops dead from 8.6 m/s in 0.01s then the deceleration would be 8.6/0.01= 860m/s/s


The Force required to do this is F=ma ......... 90kg * 860= X N

Hope this helps ........you need to know the time it takes to stop the runner and this tells you the required force to stop them, or the force the give to the thing stopping them.

It gets more complicated if you take in to account collisions (inelastic or otherwise), so don't bother


Force is something which changes the shape, speed or direction....so if the runner is at constant speed then effectively he is experiencing NO FORCE (i.e. balanced forces) in the horizontal plane.

Maybe try reading a basic newtonian mechanics book
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[Paulington]

Simple physics. In all these calculations I am talking about collisions and everything happening in 'Physicsland', a magical place where no drag forces exist and in all collisions whether elastic or inelastic transfer of energy is total and 100% efficient.

Let's say the man is an average weight of 70kg and that he is running at 8ms^-1.

Ke = 0.5mv^2

Which is calculated as thus:

0.5 x [70x(8^2)] = 2240 Joules.

That means that if that man were to hit an object he would impart 2240 joules of energy into the object.

F=ma and any of the SUVAT equations are NOT THE CORRECT WAY to calculate this as has been done above as you have asked how much force a running man would have, that is the wrong terminology.

A running man has kinetic energy which he has gained from chemical energy in his muscles. The kinetic energy you have referred to is the 'force' but it is better called 'energy'.

The best way of determining the force in Newtons that the runner hitting the object has imparted is using the equations of motion. To do this you would have to use the equations of motion, more specifically 'v = u+at' and 'F=ma'.

Firstly, you would have to know the speed the runner was doing the INSTANT before he hit the wall, that is U, V is the final speed, i.e. 0ms^-1 and T is the amount of time it took the runner to go from max speed to a complete stop after hitting the wall, around 0.1-0.4 seconds.

Then, knowing 'a', you can plug that in to F=ma where a is the answer from the equation above and m is the mass of the man in question.

Or in one single equation:

F = m[(v-u)/t].

This would give you the force in Newtons that the runner applied to the object he hit with his body.
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