View Full Version : need physics lesson

06-29-2001, 05:48 AM
This is off topic of martial arts, but an important question for me, so I put it here.

All of you physics majors,

My understanding of energy transfer is woefully inadequate, as can be seen from some of my posts. :rolleyes: So, I need help in getting the basics down.

Two cars, one going 100 mph, the other 200 mph.(I know, crude and rudimentary, sort of like me :cool: )
Both hit the imaginary brick wall.
Aside from the death and destruction, not to mention the hefty insurance hikes, how much harder will the 200 mph car hit the wall?
Only twice as hard?
How much more energy will it take to push the second car twice as fast as the first?

I realize this is simpleton stuff, but I have gotten off on the wrong track somewhere and I need to get back on. (I’ve fallen down and can’t get back up) :eek:

I only ask that you answer in english, I won’t get the quantum physics thing.
Thanks in advance. :)

kungfu cowboy
06-29-2001, 06:10 AM
Do the cars start from any particular speed from any particular distance? Is it an elastic or inelastic collision? Any more info?

E Pluribus Rectum

06-29-2001, 06:14 AM
how much harder will the 200 mph car hit the wall?

4x as hard, assuming equal masses.

How much more energy will it take to push the second car twice as fast as the first?

Not enough information.

06-29-2001, 06:21 AM
Kungfu cowboy, Braden,

I'm trying for a solid brick wall, dead stop. (pun)

You've already gone past me with the starting of the cars, I was thinking they're 'in motion'?
(Make any sense?)

Thanks for replying. :)

kungfu cowboy
06-29-2001, 06:21 AM
Yeah, were you given the mass of each car? That would be just grand!

E Pluribus Rectum

kungfu cowboy
06-29-2001, 06:25 AM
Ahhhhhhh, they DON'T stop, because it's an IMAGINARY wall, right?

E Pluribus Rectum

06-29-2001, 06:26 AM
I'd like to see a couple of Buicks do the smash up! :)

For simplicity, both would be the same.

You'll have to guess on the weight of a Buick.


"We forge our bodies in
the fire of our will." Han
from 'Enter the Dragon'

kungfu cowboy
06-29-2001, 07:00 AM
Is there a gaggle of drunk teens driving? Their weight must be included also.

E Pluribus Rectum

06-29-2001, 02:06 PM
If I had kept at it years ago I could be so much further now, Humorous meloncholly of the Self.

You are thinking Fast.

How does Faster work...try these concepts:

faster=more force of impact

Weight of cars influences force of impact

force=mass x velocity

A 500 pound car traveling 200 mph then hitting a wall might not hit the wall with the same force as an eightteen wheeler hitting the wall at 100 mph.

The same weight? 2 tons x 100 mph = 200 something or others

2 tons x 200 mph = 400 something or others

2 tons x 300 mph = 600 something or others

2 tons x 25 mph = 50 something or others.

differing weight

1 ton x 200 mph = 200 something or others

1/2 ton x 200 mph = 100 something or others

1/4 ton x 200 mph = 50 something or others

Now, the wall...

A wall a mile thick will get damaged differently that a wall six inches thick.

A wall of bricks will get damaged differently than a wall of steel, or a wall of feathers or a wall of foam.

Some would get dented. Some would break differently.

Also how wide the wall...
a wall only as big as the vehicle gets obliterated depending on the force of impact and the make-up (density~) of the vehicle. Awall a mile long might get a break in the wall, but still stand, yet get knocked down if the vehicle is going slow enough...You can put a hole in a paper towel with whipping speed without really disturbing the rest of the towel. If you go slow enough you can affect the whole towel. Also if the towel is wet or dry matters with the area of force(pin head size, fingertip size, fist nuckles size, palm heel size,knee size...)...

force depends on how heavy~ and how fast.

Damage depends on what material both are made of, how sturdy is the standing object(object to be hit) and the force og the object doing the hitting~.

Very some such,perhaps might have been, likely say some, some not.

06-29-2001, 05:33 PM

Yes, I know what you mean.

Have you seen the swung axe buried into the stump of tree trunk setting on a bunch of inverted paper cups?

Or, more closer to this forum, the sledge hammer swung down onto the stack of cement slabs supported by the martial arts student/instructor, usually suspended between two chairs at his/her head and feet?

With all these tricks and variables, do you think it is possible to truly measure the impact and effectiveness of a punch or kick with an external device? (something mechanical, without actually smacking some poor test fool upside the head)

06-29-2001, 06:32 PM
F = ma NOT F = mv

He's more interested in the kinetic energy equation though. Which is where I got my answer from.

06-30-2001, 11:49 PM
F=MA^2 describes accelleration / decelleration.

Assume both cars are the same weight (2 tons) so we can just leave mass at M as it's constant. We are computing force. So what is the A?

That would be the speed change (either 100 to 0 or 200 to 0) over the time it took to get the speed change.

The stop is never "instant". If it were, force is infinate. Feel free to plug in the numbers you see fit, force in this instance will be in poundseconds.

The simpler F=MV describes potential energy in a given moving object. M is constant, so if we simplify 100mph and 200mph as V and V*2, we find that F=MV and F=2MV so the potential force doubles.

This all varies on what you are trying to measure. But remember (EVERYONE FORGETS THIS) that when measuring F-MA^2, IT HAS NOTHING TO DO WITH THE ACCELLERATION RATE ON THE CAR COMING UP TO SPEED!!!. It has to do with the *decelleration* rate at impact.

07-01-2001, 02:44 PM
Please stop:

in terms of simple energetic consideration, you should use the simple formula:

Kinetic Energy = 1/2 * Weight * speed * speed

(using this formula, you assume that you consider the cars as the same as a point having the weight of the car)

ANd please, no : "same weight, twice faster, so twice the impact"....

07-01-2001, 06:39 PM
Oh, one of those "Here's my 2 cents, now no one repond" posts?

07-01-2001, 09:50 PM
If you'll check, Davy, I used that equation in the second reply to this thread.

07-02-2001, 08:19 AM
Thanks to all, for the answers to my questions. I (thought) knew the second car would hit 4x's as hard, but was using the wrong formula in explaining it.

My point being and as was explained to me, if you speed up your strikes (and are rooted) they'll be more powerful on impact.

Especially if you catch 'em coming in. :)

07-02-2001, 07:00 PM
Yea, that's where I thought you were going......

Netonian pointal mass physics (used for determining the force of two striking pointal masses in a vacuum, and applied to things like billiard balls and (in overly simple terms) car wrecks) has little if anything to do with the force of a punch or kick.

This has already been debated ad-nauseum. The mathematics to even attempt to describe the biomechanics involved is beyond the scope of any light tryst into the topic.

07-02-2001, 09:11 PM
remo - Jerry said it. But it's come up enough that perhaps it bears repeating. You can NOT model the physics of striking with eleventh grade physics. You can't even begin to, or give a cursory, approximate model.

07-03-2001, 08:45 AM
Jerry, Braden,

When going into minute detail, yes, I realize you both are correct. But, that hasn't stopped others from trying. :rolleyes:

Goes back to my other question: Is there a way of measuring and then improving your punches and kicks?

You probably are aware of the studies done on joint and bone breakage during the middle of last century. Some of this information is in the book "Medical Implications of Karate Blows". I believe they used weights, as in 'per foot pounds'.

The Samurai praticed cutting with their swords on cadavers. (Bodies of the executed, if I remember the story correctly.)

I'm far less adventurous, I use a focus mitt. And, my partner tells me if the punch is harder, outside, inside, etc... But, what about those times you're by yourself, has anyone found a device that can pinch hit for a workout partner? :confused:

07-03-2001, 10:44 PM
Not in minute detail, rather in even the most gross terms. Newtonian physics does not apply to striking.

Yes, there are many ways to test and get feed back on your punch (depending on what kind of force you are trying to get).

For the "pushing" force, a good heacy bag works well. See how far you can move it with a strike, try to improve that distance.

There are also devices that measure force applied, but I have not seen a cheap one (OTOH, they show up in arcades, how expensive can they be).

Some people use boards or bricks to work on other energies (breaking a far board while leaving a near one intact and such).

07-04-2001, 09:31 AM
Shouldn't all those equations be using mass instead of weight, and velocity instead of speed?

Also, what assumptions are made regarding friction?

What we do in life echoes in Eternity

07-29-2001, 05:58 AM
half m vee squared.

Since vee = 2x
vee squared = 4

therefore, 4 times :)

08-06-2001, 10:29 PM
Not exactly...

2*x^2 does not = (2x)^2

100^2 = 10000
200^2 = 40000

Therefore the impact is 4x...

Swimming is not a sport, it's something you do to keep from drowning.
Fighting is not a sport, it's something you do to keep from dying.