View Full Version : Silly Physics Question
According to my DH - according to Mythbusters - a vehicle going 30 mh hitting a stopped car creates exactly the same amount of ... impact force? as a vehicle going 30 mph hitting a vehicle coming at it at 30 mph. My "homemade physics mind" says that sounds ridiculous, 30+30 = 60. Does anyone know the answer to this, ideally including the equation?
At first thought I agreed with you. But upon further consideration, I think I see what they are saying (having recently taken a physics course.) I don't have an exact equation, but it's about each action having an equal reaction. You can't get more reaction back than what force you are applying in the first place. Think of both cars driving towards each other, but with a solid wall in the way. Even if they hit at exactly the same time, they both get 30mph of damage as each comes to a dead stop from that speed. Now, take away the wall. If they hit each other dead on, each car is acting as the wall for the other car. Each car gets back the same force. The total force is 60, but that is divided between the two cars. Does that help?
But ... if one car is going 30 and the other one is stationary, then the total force is 30 but THAT is divided by two cars as well, no?
My vote is for the 60. (but I barely have a mind at all, let alone a physics mind). :)
But ... if one car is going 30 and the other one is stationary, then the total force is 30 but THAT is divided by two cars as well, no?
In that case, I believe, whatever resistance is provided by the other car (gravity plus some friction) will exert a force on the moving car (damaging it) and the remainder of the force will be used to push the stationary car forward. Because a wall can exert a greater force than a stationary car, the wall gives the entire force back to the car (probably not quite 100% but close enough) whereas the stationary car only exerts force to the point where it moves. With the two car (1 stationary) example it wouldn't be a 50/50 split. The non-moving car would move forward, but at less than 30mph. Perhaps closer to 20? It all depends on the size of the car, tires, road conditions etc.
If a car was moving much more slowly and hit a stationary car, there is a velocity that would cause no motion to the second car. Once the force from the velocity exceeds the force of the car's resistance, the car moves.
After much thought, I've decided that you should just never run into another car (or stationary object). There. Problem solved. :~)
Okay.....here's another "critical thinking" question. If its raining out and you have to walk in the rain without an umbrella.......is it best to walk slow or fast? Do you get wetter if your run because you're running into more raindrops???
The definition of force is 1/2(mass)*velocity squared. A given vehicle traveling at 30 miles an hour, no matter what it hits always has the same amount of force to dissipate.
Hitting an immovible object (a wall) all the force has to reflect back upon itself.
Hitting a moveable, but still object, allows some of the force to be dissipated into the other object.
Two objects of equal resisiliance, mass, and velocity are so balanced that they must dissipate their force back to themselves.
In the real world, what makes a head to head crash so horrific is the ability of the more resiliant vehicle to donate some of its force into the other vehicle, making things really rough on the other vehicle.
I think I can see that in an intuitive way, Chord Ata, thanks. The more resistance the "other object" has the more force is directed back at you, up to your total force in the case of a wall, but if the opposing force is actually greater than your own (i.e. you're in a Beetle and the oncoming object is a Suburban), you're going to get all your own force plus some. So to say you're actually getting identical force directed at you to hit a stationary wall vs. an oncoming vehicle isn't technically true, unless the oncoming vehicle happens to be identical to yours in speed and weight and, er, squishiness. simplified, 30+30=60 / 2 =30, but 10+50 =60 / 2 = 30, while still retaining the truth of the equation, is a whole lot worse for the "10". In the case of the wall, it seems that there's a variable missing in the "force" equation, I guess it's called something else as the wall isn't exerting force, exactly, but can have a very variable effect depending on whether it's just a pile of cinderblocks or a rebar-reinforced concrete wall, even at the same weight. Is that "resilience"?
Force = mass * acceleration, and is expressed in SI units in "newtons", which is kilograms * meters per second^2
Momentum = mass * velocity, and is kilograms * meters per second
Kinetic energy = 1/2 * mass * velocity^2, and is expressed in SI units in "joules", which is kilograms * meters^2 * seconds ^2
Know also that momentum is conserved. And energy in a closed system is conserved, though it can change form... And that kinetic energy is conserved in perfectly elastic collisions, and not conserved in inelastic collisions.
Now kick back and think about which things concern you in the problem, and draw a picture or two.
Thank you, Bae. I think the primary concern is this fog shrouding my brain ... >8)
Seriously, I think the hiccup for me was the difference between elastic and inelastic collision. Sort of still is ... how does one measure the potential elasticity (ah, much better than "squishiness", thank you) of an object or put it into the equation? What happens to that kinetic energy if it is not conserved as motion? Heat? motion in the form of molecular or submolecular rearrangement?
Sort of still is ... how does one measure the potential elasticity (ah, much better than "squishiness", thank you) of an object or put it into the equation?
Method 1: Take a supercomputer and a few engineers, and build a finite element model, and collide the items.
Method 2: Take the two items in a lab, ram them together, measure the results :-)
What happens to that kinetic energy if it is not conserved as motion? Heat? motion in the form of molecular or submolecular rearrangement?
Heat, noise, bending/crushing/deformation of materials.
There's a reason physicists like inelastic collisions and perfectly spherical cows, whereas engineers prefer to loudly crash things together :-)
Some discussion here
http://scienceblogs.com/dotphysics/2009/04/mythbusters-crashing-two-moving-cars-or-one.php
and
http://scienceblogs.com/dotphysics/2010/05/mythbusters_energy_explanation.php
In the case of the wall, it seems that there's a variable missing in the "force" equation, I guess it's called something else as the wall isn't exerting force, exactly, but can have a very variable effect depending on whether it's just a pile of cinderblocks or a rebar-reinforced concrete wall, even at the same weight. Is that "resilience"?
That is what I am calling resilience for the purposes of this discussion. Another way to look at it is to examine the constraints on how and where the available kinetic energy is transformed.
Method 1: Take a supercomputer and a few engineers, and build a finite element model, and collide the items.
Method 2: Take the two items in a lab, ram them together, measure the results :-)
Heat, noise, bending/crushing/deformation of materials.
There's a reason physicists like inelastic collisions and perfectly spherical cows, whereas engineers prefer to loudly crash things together :-)
NOISE! Oh wow ... another open window. It Never occurred to me to wonder where all that sound energy comes from when two things collide. :doh:
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