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Today my usual chemestry teacher was sick so some guy took over the class. Apparently he was a physics teacher (he saw some guy in the hallway, and then laughed and said, "Man they pulled me away from physics and now I'm teaching chemestry." So I took the liberty to ask him a few questions when it was break time:

"Hi can I ask you a physics question?"

"Sure go ahead"

"Ok suppose I have two cars, one weighs less than one another. Each areodynamically equal."

"Um okay, so you mean 850 kg car, and 1500 kg car. So your Tercel vs my Mercedies."

"Uh sure but what I wanted to ask was, if each car had a set speed for them already like 70 km/h (40 mph). Which car would stop first?"

"Oh the Tercel for sure, F=ma it takes less force to stop it since its proportional to the mass. You don't need as much braking power as my heavy Mercerdies."

"oh no no what I meant was, if both cars were to coast."

"Coast? What does coasting mean?" (lol i knew this was going to happen that's why I didn't want to bring it up but I had to try :D )

"Well what i meant was, the two cars they are being freelanced like putting it in neutral. Going until they are stopped by areodynamic drag and friction."

"But the cars, once they go they go forever. Newton's first law states that unless a force stops upon them they will keep going and going. But that's in a perfect world you see, friction will evenually slow the cars down."

"Ya so which one would slow down first?"

"The Tercel of course, because its lighter. It needs less force for it to stop it."

"So the heavier car would go farther?"

"Yes."

Unsatisfied with my answer I went back and sat down still thinking about it. Wondering why psyshack's Accord coasted farther than his Civic. I thought his lighter Civic would cut through the air with his areodynamic body but I guess for the speeds we coast friction plays a more important role when trying to coast for long seconds.

I started giving the whatever additute since I felt that the last conversation I had didn't really enlightened me the way I hoped it would. Until he started coming towards me and then sat next to me. Peeking at my Electrovaya Tablet PC, he asked, "So how does this thing work?"

"Well since you gave me a physics lession I might as well give you a history about Tablet PC's"

(skip this part to the ***************** if you already know what a Tablet PC is, or if your just not into the computer lingo and want to skip to the main topic.)

"Wow look at this thing (the screen was standing on the keyboard), holy **** how come I never see one of these before? (No really I don't think I ever met anyone that knew what my kind of computer was. Thinking it was something so new that just came out.) Show me some tricks!"

"Ok ( I took the screen out of the keyboard and treated it like a huge notepad.) Well as you can see I can draw and such, here I'm going to show you how to convert handwriting into text. Here are some colors, my notes, and then I got lazy and started typing again. But hey if you ever get a chemical formula or some math then I can easily dot it down right there. Or if a professor releases a PDF file for the whole class I can easily edit it in this program. I like how the screen only reacts to your pen so you can rest your palm on the screen like real writing paper. Now I'm going to erase everything I just drew here with my eraser (flips the pen)."

"Wow this thing is so neat eh? How come I never heard of it if you say this technology is four years old? I never see it in Future Shop or major computer stores!"

"Well like the average consumer would say, why would I get a tablet pc when i can get a much more powerful laptop for the same price? I mean c'mon to sacraface performance (lol I think I heard this one before in the automotive industry ;) ) so you can write on the screen? People thought that was the worse idea ever so it never made it big. To make it worse the prices are usually a premium just to own a tablet pc so it never really caught on you see. But hey I'm one of the very few that took the plunge and I must say I'm pretty impressed. I mean I can keep everything that I learned into one convinent package and even store my university life in here."

"Yeah in university, every new semester I'd get so much notes I throw it all away. It was too much... it was easier to start fresh."

"Yep I've heard that one before, that's why I got this so I can keep my notes. I don't want to throw away what I could use again in the future. Plus I think carrying just this and a few textbooks is way better than textbook plus a bunch of binders."

"You must paid a lot for your computer right?"

"Well... I got mine ebay, for around 1000 it used to cost 3000 back two years ago."

"Holy ****... and it has a hard drive?"

"Yeah, a 80 gig one, its windows and all of course. See the Intel logo? The same chip used in normal laptops, its a full fledged computer. See the logo Electrovaya? That's a Canadian battery company, they used thier best technologies into this tablet pc. That's why I can almost squeeze 8 hours of battery life if I optimize it correctly. (Although I average 5 to 6, 8 is like 60 mpg for me ;)) And look at how light it is (3.5 lbs), I mean I got other stuff to carry to so every bit helps!"

"Very nice, thanks for showing it to me."

*********************************************************


He began walking away when I had to ask one more question.

"Hey wait, ok one more question about the two cars?"

He kinda gave me the "Ok here we go one more time" look and then said,

"Ok what you need?"

"Ok the two cars, if they were going at that speed ok right at that moment. And I put both cars in neutral, then which one will travel farther?"

"The Mercedies because you need a lot more force just to stop the car."

"So even if the smaller car is more areodynamic..."

"No it has nothing to do with areodynamics. Didn't you take physics before? Haven't you forgotten about F=mu? In a perfect world if nothing is going to stop those two cars, they will go forever. But we have friction in our roads so evenually that will slow the cars down and stop. Because friction is opposing the force that the car has currently until all opposing forces are canceled (or equal). However the earth's friction has a limit, and can slow down a car to a certain point before the acceleration forces overcome the earth's friction.

Now the Mercedies can travel farther because it has more mass. More force for earth to fight agaist before winning the long battle of stopping the heavy Mercedies. Since Earth has a friction limit, the Mercedies can win this battle for a very long time (ala coasting heaven) before starting to lose some sense in speed. The lighter car however, is not generating as much force as the heavier car since remember F=MA. So its easier for Earth to slow down a Tercel then a Mercedies."


"OHHHHHHH okay thank you. And F=Mu and u is equal to the coefficent of drag right?"

He walked away, and the bell rang. At least I was satisfied with my answers now :cool:

You take chem after physics up there in the Northern reaches?

...well, he's right, not with that "forces" argument he used. I take issue with his saying the the Earth is "generating" some force. And with saying that the car has its own force of some sort while coasting. Friction is the ONLY force at work here.

The Earth ain't generatin' nuthin'- gravity is a fundamental characteristic of matter. And gravity couples with mass to create the normal force on a surface, friction is directly proportional to normal force, so the friction force slowing the vehicles is proportionally the same. Mass falls out of the force equation- all objects of any mass with the same coefficient of friction will coast the same distance, in this view.

You're right- friction force, at this level we're dealing with, is basically a constant- a "limited amount." There's no battle of forces, though, on a level road. Friction wins by default. It's a matter of solving for F = (de)acceleration, which will be constant, and then counting time until velocity reaches zero, as you said.

The heavier car will coast further in the real world, of couse. But it is more properly looked at via momentum. It's greater momentum (a vector) allows it to plow through small bumps and imperfections (macroscopic cases of friction, really) with less of a penalty- relatively less of its remaining momentum is lost to bumps that jounce it around off-axis, (causing a change in momentum which takes away from momentum in the desired direction) and more if it remains directed in the desired axis of movement.

Of course, you can relate the forces involved in jouncing around to a change of momentum by integrating all the individual force vectors over time. And you can take the derivative of the car's momentum and get its time-varying "force." But that's drudge physics, not fun physics.

Putting aero back in... well, drag force goes with velocity, so now you have a differential equation where before it was simplified to an algebraic equation.

Yes, ther Mercedes will coast further, but remember that you had to invest more energy to get the mercedes up to speed than the tercel. Since there are conversion losses from fuel to kinetic energy, the mercedes must be less efficient than the tercel, even though it will coast further.

it would be sweet if there was an ideal car that allows very little weight when at a stop and then as speed builds up weight would increase to increase coasting ability. As one presses the brakes or accelerator pedals weight levels would return to normal and then heavy it up again as you coast.

Dream on ;) good description gonavy although yes there's physics up there but I did not take it this semester. It was just a sub filling in for a usual teacher that was sick on that day. Actually I have taken physics 11 twice and 12 twice and let me tell you, drabble physics ain't fun! But still important nevertheless.

Anyone with an instant mpg display knows that ten seconds of acceleration from zero to fifty-something takes a lot of gas. :eek:

Hum ! Now how did that go again?

Loved this. Thank you Phil, and hope to see more!

This reminded me of the Jeep Cherokee I had for a little while once and after driving it a month realized it was a death trap--you simply could not count on the brakes to stop it in an emergency, not because the brakes were bad but because it is like a "light truck" and it is inherently harder to stop--it would just skid a long ways very easily. As opposed to the heavier Mercedes diesel I had before that, which I could always count on to stop as it was a heavier car. Also sort of paradoxical, but in a different way, huh.

I like the way the theory was explained for your question though and it does make sense that friction would have a greater effect on a lighter vehicle more than on a heavier vehicle, due to the difference in their respective momentums. Sometimes a good way to test an "armchair scenario" is to exaggerate the controlling factor as much as possible--in this case weight--so imagine instead of your Civic you had a child's remote-controlled toy car going same speed, neck and neck with the other "real" car. Now remove all power applied to each and let them coast. One would certainly expect the asphalt to slow down the toy car much sooner than the real car.

P.S. we got rid of the Jeep quick--this was our first SUV-type vehicle and we just did NOT feel safe in it. Could easily see its tendancy to flip over if turning too quickly or impusively.

PPS By the way, you may be aware that physics people and chemistry people explain the world from vastly different viewpoints, so I would find it completely fascinating to see how a physics professor would teach a chemistry class. ;)

offtopic...

It was Dirac who said of the relativistic wave equation that it '...explains all of chemistry and most of physics...'

but no bias here :)

Without physics there is no fundamental explanation for chemistry. But without chemistry there is no life. And without life capable of thinking about it, is there any physics?

Now go and have a philosophy teacher ponder that one from their viewpoint... isn't it neat how it all wraps back on one another?

ontopic:
You are 100% correct about the RC car vs civic. We did that the other week, on a smaller scale, at my son's pinewood derby. I showed the boys WHY weight is important by sending a 2oz car down with a 5oz car, watching the lighter one bounce all over the track while the heavier one went much straighter.

Loved this. Thank you Phil, and hope to see more!

my next article is to convience normal drivers to hypermile. But comments like that make me want to finish it even more since I'm lazy to boot ~~

although gonavy is right all science is equally important as one always lead to one another.

Hi Phil:

___Another way to look at it is as a bullet. The higher mass ones even with a larger frontal area (~ the same Cd) need more powder energy to leave the barrel at the same velocity as a lighter one but travel further and more accurately in wind do to their inertia via its higher kinetic. The higher drag from the larger frontal area has an effect but the heavier ones do what they do due to the kinetic, nothing else.

___That being said, the Insight was not the coaster of the Corolla and the Corolla is not nearly the coaster as that of the Accord. At speeds > 40 mph however, the Ranger’s brick like face and Cd nearing .50 to the front as well as its extremely high RR ContiTrak’s are extremely detrimental to its glide distances even though it is within 100 #’s of the Accord.

___GoNavy, your last post on Physics vs. Chemistry and which comes first gave me a headache ;)

___Good Luck

___Wayne

lol.

chemistry and physics- its akin to chickens and eggs... but with that comes chicken waste, which leads to CNG and biodiesel!!!

finally we won't clog our pipes with fat.

A teacher recommeneded that you have a fat can so you can store any garbage fat into there instead of the sink. Since fat at cold temperatures will make it very hard and will clog up your pipes. Her husband didn't listen to her and always dumped the fat down the drain and in the winter time he had to unclogged the pipes. It was very nasty having to deal with cold grease in the winter, and now that fat can, can be your very own biodiesel :)