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i think i'm physics dumb... warning:almost off-topic(10 posts)

i think i'm physics dumb... warning:almost off-topicFrith
Dec 20, 2002 7:43 AM
OK so i remember that according to Newton if I drop 2 objects of different weights they will land at the same time. They are both affected equally by gravitational pull regardless of weight... that seems simple to me.
I was watching one of those "junk-yard wars" type shows. The mission was to use the available materials to build the best soap box racer.
One team went to work finding materials with the best aero properties and that made sense to me. The other team decided to put a big tank on the back of their racer and fill it with water. Their idea was that they would be faster to get down the hill because of the weight. They would release the water as they approached the flat section. I didn't stick around to see if they won. But the whole idea seemed to go against all physics I know (all the physics I know can be obtained by avoiding all non-mandatory physics classes in the public education system :-)
so what gives here?
oh and for the topic police: I get to the top of the hill on my bike and discover a rest station where I can take as many full water bottles as I can carry. Should I take the maximum and then deposit them at the bottom of the hill?
All about inertia/momentumTJeanloz
Dec 20, 2002 7:52 AM
1. Junkyard Wars do not happen in a vacuum, where Newton's laws work.

There are a number of reasons that a heavier bike (or Soap Box Derby) racer will go faster than a lighter one. What it really comes down to is momentum. If we assume that the frontal area of both racers is the same, the force of the air resistence on each of them is the same. But the force required to stop a 1 ton object that is in motion is much greater than the force required to stop a 1 gram object. Thus, the air resistence, which is the same for both racers, has a much greater impact on the speed of the lighter racer. This applies not only to air resistence, but also the friction in the bearings, wheels, and anything else that tries to slow down the vehicle. The downside to having a very heavy vehicle is getting it moving in the first place, but once that's done, it will not be so effected by the various frictions.
take on the waterDougSloan
Dec 20, 2002 8:02 AM
Take on the water at the top of the hill. More weight, if it does not increase air drag, will make you a little faster down the hill.

There is a 155 mile race near here that essentially goes from sea level to 9,300 feet elevation and back. About 60 miles from the finish on the way back, I have a full Camelback waiting for me. I suck on it while coasting down from 5,500 feet, which also works well to rehydrate before hitting 100 degree temps in the valley.

You can run the numbers here to evaluate the additional weight's effect on climbing or descending (a negative climb): http://www.analyticcycling.com

Doug
re: Newton apples and feathersChen2
Dec 20, 2002 8:02 AM
Newton was a smart guy but his law disregards friction. Friction is why a feather will not fall as fast as an apple or a bicycle. In a vacuum it would.
Agree with above, but also:floatch
Dec 20, 2002 8:03 AM
pssst... I love that show!
Anyway, I agree that Newton's Law only applies to objects falling in a vacuum. The above post about inertia and wind/bearing resistance is also correct. The part I don't understand is the idea of dropping the water weight near the bottom of the hill. If anything, you'd want to start with a light racer that would pick up speed easily due to lesser initial momentum, and then get heavier as it gained momentum, to help carry it's speed over the flat section. Of course, this is probably impossible.
As for the water/hill thing, if you're rolling downhill without pedalling, then the extra weight should help you maintain your speed longer on the flats, due to increased inertia. However, as soon as you get to the bottom and start pedalling again, you'll want to drink the water or pitch it to avoid pushing extra weight around.
Duh, I know that by drinking the water, you're not losing weight or getting rid of it, but just moving it around, but we are really talking about cycling here. Don't forget to drink!
dumping the waterDougSloan
Dec 20, 2002 8:11 AM
Ordinarily, I'd think you'd want to preserve the water weight to maintain momentum (speed x mass) across the flat area. I can think of several reasons to dump it, though:

1. The course actually goes up a bit

2. The weight is so much that the rolling drag is very high

3. The water is ejected from the rear somehow, causing a propelling effect

I've seen these guys, and I'm sure this was well considered.

Doug
sounds plausibleAlexx
Dec 20, 2002 8:07 AM
With the extra mass, the vehicle is less effected by the forces of drag and rolling friction. For this reason, soapbox derby cars, olympic bobsleds, etc. are limited on total weight.
Weight on the backvindicator
Dec 20, 2002 8:11 AM
In addition to the above comments, the fact that the weight is on the back of the car helps. As long as the car's center of gravity is going downhill, it is accellerating. When it hits the flats, it stops accellerating. Thus a car with a more rearward CoG will spend more time accellerating and therefore have a higher top speed at the start of the flats. I won a pinewood derby (and came in second to my brother in another) using this principal.
If they ejected the water to the rear under pressure ...Humma Hah
Dec 20, 2002 9:18 AM
... that little racer of theirs could have had a sudden surge of accelerationn that would have made your eyes pop.

Common kid's toy: water rocket, a plastic rocket you fill with half water, half air from a small pump. It accelerates to full speed in a fraction of a second and goes up a couple of hundred feet. Look up an equation called the "rocket formula". Terminal velocity is a function of exhaust velocity times ... um ... something like the log of the fully fueled to empty mass ratio.

Evil Kinevil used a similar setup in his botched attempt to jump the Snake River Canyon. That was a water rocket where steam was the pressure source. That craft would have been capable of hitting a mile or two of altitude if he hadn't pulled the ripcord at launch.
East-German Bobsled teamSnowBlind
Dec 20, 2002 9:27 AM
used to load up the team with women that looked like they were half Holstien cow, or at least had been injected with extracted hormones of said cow.
Needless to say, they had the, err, "athletic" advantage when it came to screaming downhill, as areodynamics were were still pretty good. More mass ment more pressure on the sled rails as well, improving the ablity to melt the ice under the rail and speeding up the sled
Eventually, the total mass of the riders was limited to a certian range, thus elimitating the gravity advantage.

BTW, I think the pest indication that something works is that the International commitees such as the Olympics or UCI ban said practice. Of coure it may still be stupid idea in the first place.