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Question about gravity for all the smart ones out there.....(13 posts)

Question about gravity for all the smart ones out there.....tronracer
Jun 13, 2002 11:08 AM
If I remember correctly, all else being equal, all physical matter falls at a rate of 9.8 ft/sec (or is it meters/sec?). So why would a heavier rider be able to catch a lighter rider on the descent (as a previous poster stated). Does it have something to do with momentum or inertia?
I'm by no means a physicist but...Wayne
Jun 13, 2002 11:24 AM
I think that only holds true for a vacuum and that is terminal velocity. I can't quote the formulas but this discussion just happened on the velonews tech site, go there, if you want a detailed expanation. You have to accelerate up to terminal speed and in real life, if I understood it correctly a bigger rider gains more speed on a descent due to their increased mass than they lose due to drag from their increased volume.
ACCELERATION is 9.8 m/sec*secLon Norder
Jun 13, 2002 11:26 AM
The acceration of gravity is the same for the riders, but the force of gravity is greater for the heavier rider (force = mass * acceleration). The force of wind resistance is about the same on each rider, but it is larger percentage wise (of the total force) for the lighter rider. So the lighter rider is slowed more by the wind resistance

In a vacuum a feather would fall as fast as a lead weight but it doesn't in air because of wind resistance.
See, I told you I wasn't a physicist (nm)Wayne
Jun 13, 2002 11:38 AM
Fg=Gm1+m2/d^2TopGun8614
Jun 13, 2002 2:31 PM
Im pretty sure this is why the big guy is faster. The force of gravity is Equal to G(which is 6.67x10^-11)the mass of earth, the mass of the rider, then over the distance squared. Therfore the heavier rider has a larger force of gravity on him. Then According to newton's second law of motion, A~net force/mass. therfore if you plug in the numbers i believe that should explain this occurence.
sorry, m1*m2TopGun8614
Jun 13, 2002 2:32 PM
mass to drag ratio?DougSloan
Jun 13, 2002 2:40 PM
The larger rider's mass to drag ratio is higher, thus he goes faster.

The larger mass alone will not account for greater speed, as the increased gravitational pull is offset by the increased inertia.
BZZZZT!!!grzy
Jun 13, 2002 2:53 PM
Nope.

Sure a larger mass has a larger force due to gravity, *but* it also takes more force to accelerate a larger mass (F=ma). It just so happens that all of this cancels each other out and it's not a coincidence either. What really matters is the drag as a function of mass as has been pointed out by several others here. About 6 mos. ago we had a long knock out drag out arguement and no matter how many references and cases I built I couldn't convince one guy. This also caused several people to relate the following observation: having an arguement on the internet is like running a race in the special olympics - even if you win you're still retarded! ;-)
actuallyDougSloan
Jun 13, 2002 3:18 PM
Actually, I think I was the guy, and you did convince me. I think we were having a semantics difference ("what *causes* one rider to descend faster than another?) more than a physics difference.

Doug
BZZZZT!!!Leisure
Jun 14, 2002 2:14 AM
Uh oh...he's baack
Depends on Mass, Size, Velocity, and Surface Areajose_Tex_mex
Jun 13, 2002 4:35 PM
It's a give and take depending mostly on their masses, surface areas, and velocities. At lower velocities the more massive riders (usually bigger volume wise) will have the down hill advantage. At higher speeds 40+ mph the smaller, lighter more aero riders will have their turn - as the drag goes exponential. Once the drag starts going along an exponential curve the retarding force due to the surface area will quickly take away the benefits of being more massive.

True - all things fall at the same rate - but only in the absence of air (vacuum).

Think of two spheres dropped - one is cork and the other lead. The lead sphere hits the ground first (even when neither reach their terminal velocity) because its greater mass allows it to push the air out of the way moreso than the cork ball.

Thus, if we have two riders of the same size (volume) the denser rider has the advantage. This probably will not be the case. As mass increases more than likely so to will the surface area - and the fun goes on
pizza+beer=good times for allempacher6seat
Jun 13, 2002 8:28 PM
Now that's a formula I don't mind looking at.
If there's one thing I remember from high school physics, it's that everything we learned never really occurs unless you're in space =P.
re: Question about gravity for all the smart ones out there.....zeke
Jun 14, 2002 4:22 AM
You mean a question for all those trained in physics or applied mathematics dont you?