Why do heavier people descend faster on a bike?

It's POSE on a bike. Gravity makes them fall forward faster.

if there was no wind resistance perhaps

But fatter people would probably have more wind resistance (wider chest, etc...) and would therefore descend slower not faster.

Just as a piece of balled up paper falls much more quickly than a sheet, more weight does not directly mean a faster descent, but weight does overcome whatever resistance there may be. In this case there is air resistance and wheel friction. It's the same reason why rocks sink quicly in water when most fall very slowly, I believe.

well why do you think it is then?

if it's indeed 'cause of wind resistance, then perhaps most heavier/larger people don't have enough surface area to counteract their weight/size. Like, their weight-to-surface area ratio is not the same proportion as the lighter person's.

do an experiment. get a large/wide heavy person vs a skinny heavy person. get them to be the same height if possible

The heavier person actually has more force moving down the hill F=ma where accelerration is constant for both the skinny and the fat person. The fat person has more momentum however. (Just try standing in front of the fat person) The air can be modeled as millions of tiny little molecules that slow both down. Which will be slowed down the most? The skinny person. Try throwing a a solid ball of foam 60 mph it will quickly lose its speed becuase it has very little momentum. Try throwing a baseball 60 mph it will lose its speed slowly because it has much more momentum than the foam. Basically the air has less of an effect on the fat person because the fat person has more momentum and more energy (it took the fat person more energy to get to the top).

The force is the same. The fat person has more momentum however. (Just try standing in front of the fat person) The air can be modeled as millions of tiny little molecules that slow both down. And just like you standing in front of the fat person they dont pose much resistance. Which will be slowed down the most by air? The skinny person. Try throwing a a solid ball of foam 60 mph it will quickly lose its speed becuase it has very little momentum. Try throwing a baseball 60 mph it will lose its speed slowly because it has much more momentum than the foam. Basically the air has less of an effect on the fat person because the fat person has more momentum and more energy (it took the fat person more energy to get to the top).

Doesn't the added weight increase friction between the tires and the ground and offset the benefit from the bigger rider's additional mass?

heavier bodies accelerate faster because there is more force being pressed downhill. yes, the gravity stays the same, but the mass of the object that is going down the slope is heavier, and therefore pulled downhill at a faster rate.

Um...didn't you just say F=ma?

More mass therefore equals greater Force going downhill. Or am I missing something?

It's because wind resistance is proportional to cross-sectional area at a given velocity. Cross sectional area of a rider does *not* increase linearly with weight (you don't just get wider or taller ... you also get thicker as you get heavier).

Thus, at a given velocity your *net* downhill force increases slightly more than linearly as your weight increases, which means your downhill acceleration is greater. (remember ... it's only in a vacuum that all objects accelerate at the same rate. Otherwise skydiving wouldn't be a very appealing hobby)

Of course, a lighter rider who is good at assuming an aero position can still go downhill faster than a heavier person who sits upright.

scooter4491 wrote:

Just as a piece of balled up paper falls much more quickly than a sheet, more weight does not directly mean a faster descent, but weight does overcome whatever resistance there may be. In this case there is air resistance and wheel friction. It's the same reason why rocks sink quicly in water when most fall very slowly, I believe.

OMG, rocks sink quickly because of their high density compared to water.

I was going to make fun of you for being a dumb high schooler but, I think I learned that in 3rd grade. So I guess kindergarteners get on letsrun as well.

couldn't resist!

well that is a basic diagram i just made in paint

now i'll explain

the circle represents the body of mass, the bicycle, with the smaller concentric circle the center of mass

each arrow represents a force on the object. were the object not accelerating, all the forces would cancel each other out for a net force of 0

that is not the case here. the bike is accelerating downhill so its force forward is greater than the backwards force, friction (and air resistance, which is left out to simplify things a little)

the force forward is found by using trigonometry and the force downward (acceleration is gravity in this case, so therefore f=ma becomes f=mg. The forward, downhill force is mgsin(theta) where theta (0 with line through it) is the angle of elevation.

The force of friction is equal to mu (greek letter) times the normal force. Mu is the coefficient of friction (how much friction from the surface) and is some positive number, almost always single digit. the normal force is the force perpendicular to the surface, once again using trigonometry it can be found, mgcos(theta)

sin (theta) is greater than cos (theta) and g (9.8) is generally greater than mu

knowing this one can clearly see that the only coefficient changing between the two bikes is m, or mass. It is clear that the larger mass, the greater the difference between the forward and retarding forces on the bike

hope that helped, gotta love physics

nice mu's. haha, seriously, that's some good freeform mu work.

haha thank you

i didnt think anyone would notice my handiwork

Two riders, one heavier than the other, sit at the top of a hill. They have different potential energies (PE = mass*g*height), and assume that their potential energies will be completely converted to kinetic energy at the bottom of the hill (KE = 1/2*m*v^2). So set PE = KE, and you get [g*height = 1/2*v^2) for both riders--totally independent of mass.

Theoretically, both riders should attain the same speed. In reality, if anything, it seems like the heavier rider should go slower because of greater friction (and possibly wind resistance).

ah, cushen you're right

my physics teacher always did love using PE and KE to solve problems, makes things simpler, more elegant

anyways i realized that yes, the larger rider would have a greater force exerted upon him, but as F=ma, the acceleration would be the same since the greater force over the greater mass would equal the lesser force over the lesser mass

still i have noticed while sledding that the heavier a sled, the faster it generally goes...

Lovin' branfordite's free-body diagram. nice to see someone understands physics AND ms paint.

The explanation wrote:

It's because wind resistance is proportional to cross-sectional area at a given velocity. Cross sectional area of a rider does *not* increase linearly with weight (you don't just get wider or taller ... you also get thicker as you get heavier).

Thus, at a given velocity your *net* downhill force increases slightly more than linearly as your weight increases, which means your downhill acceleration is greater. (remember ... it's only in a vacuum that all objects accelerate at the same rate. Otherwise skydiving wouldn't be a very appealing hobby)

Of course, a lighter rider who is good at assuming an aero position can still go downhill faster than a heavier person who sits upright.

We have a winner!