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Two people on top of a hill?

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Two people on top of a hill?

Old 05-22-17, 05:27 AM
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buddy
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Two people on top of a hill?

Okay you have two people on top of a hill on identical bikes. One person weights 100 lbs while the other weights 200 lbs. If all things were equal and they costed down the hill would they both go the same speed?

If they both exerted the same amount of energy would they go the same speed?

Why is it when I am going down hill with my 200 lbs it appears I am going faster than everyone else?
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Old 05-22-17, 05:34 AM
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Gravity.
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Old 05-22-17, 05:34 AM
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Originally Posted by buddy
If they both exerted the same amount of energy would they go the same speed?
I thought they were "costing" down the hill.
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Old 05-22-17, 05:36 AM
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Because you are. It's called physics. This question has been discussed here a thousand times...well almost. The knowledgeable get it. The others don't.

Simply put if your size and shape were the same as the lighter rider, you would both descend at the same speed. But your net acceleration (gravity minus aero drag deceleration) is greater than his because you get your weight at a frontal area discount.
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Old 05-22-17, 05:52 AM
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Originally Posted by rpenmanparker
The knowledgeable get it.
I took physics in middle school. Don't everyone?
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Old 05-22-17, 05:53 AM
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Originally Posted by PepeM
I took physics in middle school. Don't everyone?
I take out the trash every night. Taking physics in most cases is more like that.
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Old 05-22-17, 05:56 AM
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Every night? How much trash do you produce?
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Old 05-22-17, 06:05 AM
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Originally Posted by PepeM
Every night? How much trash do you produce?
Don't get so excited. We run the dishwasher every day too. And wash clothes several times a week.

You keep garbage in the house overnight? Eeew!
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Old 05-22-17, 06:22 AM
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Won't go downstairs to throw out a couple of napkins. No dishwasher for me. Laundry gets done every two weeks or so.
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Old 05-22-17, 06:25 AM
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Originally Posted by buddy
Okay you have two people on top of a hill on identical bikes. One person weights 100 lbs while the other weights 200 lbs. If all things were equal and they costed down the hill would they both go the same speed?

If they both exerted the same amount of energy would they go the same speed?

Why is it when I am going down hill with my 200 lbs it appears I am going faster than everyone else?
do u have ceramic(speed) bearings?
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Old 05-22-17, 06:45 AM
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Originally Posted by redfooj
do u have ceramic(speed) bearings?
Don't be rude.
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Old 05-22-17, 08:15 AM
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Originally Posted by buddy
Okay you have two people on top of a hill on identical bikes. One person weights 100 lbs while the other weights 200 lbs. If all things were equal and they costed down the hill would they both go the same speed?

If they both exerted the same amount of energy would they go the same speed?

Why is it when I am going down hill with my 200 lbs it appears I am going faster than everyone else?



The heavier person will likely go much faster


F (force) = m (mass) x a (acceleration)
a = F / m


Going downhill: F (force) = m (mass) x g (gravity) - Wind Resistance


Downhill Acceleration a (m/sec2) = g - Wind Resistance/ mass


Assume At the same speed, same aerodynamic position and same bike, roughly, Wind Resistance is proportional to surface area. The 200 lb rider is unlikely to have twice as much surface as the 100 lb rider. If both riders have exactly the same built, the surface area of the 200 lb rider is theoretically (200/100)^(2/3) - 100% =~60% more than the 100 lb rider.


In short, at the same speed, even though the gravity (g) is same, the light rider is being penalized a lot more by wind resistance.


In reality, as the speed of the heavier rider increases, the wind resistance increases even more (Resistance is proportional to ~(Velocity)^2)
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Old 05-22-17, 08:20 AM
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I was expecting a punchline.

Disappointed.
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Old 05-22-17, 08:21 AM
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Originally Posted by rpenmanparker
Because you are. It's called physics. This question has been discussed here a thousand times...well almost. The knowledgeable get it. The others don't.

Simply put if your size and shape were the same as the lighter rider, you would both descend at the same speed. But your net acceleration (gravity minus aero drag deceleration) is greater than his because you get your weight at a frontal area discount.
With all that yakety-yak you could have easily answered the question in layman's terms in half the space.
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Old 05-22-17, 08:50 AM
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Originally Posted by f4rrest
I was expecting a punchline.

Disappointed.
Here you go...


...And Raquel Welch says "Those aren't buoys!"
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Old 05-22-17, 08:50 AM
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Originally Posted by BillyD
With all that yakety-yak you could have easily answered the question in layman's terms in half the space.
Show me how.
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Old 05-22-17, 09:15 AM
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Originally Posted by rpenmanparker
Show me how.
"Gravity loves you more when you're heavier."
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Old 05-22-17, 09:17 AM
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Originally Posted by kc0bbq
"Gravity loves you more when you're heavier."
"...and wind hates you only a teeny bit more."
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Old 05-22-17, 09:20 AM
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Originally Posted by kc0bbq
"Gravity loves you more when you're heavier."
That is interestingly the exactly wrong answer.
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Old 05-22-17, 09:42 AM
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Originally Posted by rpenmanparker
That is interestingly the exactly wrong answer.
Apparently gravity sucks the fun out of everything during these seriously deep and important physics lectures.


What the 41 needs is more charts and graphs and vector analysis and calculations of normal forces.
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Old 05-22-17, 10:11 AM
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Originally Posted by kc0bbq
What the 41 needs is more charts and graphs and vector analysis and calculations of normal forces.
Only the elite, coastal members. Everyone else in the flyover areas, not so much.
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Old 05-22-17, 10:16 AM
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Originally Posted by kc0bbq
Apparently gravity sucks the fun out of everything during these seriously deep and important physics lectures.


What the 41 needs is more charts and graphs and vector analysis and calculations of normal forces.


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Old 05-22-17, 10:29 AM
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Originally Posted by buddy
Okay you have two people on top of a hill on identical bikes. One person weights 100 lbs while the other weights 200 lbs. If all things were equal and they costed down the hill would they both go the same speed?

If they both exerted the same amount of energy would they go the same speed?


Why is it when I am going down hill with my 200 lbs it appears I am going faster than everyone else?

Wait, they are exerting energy? If it was the "same amount of energy" and it was enough to offset the gravity benefit of the heavier rider, (seems it wouldn't be difficult to do this) then wouldn't the lighter rider win?
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Old 05-22-17, 10:29 AM
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Gravity (g = 9.81 m/sec2) is really the same for heavier and lighter riders. At high speed descent, lighter riders tend to experience a higher wind resistance (per unit weight). As a result, "net acceleration" going downhills (gravity - impact of wind resistance per unit weight) favors the heavier riders.
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Old 05-22-17, 11:13 AM
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E = mv^2/2 + mgh

More 'm' makes for more 'E', all else equal, and given that, more 'E' makes for more 'v'.
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