 You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
Sparked by the thread on heavy bikes.
I'm well aware of the physics, that two objects of diiferent mass should accelerate down the same hill at the same rate. But casual observation seems to suggest otherwise.
Rolling back down a fire road at the weekend alongside the wife. I easily rolled ahead of her (Me about 80kg in riding gear, her probably 60kg)
In the interest of science I stopped and swapped bikes. I rolled easily ahead other again.
The bikes in case it matters - a Blur LT with a Crossmark on the back and a Superlight with a Nevegal on the back (poth had HR's on the front)
So if heavier riders don't roll faster downhill how did I manage it on two different bikes?
Who said they don't ?
Your "physics" is a little bit out
Wind resistance and / or friction losses.
In a vacuum on a smooth surface there'd be no difference. In the real world it gets complicated as there are friction / stiction losses in the bikes and wind resistance which is proportional to surface area. A heavier person won't have a proportionally bigger frontal surface area so the lighter person has a lower weight to wind resistance ratio...
I'd think rolling resistance etc is more or less the same for both of you, but is a smaller offset relative to your mass.
good luck getting that study peer reviewed.
Your "physics" is a little bit out
So enlighten me. I'm happy to learn why I'm wrong, its why I posed the question
I'm well aware of the physics, that two objects of diiferent mass should accelerate down the same hill at the same rate
Not very aware of the physics then 😛
2 objects will accelerate at the same rate, but their terminal velocity is determined by the force acting on them ballanceing out the aerodynamic drag force. Bigger mass = bigger force = bigger terminal velocity.
I like the air resistance explanation. It makes sense.
If I can persuade the missus to haul 20kg of lead around in her Camelbak its easy to test as well.
Did I mention the conveyor belt?
That won't do it. You'll have to get her to wear a sumo suit.
Dont forget low tyre pressures will add a lot of friction.
tinas has it. F=ma (F=force, m=Mass, a=accell)
Greater mass gives greater force overcoming the drag.
i forgot you ride in air.
you stop accelerating when your weight (gravity x mass) equals drag (from air and ground).
there is a slight but.. if you're heavier you probably have more surface area too? i guess increasing rider size overall increases terminal velocity though, think of in insect riding a small bike, vs a whale.
Right, I kept getting a bit annoyed that my mate can roll down a hill and I have to pedal to keep up with him...
So we swapped bikes.. He still rolled down the hill faster than me on my bike. He was a lot heavier and bigger than me, air resistance didn't make as much difference as 20-30kg did!
The only way I can get down the hill the same as him (without pedalling and keeping up), was to tuck in about 1-6" behind his rear wheel, which I don't like doing at 30+mph.
[i]Physics and real world don't always go together... Does no one watch Mythbusters!?[/i]
Its the wind resistance mainly. Its very noticeable on the tandem On one hill on a solo I hit a terminal velocity of around 43 mph, on the tandem we are still accelerating at 50+ mph. nearly twice the weight - not much more drag
Air drag is not the only thing resisting you rolling down the hill. Rolling friction, bumps etc etc all take their toll on your downhill speed.
If you have two balls of lead, both the same diameter, but one is hollow, i.e. weigh less, and you drop them at the same time, which will hit the floor first?
Is there helium or air in the hollow one?
If you have two balls of lead, both the same diameter, but one is hollow, i.e. weigh less, and you drop them at the same time, which will hit the floor first?
Based upon what we have learned about air resistance. If the lead balls fall for long enough and have a chance to approach terminal velocity. The solid ball will hit the ground first as it will have a higher terminal velocity so will travel on average faster towards the floor
I'm sure my physics teacher told me that they'd both land at the same time, but I think he's wrong because of the greater force to overcome drag.
I can't remember, and I have A level physics and a degree in engineering!
Damo, same here, I think its the same time, but now not sure (momentum vs air friction and weight?). 😯
Which is really bugging me, my mate is bigger than me, heavier than me, rolls down a hill faster, even if we swap bikes.... hmmmm...?
If you have two balls of lead, both the same diameter, but one is hollow, i.e. weigh less, and you drop them at the same time, which will hit the floor first?
In a vacuum both will hit the floor [s]first[/s] [i]at the same time[/i] but in real life the solid ball will hit the ground first because of the greater terminal velocity for the heavier ball.
This is the problem with the feathers/lead experiment or the hollow lead ball, that we have air resitance. It was solved by doing it on the moon where there is little appreciable atmosphere to cause any drag.
I'm sure my physics teacher told me that they'd both land at the same time, but I think he's wrong because of the greater force to overcome drag.
I can't remember, and I have A level physics and a degree in engineering!
It was actualy one of the questions in the entry exam to cambridge when I applied, it's there deliberately to trip you up as the a-level sylabus teaches you that they'll hit the ground at the same time as under most everyday situations they would as they'd get nowhere near terminal velocity and calculations involving wind resistance would go beyond the a-level sylabus (which relies on GCSE maths) so you make an assumption of no wind resistance.