Interesting Encounters

[flecc]

Sorry to throw a spanner(or an apple) in the works but according to Newtons first law of motion inertia has to be considered as a force opposing that causing acceleration and I see no mention of it here.

The result is that while the force exerted on an object due to gravity increases in proportion to the objects mass so does the force required to accelerate it in exactly the same proportion. hence the heavier object will not accelerate faster, as shown with the battery rolling experiment.

Air resistance is of course a factor but all other things being equal it will affect and slow the larger rider to a greater extent.

Phew!

At least my "stiction" survives that awkward Newton bod!

He doesn't mention that.
.

 

[Ian]

At least my "stiction" survives that awkward Newton bod!
.

I'm sure Newton would have been aware of "stiction", if only from his shoes sticking to apple juice coated cobblestones. If he had investigated he'd have probably found it's effects too unpredictable to express in mathematical terms. It's more suited to the randomness of modern physics

 

[flecc]

Ah ha! Quantum.

Sounds so much posher than stiction.
.

 

[lum]

Hi Ian

Sorry to throw a spanner(or an apple) in the works but according to Newtons first law of motion inertia has to be considered as a force opposing that causing acceleration and I see no mention of it here.

The result is that while the force exerted on an object due to gravity increases in proportion to the objects mass so does the force required to accelerate it in exactly the same proportion. hence the heavier object will not accelerate faster, as shown with the battery rolling experiment.

Air resistance is of course a factor but all other things being equal it will affect and slow the larger rider to a greater extent.

I didn't say that the heavier rider would accelerate faster due to gravity. Both riders would accelerate at exactly the same speed in the absence of wind resistance (assuming rolling and mechanical resistance is the same). The acceleration due to gravity is a constant.

It is the decelerating force caused by the wind resistance which could cause the heavier object to move faster down the hill. Assuming that the area presented to the wind increases more slowly than mass for a fatter rider which I believe is a valid assumption. It certainly is for simple shapes like spheres, cubes etc etc. I am sure it is the case for a complex shape like the human body plus bike.

When people say that the force of gravity is the same for any body they are stating that the force of gravity per unit mass is the same. Force per unit mass is the same as acceleration which is just a rearrangement of Newtons second law which states Force = mass * acceleration.

I didn't explicitly mention inertia as the original post didn't and it isn't required to explain the fact that the terminal velocity of the rider is reached more quickly by the lighter rider.

The terminal velocity is reached when all forces cancel out and there is no net acceleration so it is sufficient to do a force balance to determine terminal velocity.

Wow this forum is fun, physics and electric bikes.

 

[Ian]

I see what you're getting at Lum and now I better understand your reasoning I agree. I'm not sure that was the case in my original experience mentioned in the very first post as this guy was very aerodynamic but I think there is more than meets the eye to that one.
I'm so fired up by this that I want to try an experiment, but I need a lightweight volunteer. I have a friends son in mind who's as skinny as a rake but is he brave enough to reach terminal velocity. There's also the question of dragging him away from his Playstation.

 

[flecc]

Wow this forum is fun, physics and electric bikes.

You'd be amazed Lum.

We even covered the physiological reasons for the facial characteristics of equatorial peoples recently!

It was in connection with puncture repairs, see if you can work that one out.
.

 

[lum]

... I'm so fired up by this that I want to try an experiment, but I need a lightweight volunteer. I have a friends son in mind who's as skinny as a rake but is he brave enough to reach terminal velocity. There's also the question of dragging him away from his Playstation.

Wow you are getting serious proposing an experiment

I would suggest keeping as many parameters the same as possible, same rider, same bike, same hill, but increase the mass of the rider with a weight in a rucksack or on the rack of your bike if you have one.

Keeping the same setup but putting a dense weight behind you should keep the frontal area seen by the wind and the drag coefficient as similar as possible between the two setups.

Unfortunately increased weight would mean rolling resistance would increase as the tires flatten onto the road but I don't think you can get around that.

Also you would have to carry a weight large enough to see an effect. A few kilos will be swamped by the experimental error.

 

[Ian]

Also you would have to carry a weight large enough to see an effect. A few kilos will be swamped by the experimental error.

That's why I need a lightweight volunteer, otherwise I'd have to carry something like 20kg of ballast, something I'd not be happy doing.
I was thinking of 1 bike, 1 hill, 2 riders of vastly different weight and a stopwatch.