Inclined plane: heavy and light ball

xAxis

JustinRyan:"..If this is equal for both balls the effect will be greater for the lighter ball as this is dependant on mass..."
What do you mean? What effect? Isn't the effect the air resistance? If it doesn't depend on mass, then the effect will be equal. But it does, the drag is dependent on the speed and density of object, and therefore on mass.

deep838

BruceW i wrote the 1st part wrong, I only realised that after I saw the replies, sorry for that.
And I wasn't familiar with rolling resistance! I thought that friction always acts as a dissipative force, but you say that i behaves so only sometimes. I give no comments on that for now since I know nothing about it.
When I saw this thread, I thought it will have a simple solution, but I'm forced to think otherwise now! This problem seemed so easy and basic "which will cover more distance?" something every kid asks! But I have to say that I'm feeling really disappointed about it. :(

BruceW

yeah, it is quite complicated because the problem involves dissipation, which often makes physics problems more complicated. The easy bit is when we assume zero dissipation. Then when we want to change our model to include dissipation, it usually gets more complicated.

Also, about friction, we can think about two surfaces in contact with each other. If the two surfaces have zero motion relative to each other, where they are in contact, then there is zero dissipation of energy. It is only when the two surfaces slide past each other that energy is dissipated.

A wheel is a very good example. If the wheel is rolling without slipping, then the part of the wheel touching the ground has zero motion relative to the ground. (Which might be counter-intuitive when you first hear it, but draw a diagram and think about it for a bit, and you'll realise it is true). Therefore, there is zero energy lost by friction when the wheel is rolling without slipping. So in this case, the rolling resistance must be due to other causes, not simply friction with the ground.

dipole

What's with all this "inertia" buisiness? Why not describe things in terms of something you can actually write down and measure - momentum, and energy?

JustinRyan

The effect of a force is an acceleration. The acceleration (or deceleration in this case) will be greater for the lighter mass.

I do not believe the air drag is dependant on density. Surface area & texture, volume, velocity perhaps but not density & therfore not mass.

If you throw a cricket ball, and a nurf ball of equal size, which goes further?

deep838

yeah i understand that!!

that's what we are trying to do!!

BruceW

ah, ok sorry I didn't know how much you had learned. Well, anyway, that stuff is why people talk about 'rolling resistance' instead of friction.