Question about the role of mass in inclined planes

[tovisonnenberg]

Hi! I have a question about inclined planes. In the diagram I attached, you can see that, with or without friction, mass does not affect the acceleration of the block. However, in my experience, the more people I put on my sled, the faster it goes. Why is this?

 

[russ_watters]

Sled, as in snow? Snow doesn't work like a hard, dry surface for friction. It compacts and maybe even melts a little against the sled.

 

[tovisonnenberg]

Sled, as in snow? Snow doesn't work like a hard, dry surface for friction. It compacts and maybe even melts a little against the sled.

Ah, so a combination of greater compaction and a little melting decreases the coefficient of friction?

 

[phinds]

Ah, so a combination of greater compaction and a little melting decreases the coefficient of friction?

Considerably. Every try ice-skating on a hardwood floor? Same principle

 

[berkeman]

So a related question is why wind resistance and rolling resistance are different from friction on an inclined plane. Clearly extra weight makes masses fall faster in air, and makes bike riders accelerate faster down hills...

 

[PeroK]

Hi! I have a question about inclined planes. In the diagram I attached, you can see that, with or without friction, mass does not affect the acceleration of the block. However, in my experience, the more people I put on my sled, the faster it goes. Why is this?

In general, air resistance depends on the surface area in the direction of travel. If one person is sitting behind another on the sled, then you have twice the mass and twice the gravitational force against almost the same air resistance.

For example, in the bob-sled competitions there is a maximum weight allowance.

https://adventure.howstuffworks.com/outdoor-activities/snow-sports/bobsled2.htm

"Heavier sleds go faster, so teams that do not reach the maximum occupied weight may add ballasts to make their bob heavier. Officials weigh the sleds at the end of the run to make sure they meet the weight requirement."

I guess it makes sense that all sleds are weighed up to the maximum - otherwise the heaviest team would probably win. I had no idea that teams could add ballast - but that does explain why generally there are only a few hundredths of a second between teams. The difference between first and last in a bob-sled competition is essentially experimental error!