Advice regarding sled experiment

[rlosey]

Hello. I would like to design an experiment where different sled designs are compared. Specifically, I would like to asses how sled design affects the amount of force required to pull a sled over 100 meters. Any suggestions you might have would be greatly appreciated.

 

[phinds]

What research have you done so far? What have you found out? What is your level of scientific knowledge?

 

[rlosey]

I have the sleds built. The proposed load for each sled is 75 kg. I also have a relatively flat surface to test the sled on--a flat frozen lake bed. The idea is to test traditional Indigenous sled designs, which I am very familiar with. However, I have nearly no physics experience.

 

[PeroK]

I have the sleds built. The proposed load for each sled is 75 kg. I also have a relatively flat surface to test the sled on--a flat frozen lake bed. The idea is to test traditional Indigenous sled designs, which I am very familiar with. However, I have nearly no physics experience.

There are two aspects to the force required to move a sled across ice. There is a coefficient of static friction, which determines the initial force required to get the sled moving. Once the sled is moving, there is a coefficient of kinetic friction which determines the force required to keep the sled moving at constant speed. The kinetic friction will be lower than the static friction.

In summary, you'll need an initial higher force to get the sled moving and then a lower, sustained constant force to keep the slep moving at constant speed. The sustained force is probably the one you want. The distance is irrelevant. That simply determines for how long the force must be maintained.

There will be an optimum angle above the horizontal at which to pull the sled. This depends on the coefficient of friction. For a low coefficient of friction, the optimum angle will be only slightly above the horizontal. However, if the static friction is quite high, you may find it's easier to pull the sled slightly upwards initially until you get it sliding. This might be something that you've picked up naturally. In any case, there is an optimum angle which requires the minimum force to pull each sled.

This itself could be used as a test. The better the sled is at sliding, then lower the angle at which it feels natural to pull it.

The tricky part is, of course, to get any quantitative data from an experiment on the ice. Otherwise, you are relying on your senses to determine which is easer to pull. There's no obvious way to rig up equipment that would give you any output in terms of a force.

One approach to calculating the coefficient of kinetic friction would be to get the sled moving at some constant speed and then see how long it takes to slow down and stop. If there is no equipment, an experiment could be done with two people. You get both sleds moving at the same speed and then release them at the same time. Then measure how far they slide before stopping. For a direct comparison between two sleds, you don't need any data.

If you know the starting speed and the stopping time and/or distance, you can calculate the coefficient of friction from that.

 

[Baluncore]

It will come down to the ability to get things moving, then to friction coefficient. There will also be a question of how the sled is pulled. There will be an interaction between the sled, the source of pull, and the bridle geometry.

https://www.tribonet.org/news/the-physics-behind-ice-skating-a-tribological-perspective/
You will need to log the surface composition and the temperature.

 

[sandy stone]

Wouldn't a spring balance (fish scale) give a reasonable measurement of the force needed to pull the sled?

 

[gmax137]

Wouldn't a spring balance (fish scale) give a reasonable measurement of the force needed to pull the sled?

That was my thought as well. But, you also need to consider the pulling angle and the constancy of the sled speed, as @PeroK mentioned. You will need to come up with a mechanism to ensure the speed is constant and the same between tests. I'm envisioning that the rope is wrapped around a drum that turns at a fixed speed, or something like that. If the sled speed is not constant, the spring scale reading will bounce around.