Coefficient of static friction

bendel

We did a lab in physics class to determine the coefficient of static and sliding friction. basically we used a spring scale to determine how many newtons of force was required to just get a block moving and then to keep it moving. We needed to do several trial runs with a slow, fast, and faster pull. After the block was moving, the coefficient of sliding friction were all about the same, however the coefficient of static friction became larger when we pulled faster. My teacher was unable to give me an explanation and told me that velocity should have nothing to do with coefficient of friction because it is not in the formula(Ff=uN). It makes sense to me that if you are trying to accelerate the object faster, it is going to take a lot more force to get it to move than if you are increasing the force little by little until it just barely moves. Can anyone help me understand?

tiny-tim

Hi bendel ! Welcome to PF!

I take it you mean, when you pulled more suddenly?

I suspect it was because you were pulling jerkily as you pulled harder, and perhaps the block "dug in" more.

Have you tried pulling something like a fridge (without wheels) across a floor? The more weight you let the leading edge take, the more difficult it is to move!

Yeah … try it with a fridge!

rcgldr

In the case of the faster pull, you created a larger jerk, which resulted in a higher peak force reading because of the rate of initial acceleration of the block. This had nothing to do with the static coefficient of friction.

tiny-tim

wot's a peak force?

are you saying it's a defect in the measuring instrument?

bendel

The way I'm thinking of it in my head is similar to the difference of throwing a box with an elastic rope attached off of a truck that is moving 60mph, and dragging a a box from behind the truck starting slow and accelerating. There is obviously going to be more of a stretch in the rope in the first scenario but what is the logical reason?

Lojzek

The only correct way to measure static friction is to start with a stationary object and slowly increase the force until the object starts moving. Only this way you can be sure that the force was just big enough to overcome static friction.

An experiment where you start to pull with a force big enough to cause movement has nothing to do with measuring static friction.

russ_watters

If you pull faster, and accelerate the block, you are measuring both friction and the force it takes to accelerate the block.