# Silly question

#### mom

Hello, first time posting...
I am helping a 5th grade teacher out with a physics experiment involoving friction. The first run is a block of wood and measure the difference in the length of a rubber band when you pull it across a desk and then again along sandpaper. Every thing fine so far, results as expected with the rubber band longer with the sandpaper. Next attach a second block to the first by a piece of string (like a train) and do them again. Again, everything as expected (band longer with two blocks, longest on the sandpaper). Finally, we do the last run (two blocks on top of each other). We are not surprised to see that the rubber band is the longest when two blocks are used and even longer on the sandpaper. My question is why? I understand the force to overcome friction is dependent on weight but the weight is the same in the last two situations. Shouldn't they be the same? I know it is not supposed to be dependent on surface area but I am confused. Thanks.

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surface area.

Nautica

#### mom

Surface area? Is that your answer? The few books I've looked in say friction force is independent of surface area. I just can't explain why I am getting these results. We redid the last part again using a spring that measures force on the rubber band. We got 300Newtons for the two block train setup on sandpaper and 450N for the same two blocks on top of each other. The weight stays the same in both cases the only thing that changes is the surface area in contact with the sandpaper. Anyone have any ideas? I really need this explained today. Thanks!!

#### chroot

Staff Emeritus
Gold Member
This experiment isn't being done on an incline, is it?

- Warren

#### Artman

The reason why friction isn't supposed to change is that you are varying the surface area in contact and the weight per square inch of surface area.

If that change does not remain an equal trade, (if you add to either one without removing from the other), it will not balance out.

That is what is happening to you. You have doubled the weight, however you have only reduced the surface area by ALMOST half because the weight is pressing the sandpaper into the wood and has increased the surface area per square inch in contact with the sandpaper.

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