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.