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# Experiments on rolling resistance

Finding rolling resistance is simple enough - provided you can measure pull. The way we've measured pull here is satisfyingly elegant.
9.9
We’re going to measure rolling resistance for this toy tractor. We’ll pull the tractor with a weight hung over the edge of the table like this, and see what force it takes to keep it moving. We add paper clips to the weight until the tractor keeps moving, after we give it a gentle pull. We want kinetic friction rather than static friction. For fun, we’ll see what happens if we increase the weight and arrange for it to hit the ground before the tractor reaches the end of the table. Then, we’ll let the tractor roll to a stop. Finally, we’ll weigh the tractor and the load so that we can calculate a coefficient of rolling resistance. Have you noticed a problem?
57.5
What about the friction loss when the string goes around the edge of the table? Here’s what we’ll do. We’ll take another piece of the same string and drape it over the table, so that the string doesn’t touch the ground at either end. We’ll hang the same weight on each end of the string. In this case, we’ve used paper clip hooks and washers. Next, we’ll add washers to one side until the assembly keeps moving after it’s given a start. Do you remember the difference between static and kinetic friction? We want kinetic here, steady motion. Once were happy that we’ve got the right conditions, we’ll record our measurements. We’ll also need the weight of a clip in terms of washers.
103.6
We measured this in week three. We’ll supply our data during the analysis section. You could move to the analysis now, or you could get on with the next experiment, air resistance.

Finding rolling resistance is simple enough – provided you can measure pull.

The way we’ve measured pull here is satisfyingly elegant.

### Talking points

• When we designed this experiment we discovered a problem. What do you think it was?

If you attempt the experiment, take a photo and upload it to our Through Engineers’ Eyes Padlet wall. You can include a link to your photo in the comments for this step (click on your post on the Padlet wall and then copy the web address).