Falling Washers, Rolling Cart: A Science Fair Test of Force and Mass

Science Fair Project Notes: The Washer-Powered Cart

1Eighth graders Beatriz Kolawole and Tenzin Albrecht noticed that a loaded grocery cart is much harder to get rolling than an empty one, even with the same push. Their science teacher, Mr. Desmond Ferrante, agreed to lend them equipment from the physics lab. For the school science fair, they asked: how do the net force on an object and the object's mass affect how quickly its speed changes?

2They set a low-friction cart with a mass of 0.90 kg on a level metal track. The track is 2 m long. A string tied to the cart runs over a small pulley at the end of the track and down to a light hook that starts about 1 m above the floor. Steel washers hang on the hook, and each washer has a mass of 10 g, so Earth pulls it down with about 0.1 N. The falling washers pull the cart along the track. A motion sensor at the start of the track records the cart's speed 20 times a second. First, the pair gave the cart a gentle push with no string attached. It rolled the whole length of the track and barely slowed down, so they decided to treat the pull of the washers as the net force on the cart.

3In Test 1, all ten washers started out riding on the cart. For each run, the pair moved some of the washers from the cart onto the hook and then let go. Everything the washers' pull speeds up, which is the cart, the washers riding on it and the washers on the hook, has a total mass of 1.0 kg. They ran three trials for each number of washers and recorded the cart's speed 1.0 s after letting go. The bar graph shows their averages.

4In Test 2, the pair kept 4 washers on the hook and the other 6 on the cart in every run. To change the mass, they added metal blocks to the cart, making total masses of 1.0 kg, 2.0 kg and 4.0 kg. During trial 2 of the 4.0 kg run, Tenzin's finger nudged the cart forward as he let go. The other trials went smoothly, and the pair planned two tests that each changed only one thing.