Balancing forces gravity

[Kimmie5260]

We are currently running an experiment. We put 2 tables about 12 inches apart and taped 2 rulers to them. Hung 2 cups attached by string over the rulers so 1 cup is on the ground and 1 cup is up near the rulers. Added sand to the cup on the floor and slowly added screws to the top cup until it lifted to sand cup to the top and dropped the screws to the floor. When we tried to reverse the experiment by removing most of the screws the sand cup did not fall to the floor as we had expected. It took 6 screws to lift 2 tablespoons of sand. but only 1 screw to keep in where it was. We were able to remove 5 screws without affecting the movement of the cups. Is this because of newtons law of motion (object at rest will stay at rest until stronger force acts on it.)

[HowardE]

I think you're seeing friction at work. The string is draped across two rulers and has a bit of string surface rubbing against the ruler's surface. You needed more mass of screws than you had sand mass because the friction of the string against the ruler had to be overcome as well. You'd see a more equal outcome in your experiment if you used a pulley to drape the string over instead of just the ruler.

You also saw some effect from Newton's first law. If the cups were exactly the same mass, you could move them up and down and they'd stay where you placed them. To make one pull the other down, it would have to have more mass than the other. Since you also had a good deal of extra friction, you needed even more.

Howard

[bfinio]

Hi Kimmie5260,

Howard is right that excessive friction will mess up this experiment. To improve the experiment, try reducing the friction between the string and the rulers. I can think of a few ways to do this:

1. Try using pens or pencils instead of rulers. The string may glide over them better since they have a more rounded surface.
2. Alternatively, try using a building toy like K'nex or Legos to make a pulley.

[bradleyshanrock-solberg]

You could also soap the string (rub soap on the part of the string that interacts with the rulers), if that's easier.