Physics 1205
Assignment #10



Problem A: You are watching some kids play on a metal merry-go-round at the park when you overhear one of the kids proclam, "This thing must weigh a ton." You begin to wonder how much the merry-go-round really does weigh. When the kids leave, you walk up to the merry-go-round and observe that it is about 13 feet in diameter. You stand beside the merry-go-round, and while keeping your feet in one place, push it along the outer rim as hard as you can with just your arms for about one second. Your then observe that it spins at a rate of one revolution every three seconds. You know that in the gym you can lift about 150 lbs when you push really hard with just your arms.

Problem B: You are designing a prank for a Halloween party. This prank consists of a bag filled with Jello that you will want to fall on top of your friend's head. Because you don't want to hurt your friend with the Jello bag, you want to make sure that the bag is only traveling at a rate of 3.0 m/s when it hits your friend. You attach the bag of Jello to a light string and loop the string over a pulley. The other end of the string is attached to a bag filled with sand, called the counterweight. The bag of Jello has a mass of 0.45 kg. The pulley has a mass of .33 kg and a radius of .20 m. The pulley is mounted 1.5 meters above the point where it will hit your friend on the head. What should be the mass of the counterweight so that the Jello bag will not hurt your friend when it hits?