Case 1: Pinball
Pinball is a familiar game in which a metal ball rolls around a sloping surface, bumping into objects and scoring points. You set the ball in motion by hitting it with a spring-loaded plunger and you keep the ball in motion by batting at it with plastic flippers.
1. To start the ball moving, you pull back on the spring-loaded plunger and then release it. The spring pushes the plunger forward and it collides with the stationary ball. Moments later, the ball is moving rapidly up the sloping table. Using words like force, distance, and work, show that the plunger transfers energy to the ball during their collision.
2. At one moment during the play, the ball stops motionless near the top of the table. It then begins to roll faster and faster toward the bottom of the table. Since gravity is ultimately responsible for the ball's descent, why does the ball "fall" so much more slowly on the sloping table than it would if it fell straight down?
3. One way to cheat at pinball is to prop up the front legs of the machine so that the sloping table is more nearly horizontal. The ball then descends more slowly and is much easier to play. Why does this trick slow the ball's descent?
4. At the press of a button, a plastic flipper rotates about its pivot and strikes the ball like a tiny baseball bat. When the flipper hits the ball, its rate of rotation suddenly decreases. How is the ball slowing the flipper's rotation?
Case 2: Buying Textbooks
Textbooks are on the second floor of the University Bookstore. You have to walk upstairs to get them and then carry them downstairs to the registers to buy them.
5. You are about to buy a textbook. It was brought up to the second floor on an elevator, which lifted it straight up from the first floor to the second floor. In raising that book, the elevator transferred a certain amount of energy to the book. You take that book and carry it down the stairs to the first floor. As you lower the book, the book transfers a certain amount of energy to you. Neglecting friction and air resistance, is the energy transferred to you by the book more than, less than, or equal to the energy transferred to the book by the elevator?
6. You are carrying the book smoothly across the first floor, so that it is traveling horizontally at a constant velocity. Is the book's total energy increasing, decreasing, or staying the same, and how do you know that this is the case? (Answer both items.)
7. When you slide your book across the counter to the person working the cash register, the book moves forward at a steady pace even though you are pushing it forward the whole time. Ignoring the brief period when you started the book sliding, why doesn't the book accelerate in response to your push?
8. As you are waiting for your receipt, someone drops a book off the second floor balcony and it plummets toward the first floor. Neglecting air resistance and using expressions like "decrease" and "stay the same," how do the book's gravitational potential energy, kinetic energy, and total energy change as it descends? (Discuss all three energies.)