What the Physics?

Video #2: Bouncing Balloon (Chapter 10)

Sequence:

• The video opens with a small-scale version of a water balloon bouncing.
• Break: What the Physics? At this is the point in the video where the video can pause for assessment. There can different levels of questions, depending on where the video is used—perhaps in more than one place.
  • A dropped water balloon comes to rest on landing—its kinetic energy is zero. It subsequently rebounds, rising some distance, so increasing in gravitational potential energy. What is the source of energy for the rebound?
• The video continues with a large-scale version of the demonstration. We could give additional information or questions here.
  • When the balloon lands, the rubber stretches quite dramatically. This results in enough elastic potential energy to give a sizable rebound.
• Additional questions (for assessment):
  • The balloon is dropped from a height of 10 meters. How fast is it moving when it lands?
  • The balloon rebounds to a height of 1.2 m. What fraction of the initial energy is lost to thermal energy before the rebound?

Style #2: Introductory sequence, followed by a sequence that explains what is being seen.

Video #1: Speed of Sound (Chapter 15) [NOTE: We will likely reshoot this one]

Sequence:

• The video opens with a clip of people dancing—not at all in sync with each other.
• Break: What the Physics? At this point, we pose a question:
  • Why are the dancers—who are blindfolded—unable to keep in time with each other?
• The video continues with a sequence that explains what is happening.
  • When the dancers are close together, they are able to keep in time.
  • Now, they spread out—each dancer 10 m from the dancer in front of him or her.
  • With this spacing, the sound from the speaker at the front of the line takes some time to get to the dancers at the back of the line—they lag those in front.
• Additional questions (for assessment):
  • From the front of the line to the back, there is a distance of 90 m. How long does it take sound to travel this distance?

Video #2: Floating Balloons (Chapter 13)

Sequence:

• The video opens with a clip of balloons floating in a tank.
• Break: What the Physics? At this point, we pose a question:
  • Why are the balloons—which are filled with air—floating in the tank?
• The video continues with a sequence that explains what is happening.
  • The balloons are definitely filled with air—you can see them being blown up.
  • The tank has dry ice—frozen carbon dioxide—in the bottom. This makes a gradient of density vertically through the tank. At the bottom the gas is dense; there is cold carbon dioxide. At the top of the tank, there is room temperature air.
  • The gas at the bottom of the tank is more dense than the air in the balloons, so they float!
• Additional questions (for assessment):
  • The balloons rest, stably, at a certain vertical position. If you nudge a balloon, it will bob up and down and return to its stable position. What is special about this position?

Video #3: Breaking Boards (Chapter 10)

Sequence:

• The video opens with a clip of a person breaking a board with her hand.
• Break: What the Physics? At this point, we pose a question:
  • If you want to break a board with your hand, is the key pushing hard or moving fast?
• The video continues with a sequence that explains what is happening.
  • We drop a ball that doesn't bounce; all of the kinetic energy of the ball is converted to thermal energy when it lands.
  • When the ball lands on a board, the board bends—it stores some energy as elastic potential energy. As it returns to its original shape, this energy is converted back to kinetic energy of the ball and the board; they rise up in the air.
  • The board can only store so much potential energy; beyond this point, it will break. If you drop the ball from a great enough height, the kinetic energy of the ball is greater than the energy needed to break the board. The board bends, the ball slows... but then the board breaks, and the ball continues moving.
  • When you break a board with your hand, the important feature is that your hand is moving rapidly. Your hand slows as it strikes the board and the board bends; kinetic energy of your hand is converted to elastic potential energy. But if your hand is moving fast enough, the kinetic energy of your hand is more than the maximum energy the board can take—and the board breaks.
  • When your hand hits the board, there is a force on your hand from the board—but the force necessary to break the board is less (by a factor of 12 or so) than the force necessary to break your hand. The bones in your hand don't deform much; most of the energy goes into bending the board, not bending your bones. The board breaks, but your hand doesn't.
  • Breaking three boards means three times the energy, but this only means an increase in speed of a factor of the square root of three—about 1.7. It's not as hard as it looks! (There is more to the story... some of the energy released when the boards break can help you break further boards.)
• Additional questions (for assessment):
  • What would happen if your hand wasn't moving fast enough?
  • If your hand is moving at 5 m/s, you can break 1 board. To break 4 boards, how fast must your hand move?

Video #4: Objects and Images (Chapter 18)

Sequence:

• The video opens with a clip of two toy trucks seemingly passing through each other.
• Break: What the Physics? At this point, we pose a question:
  • The trucks aren't really passing through each other; a mirror is involved. How is this done?
• The video continues with a sequence that explains what is happening.
  • There is a half-silvered mirror on the table. It reflects half the light that hits it, transmits half.
  • The yellow truck rolls in front of the mirror, the red truck rolls behind. At one point, the image of the yellow truck is at the same place as the red truck; the two overlap!
• Additional questions (for assessment):
  • Is the image of the yellow truck a real image or a virtual image?