Supplementary Problems

1.1 You leave the platform simultaneously with a friend. He walks slowly off the platform and you run off the platform, so that your initial velocity is in the horizontal direction. Who will reach the water first and why?

1.2 Explain why it is easier to use ramp to get a heavy box into a truck than it is to simply pick it up and put it into the truck.

2.1  You give a book a shove on a table.  The book slides a bit and comes to rest. 
a) Using some physics words and the concept of forces explain this behavior.
b) Using some physics words and the concept of energy provide a second independent explanation of this behavior.

2.2 Explain why ice skates blades are made of smooth steel and only touch the ground in a very small area whereas roller blade wheels are made of more sticky rubbery substances and are much wider.

2.3 In bowling you roll a ball down an alley and try to knock over 10 upright pins.  When the ball hits the pins one of the them flies into the air, making an arc before landing and coming to rest. 
a) Where does the energy come from to get the pin moving to begin with?
b) Where is the energy stored when the pin is at the top of its arc?
c) After the pin has come to rest, where is the energy?

5.1 The summer palace in St. Petersburg was designed for Czar Peter the Great in 1715. The world famous fountains are completely gravity driven, with no pumps. Consider the picture below, which shows fountains on terraces. Although each terrace is at a different level, nonetheless all the fountains reach the same height. Explain why this is so.

5.2 You take a helium balloon to the moon.  What happens to it?  Does the balloon float or sink?  Why?

5.3 Located in Israel, the Dead Sea is among the saltiest bodies of water on earth.  The people in the pictures below are floating in the Dead Sea- and they aren't using any air filled floatation devices!  Explain why they float so high.
                   

5.4 Why does the stream of water from a faucet become narrower as it falls?  Try it! (Credit to Giancoli Physics).

5.5  Why does it take pasta longer to cook in Denver than in San Francisco?  (Include an explanation of the effect of pressure on the boiling temperature of water.)

7.1. Baking a potato takes a long time, even in a hot oven, because the inside of a potato warms up very slowly.
a. What makes it hard for conduction to transfer heat to the center of the potato?
b. What makes it hard for convection to transfer heat to the center of the potato?
c. What makes it hard for radiation to transfer heat to the center of the potato?
d. Why does inserting a metal skewer through the potato help it to cook quickly?

7.2 If you are in the desert and have a warm bottle of water, you can cool it off to a temperature substantially below the air temperature by wrapping it in a wet cloth and waiting a bit.  Explain how this works.  Why wouldn't it work as well in the tropics?

7.3 A 75 degree day feels quite warm while a 75 degree swimming pool feels quite cool.  Explain. (Credit to Giancoli Physics).

7.4 If you cool a rubber racquet ball with liquid nitrogen you can shatter it by throwing it down on the ground.  Explain why.

7.5 Since the eardrum is located deep inside a person, its temperature is a good indicator of the core temperature of the person.  Ear thermometers give a readout very quickly and aren't required to touch the eardrum in order to measure its temperature.  Explain how they work and why they have these convenient properties.

7.6 When a dog gets hot, he adopts a posture like the one shown below.  Why would he do that?

7.7 Revisiting "Check your figures 1: The Cold of Deep Space".  Figure out how many food calories a day you would radiate away if floating through deep space.   The average daily consumption of food calories is about 2500 or so.  Explain the discrepancy.

7.8 As you calculated in "Check your figures 1: The Cold of Deep Space" a human body emits several hundred Watts.   Lightbulbs also typically emit about 100W (think of the wattage ratings on lightbulbs).  Even though they are emitting close to the same amount of power, we see lightbulbs, but not humans, glowing.  Give two reasons for this difference.

8.1 If you spray an aerosol continuously for a ten or twenty second you will find that the can gets quite cold.  Explain why.

8.2 At one point in the cycle of an air conditioner, the compressor rapidly compresses the working fluid to a very high temperature, much higher than the surrounding outside air temperature.  Since the goal of an air conditioner is to cool the air, why would you ever want to so dramatically heat the working fluid?

8.3 Scuba divers are told never to hold their breath while underwater.  Explain why.

8.4 Scuba divers must be careful of the "runaway ascent" in which the diver rapidly ascends to the surface in an out of control fashion.  First, explain how such a runaway ascent occurs.  Second, explain what a diver could do to stop a runaway ascent.

8.5 The special pressure regulator of an aqualung delivers air to her mouth at exactly the same pressure as that of the surrounding water. She can breathe this air easily. However, the deeper she dives, the faster she consumes air molecules from the tanks and the sooner she must return to the surface. Why does she exhaust her tanks faster by going deeper?

10.1  When you take items out of the dryer they often cling to each other- "static cling".  Explain why.

10.2  You get shocked from static electricity much more often in Colorado than in Florida.  Why?

10.3  Explain why you would probably be safe if lightning struck your car while you were in it.

10.4  Explain why you shouldn't walk around in a storm with an umbrella with a sharp metal point on top.

10.5  You are standing on a mountain top enjoying the view when your hair suddenly begins to stand on end.  This is an indication that lightning is about to strike.  Why?

10.6. Each electric outlet in a home is connected to the power company through a fuse or circuit breaker. When properly installed, current flows from the power company, through the fuse or circuit breaker, through the outlet and the appliances connected to it, and then back to the power company—a complete circuit.  If you plug too many appliances into the circuits in your home or if there is a short circuit that bypasses the appliances altogether, a fuse will "blow" and thereafter no more current will flow in that circuit.  Explain what a fuse actually is, why it "blows" and why, when blown, it inhibits flow of further current in the circuit.  Make your explanation "microscopic" and "mechanical"- that is, explain in terms of what the electrons are doing.

10.7 Repeat problem 10.6 but this time use circuit terms like voltage, resistance, current, and power as well as appropriate equations in your explanation.

11.1 If you drop a magnet over and over again it will eventually diminish in strength.  Why?

11.2  Explain why you cannot stick a permanent magnet to a piece of plastic.

11.3 You pull the wrapper off of a CD case.  You find that it now clings to your hand.  If you pick up a magnet you find that the plastic wrapper also clings to the magnet.  Does this contradict the previous problem?  Explain why or why not.

11.4  You have a coil of wire, a permanent magnet, an AC power source and a DC power source.  Using only these items give three distinct scenarios by which you can create a current in a second stationary coil of wire.

11.5 If you double the DC voltage supplied to a DC motor, what will happen and why?

11.6 If you put the batteries backward into a simple DC motor (as in Figures 11.3.3 and 11.3.4), what will happen and why?

11.7 A typical audio speaker, such as that in a radio, consists of a permanent magnet, a coil of wire, and a movable speaker cone. The coil is mechanically attached to the speaker cone so that if the coil moves, the speaker cone moves and vice versa.  The coil of wire is connected electrically to the wires emerging from the speaker.  The magnet surrounds the coil of wire and is rigidly attached to the speaker itself.  Here is a nice illustration showing this setup.  For the moving version see: http://electronics.howstuffworks.com/speaker6.htm

Imagine that you hook two such identical speakers directly together- that is you hook the wires coming from one set speaker's coil directly to the wires connected to the second speaker's coil with nothing in between.  Then you stand in front of one of the speakers and talk.  What happens and why?

11.8 Suppose two magnetic tapes have pure tones recorded on them.  One has a low frequency tone while the second has a high frequency tone exactly double the low frequency tone.  Explain how the magnetizations of the two tapes differ from one another.  Now suppose you take the tape with the low tone recorded on it and run it at double speed past the tape player's read head.  What does it sound like and why?

11.9  The read and write mechanism for a hard drive is essentially the same as that for a tape recorder.  One significant difference between the two is the size of the magnetic domains.  In a hard drive the domains are much smaller than on a tape.  (This is true in practice although it need not be so).  How does this size difference make reading and writing hard disk domains more technologically challenging?  This is the challenge faced in making hard disks smaller.

13.1 Continuing problem E9, you tune a radio receiver to the appropriate frequency and "listen" to the computer.  What frequency do you need to tune to and what would it sound like at that frequency?  In a research lab this is one way that scientists look for unwanted noise from computers near their sensitive equipment.

13.2 If you stood right next to a radio station broadcast antenna emitting a radio signal, would it be loud?  Why or why not?

13.3 A cordless microphone uses a carrier wave to carry sound information from a performer to the audio system of a theater. Each receiver has two antennas oriented at right angles to one another. Why are two antennas required?

13.4 If the inside of the picture tube's screen didn't have a thin layer of aluminum on it, the electrons would accumulate there.  Why would that be a problem?

13.5 If you hold a strong permanent magnet near the face of a color television picture tube, the image will be distorted in both shape and color.  What causes this distortion?  Even after you remove the magnet, the distortion may remain.  Why?

13.6 When you put your hand near a picture tube that has just been turned on, negative charges flow from your hand to the glass screen (that crackly feeling!).  What attracts these charges to the screen?

13.7 Your eye's refresh rate is about 30 times per second.  That is, your eye takes a picture about every 1/30th of a second.  Use this fact to explain: 1) why a wheel seems to turn backwards if it moves faster than a certain rate (what rate is that?) and 2) why the television screen would flicker if the beam drew a new picture only 10 times per second using the typical phosphors found in TV's.

13.8 What happens when you cook an unbroken egg in a microwave? Why does this happen? What other food uses this process to its advantage?

13.9 Some prepared foods come with browning sheets that contain very thin metallic layers. These sheets become hot in a microwave oven and help to brown the surface of the food. Why does a thin metallic film become so hot?

13.10 Compare how a potato cooks in a microwave oven with how it cooks in an ordinary oven.  Why does it cook faster in a microwave?

16.1 Explain how a nuclear reaction releases so much energy by considering how the fission of one nucleus releases its energy.

16.2 After a nuclear blast there is an "electromagnetic pulse" or EMP.  Explain what it is, how it is created and what its effects are.

16.3 Explain why a mushroom shaped cloud forms after a nuclear blast.

16.4 Carbon dating is a technique used to determine the ages of carbon-containing objects that obtain their carbon from contemporary plants. It's based on a radioactive isotope of carbon, 14C (carbon–14), that's present in our atmosphere in small quantities. The 14C in our atmosphere is produced indirectly by cosmic rays—high-energy particles that enter our atmosphere from elsewhere in the universe. When these cosmic rays collide with nuclei in the atmosphere, they often produce neutrons, and some of these neutrons interact with the nuclei of nitrogen atoms. When it sticks to a 14N nucleus, a neutron causes that nucleus to rearrange and release a proton. The result is a 14C nucleus. 14C is radioactive with a half-life of 5730 years. That means there's a 50% chance of each 14C nucleus spontaneously falling apart during a period of 5730 years. From the 5730-year half-life of 14C, it's possible to calculate the fraction of 14C nuclei that will survive 1 year without decaying: 99.9879%. The fraction of nuclei that survive 2 years without decaying is 99.9879% of 99.9879%, or 99.9758%. And so on. Most carbon nuclei are not radioactive. The carbon in rocks and petroleum is about 99% 12C and about 1% 13C. But because 14C is introduced into the atmosphere at a steady rate and plants obtain their carbon from the atmosphere, 14C nuclei make up a small but important fraction of the carbon nuclei in a living plant. This fraction can be measured in a modern plant to determine its value in an ancient plant while that plant was alive. After the plant died, the fraction of carbon nuclei that were 14C decreased as the 14C nuclei decayed.  Imagine that you found a fossilized fern in your backyard.  Give a step by step explanation of how you would go about figuring out how old the fossil is using the carbon dating method described here.