Physics 101: Homework Problems for Extra Credit
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To be counted for extra credit (adds up to 5% to the final grade) these problems must be turned in by the due date.
Solutions will be posted after the due date.
You are also encouraged to look at the exercises and problems in your book
Chapters 2 and 3:due Feb 8
1) Asteroids have been moving through (mostly empty) space for billions of years. What keeps them moving?
2) As you stand on a floor, does the floor exert an upward force against your feet? How much force does it exert? Why
are you not moved upward by this force?
3) For a free-falling object dropped from rest, what is its speed at the end of the 5th second of fall? What about its
acceleration at that time?
4) a) Can an object be moving when its acceleration is zero? If so, give an example.
b) Can an object be accelerating when its speed is zero? If so, give an example.
5) If it were not for air resistance , why would it be dangerous to go outdoors on rainy days?
Chapters 4 and 5:due Feb 15
1) If and when Galileo dropped two balls from the top of the Leaning Tower of Pisa, air resistance was not really
negligible. Assuming both balls were the same size, one made of wood and one of metal, which ball struck the ground
first?
2) A firefighter of mass 80kg slides down a vertical pole with an acceleration of 4 m/s2. What is the friction force that
acts on the firefighter?
3) You push a heavy car by hand. The car, in turn, pushes back with an opposite but equal force. Doesn’t this mean the
forces cancel one another, making acceleration impossible? Why or why not?
4) Suppose two carts, one twice as massive as the other, are held together and joined by a compressed spring. They will
fly apart when the compressed spring that joins them is released. How fast does the heavier cart roll compared to the
lighter cart?
Chapters 6 and 7: due Feb 22
1) When an apple falls from a tree and strikes the ground without bouncing, what becomes of its momentum?
2) You are at the front of a floating canoe near a dock. You jump, expecting easily to land on the dock. Instead you land
in the water. Explain.
3) Someone wanting to sell you a “Superball” claims that it will bounce to a height greater than the height from which it
was dropped. Can this be? (note that the ball is dropped from rest, not thrown)
4)In the absence of air resistance, a ball thrown vertically upward with a certain initial kinetic energy will return to its
original level with the same kinetic energy. When air resistance is a factor affecting the ball, will it return to its original
level with the same, less, or more kinetic energy? Does your answer contradict the law of conservation of energy?
Explain.
Chapters 8 and 9: due Mar 1
1) Why must you bend forward when carrying a heavy load on your back? (hint: consider stability)
2) If the polar ice caps of the Earth were to melt, the oceans would be deeper by about 30 m. What effect would this
have on the Earth’s rotation?
3) A friend says that since Earth’s gravity is so much stronger than moon’s gravity, rocks on the moon could be dropped
to the Earth. What goes wrong with his conclusion?
4) Most people today know that the ocean tides are caused principally by the gravitational pull of the moon. And most
people therefore think that the gravitational pull of the moon on the Earth is greater than that of the sun on the Earth.
What do you think?
Chapters 11 and 13: due Mar 15
1) a) A particular atom contains 29 electrons, 34 neutrons, and 29 protons. What is the atomic number of this element,
and what is its name?
b) If one proton is somehow added to the atom in (a), when what would its atomic number and name be? How about if
an electron was instead added? And what about if instead a neutron was added?
2) Which contributes more to an atom’s mass: electrons or protons? Which contributes more to an atom’s size?
3) If you cut your finger, why does holding it above your head reduce bleeding?
4) Why is it inaccurate to say that heavy objects sink and that light objects float? Give exaggerated examples to support
your answer.
5) The relative densities of water, ice, and alcohol are 1.0, 0.9, and 0.8, respectively. Do ice cubes float higher or lower in
a mixed alcoholic drink compared to a drink of water? What can you say about a cocktail in which the ice cubes lie
submerged at the bottom of the glass?
Chapter 14 and 15: due Mar 22
1) The pressure exerted against the ground by an elephant’s weight distributed evenly over its four feet is less than 1
atmosphere. Why, then, would you be crushed beneath the foot of an elephant, while you’re unharmed by the pressure
of the atmosphere?
2) Estimate the buoyant force that air exerts on you. To do this, you can estimate your volume by knowing your weight
and by assuming your weight density is a bit less than that of water.
3) Desert sand is very hot during the day and very cool at night. What does this tell you about its specific heat?
4) Suppose that water is used in a thermometer instead of mercury. If the temperature is at 4oC and then changes, why
can’t the thermometer indicate whether the temperature is rising or falling?
Chapter 19 and 20: due Apr 12
1) Astronomers find that light emitted by a particular element at one edge of the sun has a slightly higher frequency
than light from that element at the opposite edge. What do these measurements tell us about the sun’s motion?
2) A weight suspended from a spring is seen to bob up and down over a distance of 20 cm twice each second. What is its
frequency? Its period? Its amplitude?
3) What two physics mistakes occur in a science fiction movie that shows a distant explosion in outer space where you
see and hear the explosion at the same time?
4) Why is the sound of a harp soft in comparison with the sound of piano?
Chapter 22 and 23: due Apr 16 (*Note this is Tues, since midterm2 is on 19th*)
1) Consider two charged particles a certain distance apart, e.g. two electrons.
a) What is likely to be larger, the gravitational or the electrical force between them?
b) How would the electric force between them change if they are brought to half their original distance of separation?
What about the gravitational force?
2) If you are caught outdoors in a thunderstorm, why should you not stand under a tree? Can you think of a reason why
you should not stand with your legs far apart? Or why lying down can be dangerous? (Consider electric potential
difference).
3) Will the current in a light bulb connected to a 220-V source be greater or less than when the same bulb is connected
to a 110-V source?
4) If electrons flow very slowly through a circuit, why does it not take a noticeably long time for a lamp to glow when you
turn on a distant switch?
5) Consider a pair of flashlight bulbs connected to a battery. Will they glow brighter connected in series or in parallel?
Will the battery run down faster if they are connected in series or in parallel? Note that the brightness of a bulb
measures the power.
Chapters 24 and 25: due Fri May 3rd
1) Can an electron placed at rest in a magnetic field be set in motion by the magnetic field? What if were placed at rest
in an electric field?
2) If you had two bars of iron, one magnetized and the other not, and no other materials at hand, can you think of a way
of determining which bar was the magnet?
3) A magician places an aluminum ring on a table, under which is hidden an electromagnet (i.e. a coil of wire that may
carry a current). When the magician says “abracadabra” (and discreetly pushes a switch that turns on the current in the
coil), the ring jumps in the air. Explain his magic track.
4) Can an efficient transformer step up energy? Explain your answer.
Chapters 26 and 27: due Fri May 10th
1) What is the same about radio waves and light? What is different about them?
2) Is glass transparent to light of frequencies that match its own natural frequencies?
3) Why do objects illuminated by moonlight lack color?
4) How could you use the colored spotlights at a play to make the yellow clothes of a performer suddenly appear black?
5) If the sky on a certain planet in the solar system were normally orange, what color would sunsets be?
Chapter 31: due Tues May 14th
1) Sunburn produces cell damage in the skin. Why is ultraviolet radiation capable of producing this damage, while visible
radiation, even if more intense, is not?
2) What experiments/evidence can you cite for the wave nature of light? How about for the particle nature of light?
3) Explain the following joke: Heisenberg is out for a drive when he's stopped by a traffic cop.
The cop says, "Do you know how fast you were going?" Heisenberg says, "No, but I know where I am."