# MC Physics 10 Summer 2014 Finals (Short Answer)

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1. For the pulley system shown, what is the upper limit of weight the strongman can lift?

The upper limit he can lift is a load equal to his weight. Beyond that he leaves the ground!
2. Before the time of Galileo and Newton, some learned scholars thought that a stone dropped from the top of a tall mast of a moving ship would fall vertically and hit the deck behind the mast by a distance equal to how far the ship had moved forward while the stone was falling. In light of your understanding of Newton's first law, what do you think about this?
A stone will fall vertically if released from rest. If the stone is dropped from the top of the mast of a moving ship, the horizontal motion is not changed when the stone is dropped—providing air drag on the stone is negligible and the ship’s motion is steady and straight. From the frame of reference of the moving ship, the stone falls in a vertical straight-line path, landing at the base of the mast.
3. Two balls are released simultaneously from rest at the left end of equal-length tracks A and B as shown. Which ball reaches the end of its track first, and why?

The ball on B finishes first, for its average speed along the lower part as well as the down and up slopes is greater than the average speed of the ball along track A
4. For a freely falling object dropped from rest, what is the acceleration at the end of the fifth second of fall?
At the end of the tenth second of fall? Defend your answer.
The acceleration of free fall at the end of the 5th, 10th, or any number of seconds will be g. Its velocity has different values at different times, but since it is free from the effects of air resistance, its acceleration remains a constant g.
5. Since an object weighs less on the surface of the Moon that on Earth's surface, does it have less inertia on the Moon's surface? Why or why not?
No, inertia involves mass, not weight.
6. Three identical blocks are pulled as shown on a horizontal frictionless surface. If tension in the rope held by the hand is 30 N, what is the tension in the other ropes?

Note that 30 N pulls 3 blocks. To pull 2 blocks then requires a 20-N pull, which is the tension in the rope between the second and third block. Tension in the rope that pulls only the third block is therefore 10 N. (Note that the net force on the first block, 30 N – 20 N = 10 N, is the force needed to accelerate that block, having one-third of the total mass.)
7. If you drop a rubber ball on the floor, it bounces back up. What force acts on the ball to provide the bounce?
When the ball exerts a force on the floor, the floor exerts an equal and opposite force on the ball—hence bouncing. The force of the floor on the ball provides the bounce.
8. Suppose that two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released. What is the acceleration of the heavier cart relative to that of the lighter cart as they start to move apart?

The force on each cart will be the same, but since the masses are different, the accelerations will differ. The twice-as-massive cart will undergo only half the acceleration of the less massive cart and will gain only half the speed.
9. Would you care to fire a gun that has a bullet 10 times as massive as the gun? Explain.
No. The gun would recoil with a speed ten times the muzzle velocity. Firing such a gun in the conventional way would not be a good idea!
10. Why does the force of gravity do no work on (a) a bowling ball rolling along a bowling alley and (b) a satellite in circular orbit about the Earth?
The answers to both (a) and (b) are the same: When the direction of the force is perpendicular to the direction of motion, as is the force of gravity on both the bowling ball on the alley and the satellite in circular orbit, there is no force component in the direction of motion and no work is done by the force.
11. When the velocity of an object is doubled, by what factor is its momentum changed? By what factor is its kinetic energy changed?
When the velocity is doubled, the momentum is doubled and the KE is increased by a factor of 4. Momentum is proportional to speed, KE to speed squared.
12. Explain why a long pole is more beneficial to a tightrope walker if the pole droops.

The long drooping pole lowers the CG of the balanced system—the tightrope walker and the pole. Therotational inertia of the pole contributes to the stability of the system also.
13. When will the gravitational force between you and the Sun be greater - today at noon, or tomorrow at midnight? Defend your answer
The gravitational force varies with distance. At noon you are closer to the Sun. At midnight you are an extra Earth diameter farther away. Therefore the gravitational force of the Sun on you is greater at noon.
14. A small light source located 1 m in front of a 1-m^2 opening illuminates a ball behind. If the wall is 1 m behind the opening (2 m from the light source), the illuminated area covers 4 m^2. How many square meters will be illuminated if the wall is 3 m from the light source? 5 m? 10 m?
Tripling the distance from the small source spreads the light over 9 times the area, or 9 m2. Five times the distance spreads the light over 25 times the area or 25 m2, and for 10 times as far, 100 m2.
15. At which of the indicated positions does the satellite in elliptical orbit experience the greatest gravitational force? Have the greatest speed? The greatest velocity? The greatest Kinetic energy? The greatest gravitational potential energy?

The satellite experiences the greatest gravitational force at A, where it is closest to the Earth, the perigee; and the greatest speed and the greatest velocity at A, and by the same token greatest kinetic energy at A, and the greatest gravitational potential energy at the farthest point C.
16. Which contributes more to an atom's mass -electrons or protons? Which of these contributes more to an atom's size?
Protons contribute more to an atom’s mass, and electrons more to an atom’s size.
17. Why does the suspended spring stretch more at the top than at the bottom?

The top part of the spring supports the entire weight of the spring and stretches more than, say the middle, which only supports half the weight and stretches half as far. Parts of the spring toward the bottom support very little of the spring’s weight and hardly stretch at all.
18. Which is more likely to hurt, and why - being stepped on by a 200-lb man wearing loafers or being stepped on by a 100-lb woman wearing high heels?
Pressure would be appreciably greater by the woman, which would hurt you more.
19. Would it be slightly more difficult to draw soda through a straw at sea level or on top of a very high mountain? Explain.
Drinking through a straw is slightly more difficult atop a mountain. This is because the reduced atmospheric pressure is less effective in pushing soda up into the straw.
20. If you drop a hot rock into a pail of water, the temperature of the rock and water will change until both are equal. Does this hold true if the hot rock is dropped into the Atlantic Ocean? Explain.
The hot rock will cool and the cool water will warm, regardless of the relative amounts of each. The amount of temperature change, however, does depend in great part on the relative masses of the materials. For a hot rock dropped into the Atlantic Ocean, the change in temperature would be too small to measure. Keep increasing the mass of the rock or keep decreasing the mass of the ocean and the change will be evident.
21. Since energy is radiated by all objects, why can't we see all objects in the dark?
Human eyes are insensitive to the infrared radiated by objects at average temperatures.
22. On freezing days, frost often forms on windows. Why is there usually more frost on the bottom portions of the windows?
A temperature gradient normally exists in a room, with cooler air near the bottom. Hence frost forms on the colder part of the window, the bottom.
23. If you vigorously shake a can of chicken broth back and forth for more than a minute, will the temperature of the broth increase? Why or why not?
You do work on the liquid when you vigorously shake it back and forth, which increases its internal energy. This is noted by an increase in temperature. (In the mid-nineteenth century, James Joule, by measuring the temperature of a liquid before and after doing known work with a stirring paddle, revealed what he called “the mechanical equivalent of heat,” a discovery that led to the general law of energy conservation.)
24. A piece of iron place on a block of wood makes it float lower in the water. If the iron were instead suspended beneath the wood, would the wood float as low, lower, or higher? Defend your answer.

The block of wood would float higher if the piece of iron is suspended below it rather than on top of it. By the law of flotation: The iron-and-wood unit displaces its combined weight and the same volume of water whether the iron is on top or the bottom. When the iron is on the top, more wood is in the water; when the iron is on the bottom, less wood is in the water. Or another explanation is that when the iron is below—submerged—buoyancy on it reduces its weight and less of the wood is pulled beneath the water line.
25. If the speed of a wave doubles while the wavelength remains the same, what happens to its frequency?
Letting v = f guide thinking, twice the speed means twice the frequency.
26. A pair of loudspeakers on opposite sides of a stage emits identical tones of the same fixed frequency and wavelength. When you stand in the center aisle, you hear the sound loud and clear. Why does the volume of the sound diminish considerably when you step to one side?
When you are equally distant from the speakers, their tones interfere constructively. When you step to one side, the distance to one speaker is greater than the distance to the other speaker and the two waves are no longer in phase. They interfere destructively. (If you step far enough to one side, they will interfere constructively again.)
27. An electroscope is a simple device consisting of a metal ball that is attached by a conductor to two thin leaves of metal foil as shown below. When the ball is touched by a charged body, the leaves that normally hang straight down spread apart. Explain why they do so.

The leaves, like the rest of the electroscope, acquire charge from the charged object and repel each other because they both have the same sign of charge. The weight of the conducting metal foil is so small that even tiny forces are clearly evident.
28. A car's headlights dissipate 40 W on low beam and 50 W on high beam. Is there more or less resistance in the high beam filament compared to that of the low beam filament?
There is less resistance in the higher wattage lamp. Since power = current x voltage, more power for the same voltage means more current. And by Ohm’s law, more current for the same voltage means less resistance. (Algebraic manipulation of the equations P = IV and I = V/R leads to P = V2/R.)
29. Magnet A has twice the magnetic field strength of Magnet B (at equal distance) and, at a certain distanced, Magnet A pulls on Magnet B with 50 N of force. With how much force does Magnet B pull on Magnet A?
Back to Newton’s 3rd law! Both A and B are equally pulling on each other. If A pulls on B with 50 newtons, then B also pulls on A with 50 newtons. Period!
30. Your physics instructor drops a magnet through a long vertical copper pipe and it moves slowly compared with the drop a a nonmagnetized object. Provide an explanation for this.
As the magnet falls, it induces current that circles in the conducting pipe and is accompanied by its own magnetic field. The moving magnet is slowed by interaction with this induced field.
31. Which has the longest wavelength: Visible waves, X-rays, or radio waves?
The wavelengths of radio waves are longer than those of light waves, which are longer than the wavelengths of X-rays.
32. Why is red paint red?
Red paint is red because it reflects the red component of white light, while absorbing the other components.
33. What will happen to the image projected onto a screen by a lens when you cover one-third of the lens with a red filter, one-third with a green filter, and one-third with a blue filter?
The image will be a bit dimmer with original colors, but otherwise unaffected.
34. By how much should a pair of light rays from a common source differ in distance traveled to produce destructive interference?
By a half wavelength, or an odd number of half-wavelengths.
35. Light from an incandescent source is passed through sodium vapor and then examined with a spectroscope. (a) What is the appearance of the spectrum? The incandescent source is switched off and the sodium is heated until it glows. (b) How does the spectrum of the glowing sodium compare with the previously observed spectrum?
(a) An “absorption spectrum” is observed, with certain dark lines in a background of continuous light. (b) The “emission spectrum” will contain a few bright lines, most of which will match the lines in the absorption spectrum.
36. Sketch the spectrum you would see from an incandescent light bulb, and the spectrum you would see from a fluorescent light bulb.
• Incandescent (Blurry rainbow)
• Fluorescent (Distinct color lines in a rainbow)
 Author: elitesat ID: 279449 Card Set: MC Physics 10 Summer 2014 Finals (Short Answer) Updated: 2014-07-24 04:02:38 Tags: Physics Folders: Description: MC Physics 10 Summer 2014 Finals for short answer questions. Show Answers: