Quiz 6

1.

• Not graded

2. Speed of light and distance to Sun

• If the speed of light is 300,000 km/sec, and it takes light 8 minutes to reach Earth from the sun, how far is the Earth from the sun?

1. 144 million kilometers

2. 144 million meters

3. 40,000 km

4. 40,000 meters

5. 2.4 million kilometers

6. 2.4 million meters

Speed of light and distance to Sun

• If the speed of light is 300,000 km/sec, and it takes light 8 minutes to reach Earth from the sun, how far is the Earth from the sun?

1. 144 million kilometers 2. 144 million meters

3. 40,000 km

4. 40,000 meters

5. 2.4 million kilometers

6. 2.4 million meters

km 1

sec 80x

sec

km 000,000,144

4000,300

seconds minutes

xminute

sec 480

1

8

1

60

distancetime velocity x

3. Eclipses of Jupiter's moon

• In class I demonstrated why an observer that did not move relative to Jupiter would find that the time they measure between an eclipse of one of Jupiter's moons is different from someone who is moving either toward or away from Jupiter.

• If an observer is moving toward Jupiter, will they find that the time between eclipses is more or less than someone who is moving away from Jupiter? – More– Less

3. Eclipses of Jupiter's moon

• In class I demonstrated why an observer that did not move relative to Jupiter would find that the time they measure between an eclipse of one of Jupiter's moons is different from someone who is moving either toward or away from Jupiter.

• If an observer is moving toward Jupiter, will they find that the time between eclipses is more or less than someone who is moving away from Jupiter? – More– Less

Imagine that you are running along the circular path in a counter-clockwise direction. Someone on Jupiter is sending bullets in your direction with a machine gun. As you run from F to G, you are running into the stream of bullets. As you run from L to K, you are running away from the stream of bullets.

When you are running into the stream you get hit more frequently. When you are running away, you get hit less frequently. The guy shooting the bullets says that he sends out one bullet per rotation of its moon, which occurs at a rate of once per 42 hours.

If you were standing on the track, not moving toward or away from the stream, you would get hit with a bullet once every 42 hours.

If you say that the time per rotation is the time between when you get hit with a bullet (because the guy running the machine gun fires every time a full rotation occurred), then you would conclude that the rotations take less time as you run on the track from E to H and more time when you go from H to E.

Jupiter

Shadow of Jupiter. Every time Io (one of its moons), passes into the shadow at C, someone on Jupiter fires a bullet towards Earth.

4. Overlapping waves

In the top panel of the figure, two waves on a rope are shown to be approaching each other. At some instant in time, the waves overlap. Which of the following images best represents the state of the rope when the two waves overlap?

4. Overlapping waves

In the top panel of the figure, two waves on a rope are shown to be approaching each other. At some instant in time, the waves overlap. Which of the following images best represents the state of the rope when the two waves overlap?

Just before overlap

What is left + right?

To figure out what the rope would look like at this time, add the top wave to the bottom wave. To determine what the rope will look like, start by drawing them as dotted lines, which represents what the rope would look like if the other wave was not there.

What is left + right?

At the positions of the blue vertical lines, the amplitude of top wave is higher than bottom by about A/2 so the net amplitude will be +A – A/2 = A/2 at the positions where the blue lines intersect the horizontal.

At the position of the red vertical line, the amplitude of top wave is equal and opposite to the amplitude of bottom wave so the net amplitude is A-A = 0 at the position where the red line intersects the horizontal.

A

A

What is top + bottom?

Repeat the procedure of adding the top to the bottom at many points along the rope to figure out what the net wave will look like.

A/2

Related question

What happens when these waves overlap?

5. Electromagnetic radiation energy

• The equation that tells us how much energy electromagnetic radiation has is E = hc/, where is the wavelength. According to this formula, which of the following will have the most energy?

1. A red photon2. A green photon3. A violet photon4. A yellow photon

5. All are the same

5. Electromagnetic radiation energy

• The equation that tells us how much energy electromagnetic radiation has is E = hc/, where is the wavelength. According to this formula, which of the following will have the most energy?

1. A red photon2. A green photon3. A violet photon4. A yellow photon

5. All are the same

Violet has the smallest wavelength.

6. Electromagnetic radiation speed

• Which of the following forms of electromagnetic radiation has the highest speed in empty space?

(A) A violet photon(B) A green photon(C) A red photon(D) A yellow photon(E) All are the same

6. Electromagnetic radiation speed

• Which of the following forms of electromagnetic radiation has the highest speed in empty space?

(A) A violet photon(B) A green photon(C) A red photon(D) A yellow photon(E) All are the same (and equal to c)

fc Applies to all electromagnetic waves!

7. Photons and Electromagnetic Radiation

• What is the difference between light and electromagnetic radiation?

1. A light photon travels faster

2. They are the same thing. What we call visible light is electromagnetic radiation with a frequency that our eyes can detect.

3. A light photon travels slower

4. Electromagnetic radiation can interact with electromagnets while light cannot

7. Photons and Electromagnetic Radiation

• What is the difference between light and electromagnetic radiation?

1. A light photon travels faster

2. They are the same thing. What we call visible light is electromagnetic radiation with a frequency that our eyes can detect.

3. A light photon travels slower

4. Electromagnetic radiation can interact with electromagnets while light cannot

Overlapping waves 2• In the top panel of the figure, two waves on a

rope are shown to be approaching each other. At some instant in time, the waves overlap. Which of the following images best represents the state of the rope when the two waves overlap?

• A• B• C• D

Overlapping waves 2• In the top panel of the figure, two waves on a

rope are shown to be approaching each other. At some instant in time, the waves overlap. Which of the following images best represents the state of the rope when the two waves overlap?

• A• B• C• D

Draw dotted lines to show what rope would look like of other wave was not there (dotted lines are on top of each other in this case). Result when both waves overlap is sum of both waves.

9. Rotating mirror and the speed of light

• If the rotating mirror is rotated more slowly, what would happen to the deflection angle?

1. The deflection angle would decrease2. The deflection angle would increase

9. Rotating mirror and the speed of light

• If the rotating mirror is rotated more slowly, what would happen to the deflection angle?

1. The deflection angle would decrease2. The deflection angle would increase

As the light travels along 2 and then back along 3, the rotating mirror rotates. As a result, the angle that the beam is reflected back towards the light source is not the same as the angle that it was reflected when it went towards the stationary mirror. This results in a deflection angle. If the rotating mirror is rotated more slowly, (or stopped, for example) then the beam would be reflected right back at the light source and there would not be a deflection angle.

10. Surface Area of a Sphere 2

• How much more paint would it take to paint the surface of a sphere of radius = 4 meters than a sphere of radius = 2 meters?

1. Four times as much paint2. Two times as much paint3. Sixteen times as much paint4. Eight times as much paint

10. Surface Area of a Sphere 2

• How much more paint would it take to paint the surface of a sphere of radius = 4 meters than a sphere of radius = 2 meters?

1. Four times as much paint2. Two times as much paint3. Sixteen times as much paint4. Eight times as much paint

10. Surface Area of a Sphere 2

)4(4)22(4)2(4Area1 2

24Area r

If the radius of a sphere doubles, what happens to the volume?Approach: Choose r1 = 2 and r2 = 4

Solution: Area2 is 4 times bigger than Area1

You don’t need to know the exact formula, but you should know that the surface area depends on r2 (because r2 has unit of area).

)16(4)44(4)4(4Area2 2

11. Pop a cap in ya

• Suppose someone is shooting bullets at you with a machine gun that fires a bullet every 10 seconds. If you run toward the stream of bullets, the frequency at which you get hit will

1. increase2. decrease3. stay the same

11. Pop a cap in ya

• Suppose someone is shooting bullets at you with a machine gun that fires a bullet every 10 seconds. If you run toward the stream of bullets, the frequency at which you get hit will

1. increase2. decrease3. stay the same

12. Using Jupiter to Measure the Speed of Light

The diagram below was used to explain the observation that the time between disappearances of one of Jupiter's moons (because it went into Jupiter's shadow) depended on the relative motion of Earth with respect to Jupiter.

Suppose that the time it takes for Jupiter's moon to do a full rotation is two days. Every time the moon goes through point C, someone on Jupiter sends a bullet towards Earth. Note that Jupiter moves very slowly so you can assume that it does not move in its orbit in the time span of two days.

During the time Earth is moving from F to G, the person on Earth will get hit by a bullet

1. less often than once every two days (for example, once every two days plus a few minutes).2. exactly every two days.3. more often than once every two days (for example, once every two days minus a few minutes).

1. less often than once every two days (for example, once every two days plus a few minutes).2. exactly every two days.3. more often than once every two days (for example, once every two days minus a few minutes).

13. Using Jupiter to Measure the Speed of Light 3

Suppose we put a spacecraft at point E and it does not move. If someone on Jupiter fires a bullet at E every second, how often will the spacecraft get hit with a bullet?

Note that Jupiter moves very slowly so you can assume that it does not move in its orbit in the time span of a second.

13. Using Jupiter to Measure the Speed of Light 3

1. once per second2. once every 5 seconds3. once every 0.5 seconds

The frequency at which you get hit only changes if you are moving towards or away from the bullet stream (signal).

14. Mass and Weight 3

• If the mass of the Earth was twice as large but its radius was the same, what would happen to your mass and weight?

1. Your mass would decrease and your weight would be the same.2. Your mass and weight would both be unchanged. 3. Your mass would increase and your weight would be the same.4. Your mass would be the same and your weight would increase.5. Your mass and weight would both increase.

14. Mass and Weight 3

• If the mass of the Earth was twice as large but its radius was the same, what would happen to your mass and weight?

1. Your mass would decrease and your weight would be the same.2. Your mass and weight would both be unchanged. 3. Your mass would increase and your weight would be the same.4. Your mass would be the same and your weight would increase.5. Your mass and weight would both increase.

14. Mass and Weight 3

2Force

r

GMm

Your mass (which depends on the number of atoms and molecules in your body) would not change. The radius did not change. The only thing that changes in the force equation is M.

Your weight depends on how much Earth is pulling down on you (which is equal in magnitude to how much you are pulling Earth toward you).

2

)(Force1

r

mMG

2

)2(Force2

r

mMG

15. Newton's Law of Gravity 3

• According to Newton's Law of Gravity, if the mass of each object increased by a factor of four, the force that object 1 exerts on object 2

1. increases by a factor of four2. increases by a factor of 16.3. decreases by a factor of 4.4. decreases by a factor of 2.5. increases by a factor of 2.6. does not change.

15. Newton's Law of Gravity 3

• According to Newton's Law of Gravity, if the mass of each object increased by a factor of four, the force that object 1 exerts on object 2

1. increases by a factor of four2. increases by a factor of 16.3. decreases by a factor of 4.4. decreases by a factor of 2.5. increases by a factor of 2.6. does not change.

14. Mass and Weight 3

2Force

r

GMm

2Force1

r

GMm

2216

)4)(4(Force2

r

GMm

r

mMG