Flux and Hostile Aliens

Here is an eclectic mix of problems to get you thinking. Enjoy!

1. Consider this tapering pipe with incompressible water flowing to the right.

A)How does the flux through

A1 compare with the flux through A2? Let flux be defined as we did in the lab as the volume of water flowing through an area per time.

B) How does the speed of the water v2 at A2 compare with the speed v1 at A1? Find the ratio v2/v1.

C) How does the flux through A3 compare with A1 and A2?

D) What is the flux though A4?

2. Hostile aliens vaporize the Sun. (The Sun is normally about 8 light minutes away.)

A) Where is the Sun’s light 4 minutes after the Sun is vaporized*? Sketch all the light from the Sun in cross-section. (Use yellow ink!) Note the Sun’s original location.

HW #1

A2

A1

A4

A3

1

B) Do earthlings know anything has happened to the Sun 4 minutes after vaporization? How long after vaporization will it take before earthlings know that the Sun is gone?

C) Speculate about what will happen to the Earth as time goes by.

*Assume that “vaporized” means instantly removed from our universe.

AliensNot to Scale!

Sun’s former location

EARTH

3. Now let’s change gears and try to get a handle on the scale of the Milky Way Galaxy we live in. Our sun is one of about two hundred billion stars in a disc that is 100,000 light years* across. Reduce the Sun (1 million miles across) to the size of the period at the end of this sentence. How far across will our scale galaxy be?

2

Take a rough guess before you do any math and record your guess here:

Now do the math, using estimates and approximations. Explain each step!

*A light year is the distance light travels in a year. Light travels 186,000 mi/s (3.0 x 108 m/s.)

Scale Milky Way Diameter Guess =

Scale Milky Way Diameter Calculation = =

(alg) (#)

3. Now let’s change gears and try to get a handle on the scale of the Milky Way Galaxy we live in. Our sun is one of about two hundred billion stars in a disc that is 100,000 light years* across. Reduce the Sun (1 million miles across) to the size of the period at the end of this sentence. How far across will our scale galaxy be?

2MASS map of half a billion stars in our Milky Way Galaxy

3