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Day 10) Hubble’s Law 9.7
Name __________________
Measuring Speed
1) Measure and calculate the speed of a student walking around the classroom without leaving your seat.
Describe how you did this.
v = d/t = 2r/T. They need to estimate the distance and use phones to measure the time.
2) This same technique can be used to measure the speed of the Earth as it orbits the sun, but it is hard to
measure speeds of very distant objects because they don’t appear to move as much as close objects.
Instead astronomers use the Doppler Shift.
a) Watch Example of Doppler Shift using car horn. https://www.youtube.com/watch?v=a3RfULw7aAY
What happens to sound as it comes towards you? Higher pitch Moves away from you? Lower pitch. Where
else have you heard this? (Ambulance, race cars going around a track.)
b) Watch The Doppler Effect: What does motion do to waves? youtube.com/watch?v=h4OnBYrbCjY
What color would a blue light appear if it were moving toward you?
A) violet
B) blue
C) green
D) red
A) It is ‘blue-shifted’ which means shifted toward the blue end of the spectrum - not to blue light.
Demonstrate this technique with a speed gun.
3) The table below shows one of the spectral lines for calcium. Use a ruler to draw a line from the spectral
line to the speed scale at the bottom. Record the speed in the table and then graph the data.
a) How are the speeds and distances related? The farther they are, the faster they are moving.
b) Graph the data and draw a line of best fit and calculate the slope using this line. Show your steps on the
graph. Don’t forget units and powers of ten. It will be around 4 x 1017 s. They should use the line and the
largest rise and run.
c) Convert the units of the slope to years. It should be around 14 x 109 years.
d) What is the meaning of the slope? The age of the universe!
Edwin discovered this relationship 80 years ago. However, he graphed the speed on the vertical axis. This
has switched to make it easier for the students. The slope is the age of the universe.
4) Watch Alice and Bob: Is that star really there? https://www.youtube.com/watch?v=F2VOMnVyY1s
How is looking far away like looking back in time?
5) The light from all of these galaxies is red-shifted. Which way are all the galaxies moving? Why?
All of the galaxies (except Andromeda) are moving away. Why? This is the subject of the next class.
6) This linear relationship gives third way to measure distance. Explain.
The amount of red shift gives speed. The speed gives distance.
Textbook Consolidation: Read pages 393-394 and answer questions 1, 3, 4, on page 397