212 - Problem Set #14

Physics 212 Fall 2008 The Very Last Problem Set Read: 28.1 – 28.4, 28.6, 28.7 and do the following. Due Wednesday, December 10th. 1)

2)

3)

An Unidentified Flying Object has a red flashing light mounted at the bottom of its fuselage. The light flashes every 1.5 s. You are out in the desert drinking heavily with your posse. You see the flying saucer and observe that the light flashes every 2.5 s. Assuming that your inebriated state is not the cause of the time difference, calculate the velocity of the UFO. Skid purchases a new Mustang that can achieve velocities approaching the speed of light. Skid drives by you in the Mustang at 0.9c relative to the Earth. As it passes, you determine that the length of the car is 1.4 m. When Skid eventually comes back and stops to chat with you what length do you measure the car? If astronauts could travel at 0.95c, we on Earth would say it takes 4.42 years to reach Alpha Centauri, which is 4.2 lighters away. The astronauts disagree. (a) How much time passes relative to the astronauts? (b) What is the distance relative to Alpha Centauri relative to the astronauts?

4)

A friend in a starship travels past you at a high speed. He tells you that his ship is 20 m long and that the identical ship you are in is 19 m long. Relative to you, (a) how long is your ship, (b) how long is his ship, and (c) what is the speed of his ship?

5)

An electron is accelerated from rest to speed of 0.99c. What amount of work is done on the electron?

7)

Calculate the velocity of a proton when its total energy is equal to four times its rest energy.

8)

A spacecraft from Barnard’s Star approaches Earth and launches an exploration vehicle. After the launch, an observer on Earth sees the spacecraft approaching at a speed of 0.5c and the exploration vehicle approaching at a speed of 0.7c. What is the velocity of the exploration vehicle relative to the spacecraft?

9)

Greezy-X is walking down Lincoln Ave. in Chicago after a late night at The Metro. He notices Small Spaceship #1 approaching from the right at 0.75c and Small Spaceship #2 approaching from the left at 0.65c. What is the relative velocity between the two spaceships as measured by an alien on one of them?

10) An electron moves to the right with a speed of 0.9c relative to the laboratory frame of reference. A proton moves to the left with a speed of 0.7c relative to the electron. Calculate the speed of the proton relative to the lab frame.

ANSWERS: 1) 0.8c 2) 3.21 m 3) a) 1.38 years b) 1.31 light-years 4) c) 0.31c

5)5.0 x 10 6)

6)

A proton moves with a speed of 0.95c. Calculate its (a) rest energy, (b) total energy, and (c) kinetic energy.

-13 J a) 939 MeV b) 3.0 GeV c) 2.06 GeV

7) 0.968c 8) 0.31c 9) 0.94c 10) 0.54c