Week 2 review quiz The due date for this quiz is Wed 24 Apr 2013 12:29 PM IST +0530.

This Week 2 review quiz has 12 multiple-choice questions to answer. You may take the quiz up to three times for credit. (You may continue to take it after the third time, but your scores will not count. Also note that the questions may be different each time you take it.) Your highest score of the first three (or fewer) times you take it will count as your final score. The regular deadline to take the quiz is Tuesday, April 23 at 11:59 pm Pacific time. You may take it after that up until Monday, April 29 at 11:59 pm Pacific time, but there will be a 10% late penalty assessed on that particular score if it is one of the first three times you take it. Note: In questions involving world lines and the like, assume that all persons involved agree on which direction is "to the right" (positive x direction) and which is "to the left" (negative x direction). Also assume that, if time is involved, their clocks are synchronized and all use the same time for t = 0.

Question 1 What is the simplest scientific meaning of the phrase “time is the fourth dimension”? That there are four dimensions of time, only one of which we experience. That time travel is possible. That an event that occurs in space and time can be described by three coordinates (x,y,z) representing the spatial location and a fourth coordinate representing the time the event occurs. That an event that occurs can be specified using a lattice of four clocks.

Question 2 Consider Bob on Earth. At time t = 0, Alice flies by in her spaceship at constant velocity, heading to the right. Which statement below is correct in describing who is at rest (not moving)? Because Bob is on Earth and Alice is flying by in a spaceship, Bob is at rest but Alice is not. Both Bob and Alice may consider themselves at rest, each in their own frame of reference. Because Alice is in a spaceship that can stop moving, she can consider herself at rest, but only when she's not moving with respect to the Earth. Because Bob is on Earth, and the Earth is constantly moving around the sun, Bob cannot consider himself at rest.

Question 3 Consider person #1 moving to the right (positive x direction) at a constant velocity v with respect to person #2. Assume that at time t = 0 the two were side by side. Each person measures distances using the same units (such as meters), and each person uses their own location as the origin for their measuring system. Let the measurements of person #1 be represented by x1 and the measurements of person #2 be represented by x2. Consider an event that happens at some position x2 and time t (after time t = 0), according to person #2. Person #1 measures the location of this event at position x1. Would the value of x1 be greater than or less than x2? x1 will be greater than x2 x1 will be less than x2

Question 4 Which world line represents an object that is moving with a varying velocity along the x axis?

Question 5 Consider Bob on Space Station Omega in deep space. At time t = 0, Alice flies by in her spaceship at constant velocity v, heading to the right, while Kris flies by in another spaceship at the same velocity as Alice, but heading left. Which diagram represents the world lines of Alice and Bob in Kris’s frame of reference?

Question 6 Consider Bob on Space Station Omega in deep space. At time t = 0, both Alice and Kris fly by Bob heading to the right at the same velocity, with Alice in the lead. (Alice passes by Bob at t = 0, with Kris slightly behind her.) Which diagram represents the world line of Alice in Kris’s frame of reference?

Question 7 Consider a train with three cars, moving to the right past Alice, who is on a train station platform. The front of the first car reaches Alice at time t = 0. In Alice’s frame of reference, which set of world lines represents the world lines of the three cars? (Let a world line represent the front of the car.)

Question 8 Consider Bob on Space Station Omega. At time t = 0, Alice flies by in her spaceship at constant velocity, heading to the right. She then quickly reverses direction and heads to the left and passes Bob. Which diagram represents Alice’s world line in Bob’s frame of reference?

Question 9 What is the relationship between the velocity of an object moving at constant velocity along the x axis and the slope of its world line? velocity = slope/"rise" velocity = slope velocity = slope/"run" velocity = 1/slope

Question 10 Which world line represents an object moving to the right along the x axis and slowing down (decelerating)?

Question 11 What is the inertia of an object? It is equivalent to the velocity of an object. It refers to whether the object is moving up or down. It is a combination of the velocity of an object and the direction it is moving. It is the object’s resistance to being put into motion if it is at rest, or to having its velocity changed if it is moving at some velocity.

Question 12 Alice creates a machine that can shoot basketballs in a horizontal direction. Using a radar gun, Bob measures the speed of the basketball coming from the machine when it is stationary and finds that it is 30 km/hour. If Alice then puts the machine on top of a car, which she is driving at 10 km/hour away from Bob, and shoots a basketball behind her and toward Bob, what would be the horizontal speed of the ball as Bob measures it, and which direction is it going? (Neglect any effects of air resistance or other sources of friction.) 20 km/hour, toward Bob 30 km/hour, toward Bob 20 km/hour, away from Bob 10 km/hour, away from Bob

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