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Test 1 Fall 2008

# Test 1 Fall 2008 - PHYS 2211 Test 1 Name(print)Key-EFG...

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PHYS 2211, Test 1, September 18, 2008 Name (print)Key-EFG Instructions Read all problems carefully before attempting to solve them. Your work must be legible, and the organization must be clear. You must show all work, including correct vector notation. Correct answers without adequate explanation will be counted wrong. Incorrect work or explanations mixed in with correct work will be counted wrong. Cross out anything you don’t want us to read! Make explanations correct but brief. Don’t write a lot of prose. Include diagrams! Show what goes into a calculation, not just the final number, e.g.: a · b c · d = (8 × 10 - 3 )(5 × 10 6 ) (2 × 10 - 5 )(4 × 10 4 ) = 5 × 10 4 Give standard SI units with your results. Unless specifically asked to derive a result, you may start from the formulas given on the formula sheet, including equations corresponding to the fundamental concepts. If a formula you need is not given, you must derive it. If you cannot do some portion of a problem, invent a symbol for the quantity you can’t calculate (explain that you are doing this), and use it to do the rest of the problem. Honor Pledge “In accordance with the Georgia Tech Honor Code, I have neither given nor received unauthorized aid on this test.” Sign your name on the line above

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Problem 1 (25 Points) 0 2 4 6 8 10 100 80 60 40 20 0 20 40 60 80 100 t p y (t) (a, 6 pts) Consider the parabolic motion (in the x-y coordinate system) of a cannonball in the presence of the Earth’s gravitational field. If the Y-component of the momentum, p y , changes with time, t , (as shown in the graph above) why is p y changing sign? Define our system to be the cannonball and the surroundings to be the earth. -→ F net = 0 , - mg, 0 and by the momentum principle, Δ p y = - mg Δ t , we see that p y is changing with time. From the graph we see that the initial momentum of the cannonball, p y, 0 , is positive. The momentum of the cannonball will remain positive until the ball reaches the top of its trajectory because it is raising along the positive Y-direction. The change in sign occurs when the ball travels along the direction of the net force, which is the negative Y-direction. (b, 6 pts) Why does p y depend linearly on time and why is the slope negative? p y will depend linearly on time because gravity and the mass of the cannonball are constant. This implies that the solution to the momentum equation given in (a) is p y ( t ) = - mgt + p y, 0 .
(c, 6 pts) On the graph below, draw the time evolution of the Y-component of the cannonball’s position.

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Test 1 Fall 2008 - PHYS 2211 Test 1 Name(print)Key-EFG...

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