PHYS 101 2010 Midterm

# PHYS 101 2010 Midterm - PHYS 101 MIDTERM October 22nd 2010...

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Phys 101 Midterm, Oct 22 nd , 2010 PHYS 101 MIDTERM October 22 nd , 2010 The exam comprises two parts: 6 short-answer questions , and 4 problems . Calculators are allowed. Answer all the short-answer questions with a few words or a phrase, but be concise, please! For the problems, your grade will be calculated with the best three problems . Show your work. The short answer problems are worth four points each, and the problems are worth 10 points each. Put all answers in the red and white answer booklets provided, and please return this exam with your exam booklet. Good luck ! WITH SOLUTIONS !! Short answer questions (answer all): you should not need to do any calculations for these questions. Answer in a few words, a short phrase, or a simple sketch. 1) [4 pts] To convey an impression of power, a car ad claims that “the force of the car’s acceleration pushes you back in your seat”. Is this a correct description of the physics of the situation? If not, explain briefly. Solution : The acceleration does not “push you back in your seat” – you are moving with the car (and hence accelerating), and the seat is providing the force (in the forward direction) for this acceleration. The reaction force is you ‘pushing back’ on the seat. 2) [4 pts] Two untethered (unattached) astronauts on a space walk decide to have a game of catch, lobbing a ball back and forth. Describe what happens as the game progresses (that is, as each astronaut in turn catches, and re-throws, the ball). Would it be safest for the astronauts to do this with a ping-pong ball (remember, there’s no atmosphere so there’s no air friction) or a medicine ball (a medicine ball is a very massive ball)? Solution : As an astronaut catches the ball, they acquire velocity “outward” (away from the center of mass of the two astronauts”) due to conservation of momentum in the collision. When they re-throw the ball, they again acquire more velocity

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backwards (conservation of momentum again), and so on. On each exchange, both astronauts move further apart, and faster apart. This would be safer (assuming large distances are not good, since the astronauts must eventually return to their spacecraft) with a ping-pong ball, where the momentum is much smaller (assuming the two balls are thrown at the same speed, or the same energy). 3) [4 pts] A projectile is launched over horizontal ground at an angle between (but excluding) 0 o and 90 o . a) Is there any point on the trajectory where velocity and acceleration are parallel ? If so, where? b) is there any point on the trajectory where velocity and acceleration are perpendicular to each other? If so, where? Solution
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## This note was uploaded on 11/02/2011 for the course PHYS 101 taught by Professor Prof during the Fall '08 term at McGill.

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PHYS 101 2010 Midterm - PHYS 101 MIDTERM October 22nd 2010...

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