exam01_sol_practice_f04 - MASSACHUSETTS INSTITUTE OF...

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MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics Physics 8.01T Fall Term 2004 Practice Exam 1 Solutions Concept Questions: Briefly answer the following questions. 1. When a car accelerates forward, which force is responsible for this acceleration? State clearly which body exerts this force, and on which body the force acts. Answer: The friction force of the road acting on the wheels accelerates the car forward. 2. A weightlifter and a barbell are both at rest on a large scale. The weightlifter begins to lift the barbell, ultimately holding it motionless above her head. Does the scale reading ever differ from the combined weight of the two bodies at any time during the lift? Explain. Answer: Choose positive direction upwards. The motion of the barbells is as follows: Stage 1: Barbells are being accelerated upwards as weightlifter jerks them from the floor. In this stage, total total y Nm gm a −= , Since acceleration is positive upwards, total total total y Nmgma mg =+ > so scale shows increase. Stage 2: Barbells move at constant velocity. Since acceleration is zero, total Nmg = so scale doesn’t change. Stage 3: Barbells are being decelerated as the weight lifter brings them to rest above head. Since acceleration is negative upwards, total total total y < so scale shows decrease. 3. As a space shuttle burns up its fuel after take-off, it gets lighter and lighter and its acceleration larger and larger. Between the moment it takes off and the time at which it has consumed nearly all of its fuel, is its average speed larger than, equal to, or smaller than half its final speed? Explain why. Answer: In figure 1, the graph of velocity vs. time for both cases are shown, constant acceleration and non-constant acceleration.
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Figure 1: velocity graphs for constant and non-constant acceleration When the acceleration is constant, the velocity is a linear function of time. Therefore the average velocity is given by (see figure 1) () ,0 , 11 22 xx x x f vv t v v =+ = . Hence for an object starting at rest, the average velocity is one half of the final velocity.
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This note was uploaded on 10/15/2011 for the course PHYSICS 0207239 taught by Professor Mamar during the Spring '09 term at Jordan University of Science & Tech.

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exam01_sol_practice_f04 - MASSACHUSETTS INSTITUTE OF...

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