# HW7.sol - homework 07 RAHMAN, TARIQUE Due: Feb 15 2008,...

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homework 07 – RAHMAN, TARIQUE – Due: Feb 15 2008, 3:00 am 1 Question 1, chap 5, sect 1. part 1 of 1 10 points An elevator is being lifted up an el- evator shaft at a constant speed by a steel cable as shown in the ±gure be- low. All frictional e²ects are negligible. steel cable Elevator going up at constant speed In this situation, forces on the elevator are such that 1. the upward force by the cable is smaller than the downward force of gravity. 2. the upward force by the cable is greater than the downward force of gravity. 3. the upward force by the cable is equal to the downward force of gravity. correct 4. the upward force by the cable is greater than the sum of the downward force of gravity and a downward force due to the air. 5. None of these. (The elevator goes up because the cable is being shortened, not be- cause an upward force is exerted on the eleva- tor by the cable.) Explanation: Since the elevator is being lifted at a con- stant speed, the net force on it is zero, there- fore, the upward force by the cable is equal to the downward force of gravity. Question 2, chap 5, sect 1. part 1 of 1 10 points A space probe is carried by a rocket into outer space where it continues to move on its own in a straight line. What keeps the probe moving? 1. Nothing keeps the probe moving. The probe will eventually stop. 2. The gravitation forces from di²erent stars and planets keep the probe moving. 3. There must be a propeller that keeps the probe moving. 4. None of the above 5. Nothing speci±c keeps the probe moving. In the absence of forces it would continue moving in a straight line. correct Explanation: According to the law of inertia, every ob- ject remains at rest, or remains in motion with constant velocity in a straight line unless ex- ternal forces act on it. Thus inertia keeps the probe moving in outer space. Question 3, chap 5, sect 1. part 1 of 1 10 points Start a ball rolling down a bowling alley and you’ll ±nd it moves slightly slower with time. Does this violate Newton’s law of inertia? Defend your answer.

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## This note was uploaded on 02/03/2010 for the course PHY 303K taught by Professor Turner during the Fall '08 term at University of Texas.

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HW7.sol - homework 07 RAHMAN, TARIQUE Due: Feb 15 2008,...

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