Practice Homework 4 Solutions

# Practice Homework 4 Solutions - practice 04 – ALIBHAI,...

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Unformatted text preview: practice 04 – ALIBHAI, ZAHID – Due: Feb 11 2007, 4:00 am 1 Question 1 part 1 of 2 10 points Three objects can only move along a straight, level path. The graphs below show the position d of each of the objects plotted as a function of time t . d t I d t II d t III The magnitude of the velocity bardbl vectorv bardbl of the object increases in which of the cases? 1. I and II only 2. I only 3. III only correct 4. I, II, and III 5. II only 6. II and III only 7. I and III only Explanation: Case I: The object moves at constant speed. Case II: The object remains at rest. Case III: The speed of the object increases with time; i.e. , constant acceleration. Thus, the magnitude of the velocity of the object increases only in case III. Question 2 part 2 of 2 10 points The sum of the forces summationdisplay F i = 0 on the object is zero in which of the cases? 1. II and III only 2. I and III only 3. II only 4. I only 5. III only 6. I and II only correct 7. I, II, and III Explanation: When the sum of forces on the object is zero, the acceleration of object is zero by Newton’s 2nd law. In cases I and II, the velocity of the object doesn’t change with the time, so the sum of the forces on the object is zero. In case III, the object is accelerating, so the sum of forces on the object is not zero. Question 3 part 1 of 1 10 points A block of mass m accelerates with acceler- ation g up a frictionless plane that is inclined at an angle α above the horizontal. The force F o that pushes the block is at an angle β above the horizontal. Find the force F o . m α β F o 1. mg 1 + sin β cos( β − α ) 2. mg 1 cos( α + β ) 3. mg 1 + sin α cos( β − α ) 4. mg 1 − sin β cos( α + β ) 5. mg 1 − sin β cos( β − α ) practice 04 – ALIBHAI, ZAHID – Due: Feb 11 2007, 4:00 am 2 6. mg 1 + sin α cos( α + β ) correct 7. mg 1 + sin α cos( α − β ) 8. mg sin β cos( α + β ) 9. mg 1 + sin β cos( α − β ) 10. mg 1 + sin β cos( α + β ) Explanation: The acceleration up the plane is given, a = g . The angle that F o makes with respect to the inclined plane is α + β . So the component of F o which is parallel to the inclined plane is F o cos( α + β ) Therefore F o cos( α + β ) − mg sin α = ma = mg F o = mg 1 + sin α cos( α + β ) Question 4 part 1 of 3 10 points The following 3 questions refer to the colli- sions between a car and a truck whose weight is much heavier than the car ( M ≫ m ). For each description of a collision below, choose the one answer from the possibilities that best describes the size (or magnitude) of the forces between the car and the truck. m M v v They are both moving at the same speed when they collide. 1. The truck exerts the same amount of force on the car as the car exerts on the truck....
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## This note was uploaded on 10/13/2009 for the course PHY 303K taught by Professor Turner during the Spring '08 term at University of Texas.

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Practice Homework 4 Solutions - practice 04 – ALIBHAI,...

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