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Unformatted text preview: ConcepTest 7.1 To Work or Not to Work
Is it possible to do work on an object that remains at rest? 1) yes 2) no ConcepTest 7.1 To Work or Not to Work
Is it possible to do work on an object that remains at rest? 1) yes 2) no Work requires that a force acts over a distance. distance If an object does not move at all, there is no displacement, and therefore no work done. displacement done ConcepTest 7.2a Friction and Work I
A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction? 1) friction does no work at all 2) friction does negative work 3) friction does positive work ConcepTest 7.2a Friction and Work I
A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction? 1) friction does no work at all 2) friction does negative work 3) friction does positive work Friction acts in the opposite direction to the displacement, so the work is negative. Or using the negative definition of work (W = F d cos ), since = 180o, then W < 0. f N displacement Pull mg ConcepTest 7.2b Friction and Work II Can friction ever do positive work? 1) yes 2) no ConcepTest 7.2b Friction and Work II Can friction ever do positive work? 1) yes 2) no Consider the case of a box on the back of a pickup truck. If the box moves along with the truck, then it is actually truck the force of friction that is making the box move. move ConcepTest 7.2d Tension and Work
A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension? 1) tension does no work at all 2) tension does negative work 3) tension does positive work ConcepTest 7.2d Tension and Work
A ball tied to a string is being whirled around in a circle. What can you say about the work done by tension? 1) tension does no work at all 2) tension does negative work 3) tension does positive work No work is done because the force acts in a perpendicular direction to the displacement. Or using the definition of work (W = F d cos ), since = 180o, then W < 0. T v Followup: Is there a force in the direction of the velocity? ConcepTest 7.4 Lifting a Book
You lift a book with your hand in such a way that it moves up at constant speed. While it is moving, what is the total work done on the book? 1) mg r 2) FHAND r 3) (FHAND + mg) r 4) zero 5) none of the above r FHAND v = const a=0 mg ConcepTest 7.4 Lifting a Book
You lift a book with your hand in such a way that it moves up at constant speed. While it is moving, what is the total work done on the book? The total work is zero since the net force acting on the book is zero. The zero work done by the hand is positive, while the work done by gravity is negative. The sum of the two is zero. Note that the kinetic energy of the book does not change either! 1) mg r 2) FHAND r 3) (FHAND + mg) r 4) zero 5) none of the above r FHAND v = const a=0 mg Followup: What would happen if FHAND were greater than mg? ConcepTest 7.6a Free Fall I
Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? 1) quarter as much 2) half as much 3) the same 4) twice as much 5) four times as much ConcepTest 7.6a Free Fall I
Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? 1) quarter as much 2) half as much 3) the same 4) twice as much 5) four times as much Consider the work done by gravity to make the stone fall distance d: KE = Wnet = F d cos KE = mg d Thus, the stone with the greater mass has the greater KE, which is twice as big for the heavy stone. KE Followup: How do the initial values of gravitational PE compare? ConcepTest 7.6b Free Fall II
In the previous question, just before hitting the ground, what is the final speed of the heavy stone compared to the light one? 1) quarter as much 2) half as much 3) the same 4) twice as much 5) four times as much ConcepTest 7.6b Free Fall II
In the previous question, just before hitting the ground, what is the final speed of the heavy stone compared to the light one? 1) quarter as much 2) half as much 3) the same 4) twice as much 5) four times as much All freely falling objects fall at the same rate, which is g. Since the acceleration is the same for both, and the distance is the both same, then the final speeds will be the same for both stones. same ConcepTest 7.8a Slowing Down
If a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr? Assume that the braking force is the same in both cases. 1) 20 m 2) 30 m 3) 40 m 4) 60 m 5) 80 m ConcepTest 7.8a Slowing Down
If a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr? Assume that the braking force is the same in both cases. 1) 20 m 2) 30 m 3) 40 m 4) 60 m 5) 80 m F d = Wnet = KE = 0 1/2 mv2 thus: F d = 1/2 mv2 Therefore, if the speed doubles, doubles the stopping distance gets four times larger. larger ConcepTest 7.8b Speeding Up I
A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph. Which takes more energy, the 030 mph, or the 3060 mph? 1) 0 30 mph 2) 30 60 mph 3) both the same ConcepTest 7.8b Speeding Up I
A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph. Which takes more energy, the 030 mph, or the 3060 mph? 1) 0 30 mph 2) 30 60 mph 3) both the same The change in KE (1/2 mv2 ) involves the velocity squared. squared So in the first case, we have: 1/2 m (302  02) = 1/2 m (900) In the second case, we have: 1/2 m (602  302) = 1/2 m (2700) Thus, the bigger energy change occurs in the second case. case Followup: How much energy is required to stop the 60mph car? ConcepTest 7.9b Work and Energy II
A golfer making a putt gives the ball an initial velocity of v0, but he has badly misjudged the putt, and the ball only travels onequarter of the distance to the hole. If the resistance force due to the grass is constant, what speed should he have given the ball (from its original position) in order to make it into the hole? 1) 2 v0 2) 3 v0 3) 4 v0 4) 8 v0 5) 16 v0 ConcepTest 7.9b Work and Energy II
A golfer making a putt gives the ball an initial velocity of v0, but he has badly misjudged the putt, and the ball only travels onequarter of the distance to the hole. If the resistance force due to the grass is constant, what speed should he have given the ball (from its original position) in order to make it into the hole? 1) 2 v0 2) 3 v0 3) 4 v0 4) 8 v0 5) 16 v0 In traveling 4 times the distance, the resistive force will distance do 4 times the work. Thus, the ball's initial KE must be work 4 times greater in order to just reach the hole  this requires an increase in the initial speed by a factor of 2, 2 since KE = 1/2 mv2. ...
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This note was uploaded on 05/13/2010 for the course PHYSICS 53L taught by Professor Mueller during the Fall '07 term at Duke.
 Fall '07
 Mueller
 Physics, Force, Work

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