Energy+and+Work

Tuesday



Wednesday

EXTRA CREDIT QUESTION (5 pts):

Thursday

Conservation of Energy problems (there is an answer key at the bottom of page 2)

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Monday



Tuesday



Wednesday

Book work: page 170 (1-4), 171 (1-6), 193 (1-10)

Answers:

__**Page 170**__ 1) 1.5 * 10^7 J (or 15,000,000 J) 2) 700 J 3) 1586 J 4) 1.13 meters (thanks Olivia, Kate, Maddy, and Kate)

__**page 171**__ 1) no, no, yes 2) the neighbor did TWICE the work you did 3) negative, positive, negative 4) yes, no, yes 5) 8280 J, 7920 J, 360 J 6) 0.9702 J - 0.462 J = 0.5082 J

__**page 193**__ 1) No, that would mean a change in kinetic energy 2) no, no, yes POSITIVE, yes NEGATIVE 3) No, because regardless of the angle, you have to provide the same amount of work to get it to the top. 4) Tension does not because the force due to tension is never in the direction of motion. Gravity DOES because it works in the Y direction and there is a change in the Y direction. 5) The one with the longer skid marks was going 1.41 times as fast. (you consider that both cars went from some KE down to 0 due to the force of the brakes. If the brakes have the same F but D is twice as long for the one with longer skid marks then you have to assume the work was TWICE as much. Since, in this case, work = change in KE, if the total KE was twice as much and the masses are the same, it must have all come from a higher speed. So, what V squared equals 2? 1.41. 6) Yes he is because he's exerting a force and the ball is moving in that direction. He stops doing work on the ball the moment the ball leaves his foot. During the flight air resistance and gravity will do work on the ball. 7) The person did 52.92 J of work when going vertically and none while moving horizontally.Gravity did -52.02 J of work while the object was moving upward. 8) 2,592,000 J of work. (the total force up - weight of rocket down = ma and "a" should be 1) 9) 47.5 J 10) 6230 J, friction supplies -6230 J of work because it's a constant velocity so there can't be any acceleration. 0.35 is the coefficient of friction. (you know frictional work was 6230, work equals Fd. So, you can find the force of friction pretty easily. After that you know that force of friction = coefficient of friction * normal force. Normal force is 70 N. Viola!)

__**PRACTICE TEST**__

__Conceptual Questions__

1) How are work and force similar? How are they different? 2) Use dimensional analysis to show that you get the same unit (fundamental units) using the equation for PE as you do for KE and work. (not the Joule, I'm talking about the fundamental units here) 3) Explain how the law of conservation of energy explains the transfer of energy during an object's free-fall. 4) If you increase an objects velocity by 2.5 times it's previous velocity, how much more energy does it have? 5) What is "power" and how is it related to work and forces? 6) If the net work on an object is zero, is it moving or not? Or do we not have enough information to know? 7) A rocket shot upward not only gains gravitational potential energy but it also gains kinetic energy due to it's increasing velocity. Why isn't this a violation of the conservation of energy? Where is the extra energy coming from? 8) Under what circumstances would you need to find the X or Y components of a force to determine the work done? Give an example. 9) Under what circumstances would you set the force of friction equal to the force pushing on the box? In general, under what circumstances do you set the force and opposing forces equal to each other? (this could be a lifting force vs. gravity, a pushing force vs. friction, etc) 10) Is work being done on an object that has reached terminal velocity? Give an explanation of your answer.

__Problems__

11) How much work does it take to push a 15 kg crate 9 meters across a carpeted floor with constant velocity? The coefficient of kinetic friction is 0.68. 12) Use conservation of energy to calculate how fast a 35 kg. skateboard will go on the horizontal part of a 7 m high half-pipe if she loses 25% of her energy to friction. How high will she go on the other side if she loses an additional 15% of her remaining energy to friction as she goes up the other side. 13) Use the concepts of work and energy to show how the potential energy of a box at the top of a ramp comes from the work taken to push it there. 14) Two superhero kids are pushing a stalled car (2500 kg) 100 meters up a slight 9 degree hill. One of the kids is pushing with twice the force of the other. If the coefficient of rolling friction between the tires and the road is 0.05 and they're able to accelerate the car at 2 m/s2, How much work does each superhero kid do to get the car to the top? If the weaker of the superhero kids dies on the way up, by what percentage does the remaining kid need to adjust his force in order to continue pushing the car with a constant velocity? 15) The remaining superhero kid goes back in time to the point where the other kid died and together they successfully push the car up the hill with the previously mentioned acceleration. What is the power each superhero kid has to provide with their superhero legs?

__**ANSWER KEY**__

1) work and force are similiar because they both involve forces that cause accelerations. They're different because you can have a force that doesn't cause motion but you can't have work without motion. 2) (you can do without a key for this one) Show that PE, KE, and work all end up with (kg*m^2)/s^2 3) As the object falls, it is transferring some of it's PE into KE, which decreases the height and increases the velocity. At the end it's all KE but always equal to the original amount of total E. 4) 6.25 times as much 5) power is the rate at which work is being done, and work involves forces. 6) Not enough information to know. It could be moving at a constant velocity or it could be standing still. 7) The potential chemical energy inside the rocket fuel is released and added to the entire system's energy, this giving it additional PE as well as KE. 8) If the force isn't parallel to the objects motion. For example, if I was pulling a heavy sled with a rope over my shoulder. I'd be pulling in both the X and Y directions. 9) If there was constant velocity (zero acceleration). In general, if there is no net force--meaning no acceleration, you set both "sides" of the forces equal to each other. 10) Yes, because the force of gravity is still pulling the object down and it is still moving a distance. The NET work may be zero but gravity is still doing work on the object, it's just being balanced out the the work done by air resistance. So, this is a tricky one because the NET force is zero, meaning the net work is zero, but that doesn't mean forces aren't doing work on the object, they're just balancing each other out.