Homework 3U Energy

Unit 3 Energy: Homework L1 Work Read from Lesson 1 of the Work, Energy and Power chapter at The Physics Classroom: http://www.physicsclassroom.com/Class/energy/u5l1a.html http://www.physicsclassroom.com/Class/energy/u5l1aa.html 1. An impulse is a force acting over some amount of time to cause a change in momentum. On the other hand, work is a ______________ acting over some amount of ___________________ to cause a change in __________________. 2. Indicate whether or not the following represent examples of work. Work Done? a. A teacher applies a force to a wall and becomes exhausted. Explanation: Yes or No? b. A weightlifter lifts a barbell above her head. Explanation: Yes or No? c. A waiter carries a tray full of meals across a dining room at a constant speed. Explanation: Yes or No? d. A rolling marble hits a note card and moves it across a table. Explanation: Yes or No? e. A shot-putter launches the shot. Explanation: Yes or No? 3. Work is a ______________; a + or - sign on a work value indicates information about _______. a. vector; the direction of the work vector b. scalar; the direction of the work vector c. vector; whether the work adds or removes energy from the object d. scalar; whether the work adds or removes energy from the object

4. Which sets of units represent legitimate units for the quantity work ? Circle all correct answers. a. Joule b. N x m c. Foot x pound d. kg x m/sec e. kg x m/sec 2 f. kg x m 2 /sec 2 5. Indicate whether there is positive (+) or negative (-) work being done on the object. a. An eastward-moving car skids to a stop across dry pavement. b. A freshman stands on his toes and lifts a World Civilization book to the top shelf of his locker. c. At Great America, a roller coaster car is lifted to the peak of the first hill on the Shock Wave. d. A catcher puts out his mitt and catches the baseball . e. A falling parachutist opens the chute and slows down. 6. Before beginning its initial descent, a roller coaster car is always pulled up the first hill to a high initial height. Work is done on the car (usually by a chain) to achieve this initial height. A coaster designer is considering three different angles at which to drag the 2000-kg car train to the top of the 60-meter high hill. Her big question is: which angle would require the most work? _______________ Show your answers and explain. Angle Force Distance Work 35° 1.15 * 10 4 N 105 m 45° 1.41 * 10 4 N 84.9 m 55° 1.64 * 10 4 N 73.2 m

e. A 2-kg object is pulled upward at constant speed by a 20-N force for a vertical displacement of 5.0 m. Wtens = • • cos( ) = J Wgrav = • • cos( ) = J Wtotal = J f. A 2-kg tray of dinner plates is held in the air and carried a distance of 5.0 m to the right. Wapp = • • cos( ) = J Wgrav = • • cos( ) = J Wtotal = J 9. When a force is applied to do work on an object, does the object always accelerate? _________ Explain why or why not 1. Determine the work done in the following situations. a. Jim Neysweeper is applying a 21.6-N force downward at an angle of 57.2° with the horizontal to displace a broom a distance of 6.28 m. b. Ben Pumpiniron applies an upward force to lift a 129-kg barbell to a height of 1.98 m at a constant speed. c. An elevator lifts 12 occupants up 21 floors (76.8 meters) at a constant speed. The average mass of the occupants is 62.8 kg. L2 Energy Intro

1. A 2000 kg truck is travelling at 80 km/h. What is the kinetic energy of the truck? (4.9x10 5 J) 2. What speed would the truck in the above problem, have if its kinetic energy was cut in half by applying the brakes? (54 km/h) 3. A man decides to climb an office tower using the stairs. If the floors are 3.8 m apart, how much gravitational potential energy would the man have relative to the ground floor if he made it to the a) fifth floor? b) tenth floor? c) the first basement level? (a) 41J, (b) 92J (c) -10J 4. What percentage of its gravitational potential energy does a squash ball lose if it falls from 3.0 m and returns to a height of 0.76 m after bouncing once? (75%) 5. A 1300 kg car starts from rest at a stoplight and accelerates to a speed of 14 m/s over a displacement of 82 m. T/I (a) Calculate the net work done on the car. [ans: 130 kJ] (b) Calculate the net force acting on the car. [ans: 1.6 3 10 3 N] L3 Conservation of Energy

air every two seconds, what is its power?(1.50 x 10 3 W) pulley system. The mass of the engine is 170.0 kg. By pulling as a team, the students can exert a force of about 1.72 _ 10 3 N to lift the engine to the necessary height of 2.20 m. a) How much “useful work” was done by the students? (3.66 x 10 3J) b) How much work was done in total to lift the engine? (3.78 x 10 3 J) c) What was the overall efficiency of the students in lifting the engine?(96.8 %) d) One of the students recommends that all of her friends who helped lift the engine should receive a final grade equal to their percent efficiency. The shop teacher claims that this would be unfair to the students because none of them could ever achieve 100% efficiency. Explain. 2. How much electrical energy is used by a 100 W light bulb if it was accidentally left on for 8.0 h? (2.9 x10 6 J) 3. How long would it take a 1.00 kW electric motor on a conveyor belt to do 750 J of work? (0.750s) 4. A 613.0 kg mass is placed on a forklift that can generate 950 W of power. What is the constant speed of the forklift while lifting this load? (0.158 m/s) 5. Water is pumped up to a water tower, which is 92.0 m high. The flow rate up to the top of the tower is 75 L/s and each litre of water has a mass of 1.00 kg. What power is required to keep up this flow rate to the tower? (6.8 x 10 4 W) 7. A karate blow can transfer 35.0 J of total energy to kinetic energy. If this transfer is only 25% efficient, what maximum velocity can the 70.0 kg target ever reach? (0.5 m/s) L6 Work Force Energy Power Difference 1. http://www.physicsclassroom.com/Concept-Builders/Work-and-Energy

This link will take you to 4 sets of concept building questions (shown below). Answer all the questions from each set. Name that Energy – 24 Questions What's Up (and Down) With KE and PE? – 36 Questions Energy Rankings – 24 Questions Work Concept Builder- 36 Questions 2. Use the following words to create a concept map/flow chart, showing the connection between them: Acceleration, mass, force, work, energy, power L7 Thermal Energy 1. How much thermal energy is required to raise the temperature of 2.0 kg of water by 10.0 °C? T/I [ans: 8.4 x 10 4 J] 2. A glass window with a mass of 20.0 kg is heated to a temperature of 32.0 °C by the Sun. How much thermal energy is released by the window to the surroundings as it cools to 5.0 °C at night? T/I [ans: 4.5 x 10 5 J] 3. An aluminum block absorbs 1.0 x 10 4 J of energy from the Sun when its temperature increases by 5.0 °C. What is the mass of the block? T/I [ans: 2.2 kg] L8 Practice Conservation of Energy 1. An amusement park has a slide for which participants are given a cloth sack to sit on. The t0p of the slide is 6.0 m high. (a) Determine the speed attained at the bottom of the slide by a 30 kg child. Assume that the child starts from rest and make the unrealistic assumption that friction can be ignored in this case.(ans: 11 m/s) (b) Repeat your calculations for (a), but this time assume that 80% of the gravitational potential energy that the child has at the top of the slide is required to overcome friction, and is therefore lost as heat energy. (ans:4.8 m/s) 2. A bungee jumper has a mass of 73 kg and falls a distance of 120 m before the bungee catches and sends the jumper upward. Use the law of conservation of energy to calculate the speed of the jumper just before the bungee catches (ans:48 m/s) 3. A student throws a 0.22 kg rock horizontally at 20.0 m/s from 10.0 m above the ground. Ignore air resistance.

9. Find v at midpoint 10. A beach ball is thrown straight up with a speed of 10m/s from a point 2m above the ground. a) Assuming no air resistance, calculate how far up will the ball go. (ans: 7m) b) At what speed will the ball eventually hit the ground? (ans: 10m/s) 11. Several children, pretending that they are playing in the jungle, suspend a rope from an overhead tree limb. A child of mass 40 kg running at 8.0 m/s grabs the rope and swings off the level ground. (a) What maximum height does the child reach? (ans: 3.3m) (b) How fast would a 30 kg child have to run to reach the same height as the 40 kg child? (ans: 8.0 m/s) L9 Conservation of Thermal Energy – Heat Exchange 1. Sun-Young places a 2.0 kg block of aluminum that had been heated to 100.0 °C in 1.5 kg of ethyl alcohol with an initial temperature of 18.0 °C. What is the final temperature of the mixture? T/I [ans: 45 °C] 2. A metal bar with a mass of 4.0 kg is placed in boiling water until its temperature stabilizes at 100.0 °C. The bar is then immersed in 500.0 mL of water with an initial temperature of 20.0 °C. The mixture reaches a temperature of 35.0 °C. What is the specific heat capacity of the metal bar? T/I [ans: 1.2 x 10 2 J/ kg . °C)] 3. A bar of pure gold is heated to 95.0 °C. The specific heat capacity of gold is 1.29 x 10 2 J/ kg °C). The gold is placed into 500.0 mL of ethyl alcohol initially at a temperature of 25.0 °C. The final temperature of the mixture is 27.0 °C. What is the mass of the gold? (ans;220g) 4. Danielle cools a 2.0 kg metal object to a temperature of -25 °C. She places the metal in 3.0 L of pure water initially at a temperature of 40.0 °C. The final temperature of the mixture is 36.0 °C. What is the specific heat capacity of the metal?( ans: 4.1 x 10 2 J/ kg . °C)] 5. A 1.50 3 102 g piece of brass (specific heat capacity 3.80 x 10 2 J/ kg . °C)] is submerged in 400.0 mL of water at 27.7 °C. What is the original temperature of the brass if the mixture has a temperature of 28.0 °C? (ans: 36.8 °C)