Energy ~ Learning Guide Name:

Transcription

1 Energy ~ Learning Guide Name: Instructions: Using a pencil, answer the following questions. The Pre-Reading is marked, based on effort, completeness, and neatness (not accuracy). The rest of the assignment is marked, based on effort, completeness, neatness, and accuracy. Every time you see a bold word, make sure you refer back to your "Submission Requirements." Do your best! Pre-Unit Thoughts: Why do you think it has been said that "all modern political decisions are related to energy?" Work and Energy: 1. Potential energy can be viewed as stored energy. This type of energy becomes able to do work most commonly when it changes forms. For example, gravitational potential energy means that an object s position in a gravitational field wants to fall. Once it falls it can do work as its potential energy is converted into kinetic. This is how hydro-electric dams work. They transfer the potential energy of water into kinetic energy as it falls. Then the kinetic energy is transformed into electric energy as the moving water turns the turbines and create electricity. Examine the list of energies presented in the notes. Pick out the energies that you would consider to be potential energies. Beside each choice briefly describe how this energy is stored and must be transformed to do work (you may use an example). Type of Potential Energy Why it is Potential Page 1 of 8

2 2. Work is a scalar quantity. However it is acceptable in physics to have negative work. What would this mean? Provide an example of negative work. 3. The formula for the potential energy stored in a spring is given by U s = 1 2 k( x)2. It takes work to load a spring up either by compressing it or expanding it a distance x. The graph Force vs Length Change of a real spring is shown below. a. Derive the equation for spring potential energy using the above graph. You must remember that any work done by the force goes into changing the potential energy of the spring. b. Explain the ½ in the equation using the fact that W = F avg d. (Note: Favg=average force over d) Page 2 of 8

3 4. If you push a 75 N block along a floor a distance of 4.2 m at a steady speed, and the coefficient of kinetic friction is 0.40, how much work will you do on the block? Show all work. (ans. 130 J). 5. How much work will a 4.0 HP (3 kw) motor do in half an hour? Show all work. (ans. 5.4 x 10 6 J ). Conservation of Energy: 1. Two rocks are thrown off of a cliff with the same initial speed, v. The first rock is launched with at an angle Ө directed below the horizontal as shown below as 1, while the second rock is launched above at the same angle. a. Predict how the final speeds of each rock compare just before they strike the water. Provide a brief explanation justifying your prediction. Page 3 of 8

4 b. Measurements are taken and it turns out that both rocks strike the water below at precisely the identical speed. Justify using a conservation of energy argument. Use equations to aid your explanation. c. Justify using a kinematics argument (projectile motion). The diagram below might help your explanation. d. Both arguments above assume that we are ignoring air resistance. Which rock would strike the water with a larger final velocity if we include air resistance. Use a conservation of energy argument to help with your solution. Page 4 of 8

5 2. Atwood s Machine revisited: Earlier in the dynamics unit we studied the physics of gravitypowered pulley problems such as Atwood s Machine depicted below. a) The two masses in the Atwood's machine shown are initially at rest at the same height. After they are released, the large mass, m 2, falls through a height, h, and hits the floor, and the small mass, m 1, rises through a height h. What is the speed of the masses just before m 2 strikes the floor? (i) Assume friction is negligible and describe in words the energy transformations involved in this system. Be sure to state the initial energy state and the final. (ii) Write out the conservation of energy equation you would use to solve this question. Please use the variables depicted in the diagram along with g. Page 5 of 8

6 b) Evaluate your answer to part (a) for the case where h= 1.2 m, m 1 =3.7 kg, and m 2 =4.1 kg. Show all work. (ans. 1.1 m/s) c) Resolve this question using what we learned in dynamics and kinematics (remember: the link between dynamics and kinematics is acceleration). 3. A pendulum bob is moving 1.8 m/s at the bottom of its swing. To what height above the bottom of the swing will the bob travel? Show all work (ans m). 4. Tarzan grabs a vine 12 m long and swings on the end of it, like a pendulum. His starting point is 5.0 m above the lowest point in his swing. How fast is Tarzan moving as he passes through the bottom of the swing? Show all work (ans. 9.9 m/s). Page 6 of 8

7 Thermal Energy: 1. You have six Styrofoam cups containing the same amount of water at 20 C. You also have six copper blocks whose masses and initial temperatures vary as shown below. One block goes into each cup. (Assume the mass of the water is between 500 g and 1000g.) Rank these cups according to the maximum temperature of the water after the block is added. Highest Lowest Or, all of the cups have the same maximum temperature. Carefully explain the reasoning behind your ranking. Page 7 of 8

8 2. The first statement of the 2nd law of thermodynamics - heat flows spontaneously from a hot to a cold body - tells us that an ice cube must melt on a hot day, rather than becoming colder. If the flow of heat was from cold to hot describe what would happen an ice cube placed into your hand (be sure to describe both parts of the system your hand and the ice). 3. Evaporative cooling is the reason why hot coffee cools over time. Look up evaporative cooling and use this concept to explain why the water freezes when the pressure drops. Refer to Energy > Lab Resources > Video: Freezing Boiling Water. 4. A kg block of iron is heated from 295 K to 325 K. How much heat is absorbed by the iron? Show all work. (ans. 5.4 x 10 3 J) Page 8 of 8