Topic 4.3 – Conservation of Energy Physics Unit 4: Work, Energy, and Power Big Idea: Energy is the ability to create change. This requires work to be done. Mechanical energy, which is the sum of the potential and kinetic energy, is conserved in a system. Topic 4.1 – Work and Power Topic 4.2 – Kinetic and Potential Energy Topic 4.3 – Conservation of Energy

I can describe energy transfer vs conversion events. Learning Goal: You will understand how the law of conservation of energy explains the behavior of systems of objects. Success Criteria: You will know you have met the learning goal when you can truthfully say: I can describe energy transfer vs conversion events. I can use the law of conservation of energy to calculate the energy changes in objects. Image(s) from Bing Images

Success Criteria 1: I can describe energy transfer vs conversion events. As you’ve learned before, energy cannot be created or destroyed. It can only be transferred between objects or converted between kinetic and potential energy. In an energy conversion event, energy is converted between kinetic and potential energy within one object. When a pendulum swings, it trades speed (KE = ½mv2) for height (PE = mgh). In an energy transfer event, one object loses energy and one object gains energy. A collision is a typical energy transfer event. In both energy conversion and transfer events, the total number of joules of energy remains the same.

Success Criteria 1: I can describe energy transfer vs conversion events. Task 4.3.1: a) Write T for transfer or C for conversion: ____ an object falls, losing height and gaining speed. ____ you throw a ball. ____ a hot object loses heat to a cold object ____ you pull stretch a rubberband b) Describe what is happening in each case, and classify them as energy conversion or transfer events (Ex: the object loses KE and gains PE in a conversion event) A rock falls off of a cliff. A car speeds up. A stretched rubber band launches a paperclip c) Describe where energy comes from and where energy goes when you burn a candle.

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. As we briefly discussed earlier, the sum of the kinetic energy and the potential energy is the mechanical energy (Em, or ME), which is the energy available to do work. ME = KE + PE = ½mv2 + mgh For objects that are not being lifted or propelled, such as projectiles or roller coasters, Ek and Ep can be converted between each other by velocity being exchanged for height or vice versa. For example: Pendulum: Projectile:

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. Task 4.3.2: Open the Energy Skate Park Basics simulation at phet.Colorado.edu. Click on the intro tab, enable the bar graph, and answer these questions. a) When is the kinetic energy of the skater the highest? b) Does kinetic energy appear to correlate with height or speed? c) When is the potential energy of the skater the highest? d) Does potential energy appear to correlate with height or speed? e) What happens to the total energy as the skater does up and down the ramp? f) What are two things you can do to start the skater with more total energy? g) If you click on the friction tab, and enable the bar graph, what happens to the thermal energy as the skater loses kinetic and potential energy? h) What does friction do to the kinetic energy of an object? i) Based on this, why do your hands get hot when you rub them together?

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. Task 4.3.3: Complete this table. KE PE ME a) 300 J 500 J b) 1678 J 245 J c) 9000 J 13000 J d) 328 kJ 1105 kJ e) 1145.7 kJ 45.7 MJ f) 612 GJ

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. Task 4.3.4: Find the wanted information. a) What is mechanical energy? Explain both in words and using an equation. b) Explain the difference between potential energy and kinetic energy. c) A pitcher throws a 0.182 kg softball at 29m/s. When the ball is 1.34 m off the ground, what is the mechanical energy of the ball? d) A 0.38 kg rock is thrown into the air. What is the ME of the rock if at a particular instant it is travelling at 21 m/s and it is 4.8 m above the ground? e) I fall out of an airplane. If my kinetic energy after falling for several seconds is 93,750 kJ and my potential energy due to my height is 735,750 kJ, what is my ME? f) You slide a 1.34 kg book off a table with 17.1 J of KE. Once it falls by 0.62 m, how fast will it be going?

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. Task 4.3.5: Find the wanted information. g) Explain the concept of conservation of energy. How does this allow us to figure out the mechanical energy, potential energy, or kinetic energy when we know two of the three energies? h) A pumpkin being held 130 m above the ground has PE of 10840 J. What is its mass? i) A 0.09 kg bullet travelling at 340 m/s has the same kinetic energy as a 970 kg car travelling at what velocity? j) How much kinetic energy would a 2200 kg truck have after being accelerated with a 7030 N net force for 3.5 seconds? k) When you do work on an object be lifting it against the force of gravity, are you adding potential energy or kinetic energy? l) Where does the energy it takes for you to lift an object against the force of gravity come from? Where does that energy come from? Where does that energy come from? Where does that energy come from? Where does that energy come from?

Success Criteria 2: I can use the law of conservation of energy to calculate the energy changes in objects. Task 4.3.5: A 0.60 kg ball, starting from rest and a height of 3.2 m, rolls down a frictionless ramp. It reaches the bottom 3.0 seconds later. Plot the potential energy, kinetic energy, and total mechanical energy of the ball at t: 0 s, 1 s, 2 s, and 3 s (Hint, you’ll have to use the kinematic equations to determine the acceleration)

Task 4.3.6: Write at least 8 things you learned in this topic (1/2 point each). If you do this in your notebook, please do it in list form rather than paragraph form. Image from Bing Images