Unit 3 Energy and Society Work and Energy

Transcription

1 Unit 3 Energy and Society Work and Energy Today's goal: I can identify energy issues that exist and can articulate the connection between issues and work and energy. Energy Issues: Definitions: Energy Work Energy Transfer 1

2 The requirements for work: Example: A worker pushes a 20 kg box for 5 m with 150 N. How much work is done? Example: A child pushes a toy tractor 5 m [R] with a force of 10 N [R]. How much work is done? 2

3 Example: Jenn is pushing her stalled car with 200 N of force but the car does not move. How much work is done? Example: A worker pulls a cart with a force of 40 N [R]. If the handle of the cart is at a 50 degree with the horizontal, how much work is done if they pull the cart for 20 m? Homework: pg 221 #1 3 pg 241 #

4 Lesson28.notebook Gravitational Potential Energy Today's goal: I can explain, calculate and compare gravitational potential and kinetic energy. Gravitational Potential Energy (GPE) is a common form of energy. This energy is the result of an objects vertical position. The equation can be derived from Newton's Second Law: Example: Find the GPE of a 20 kg hanging pot located 5 m above the ground. Example: If a 5 kg ball falls from 15 m to 5 m, how much GPE was lost? 1

5 Lesson28.notebook Example: A roller coaster is shown. Answer the following: A) What is the GPE at point A. B) What is the change in GPE from point A to B? C) What is the GPE at point B? Kinetic Energy Kinetic Energy (KE) is the energy contained in a moving object. We can derive the equation for kinetic energy using our equation for work and our Kinematics equations: 2

6 Lesson28.notebook Example: How much KE does a 1 kg ball have travelling at 10 m/s? Example: If a curler uses the same amount of energy on an adult stone (20 kg) and a children's stone (10 kg), what would the difference be in velocity? Example: How much KE does a 100 kg person have if: A) They run at 4 m/s? B) They run at 2 m/s? C) Why is the relationship between A and B not linear (ie. Half?) Homework: page 243 #

7 Lesson29and30.notebook Conservation of Energy Today's goal: I can explain the conservation of energy and make the connection between gravitational potential and kinetic energy in a closed system. The Law of Conservation of Energy states that energy cannot be created or destroyed, but simply changes from one form to another. No energy is lost. A Falling Rock (10 kg) from a total height of 10 metres. Some examples: 1

8 Lesson29and30.notebook Example: As water in a river approaches a 5.7 metre vertical drop, it average speed is 5.1 m/s. For each kilogram of water, determine: A) The total energy of each kilogram at the top of the falls. B) The total energy at the bottom of the falls. C) The velocity of the water prior to impact. Example: A student shoots an arrow straight up at 30 m/s. If the arrow has a mass of 0.04 kg, answer the following: A) What is the maximum height of the arrow? B) If the gym ceiling is 15 m high, what velocity will the arrow strike the ceiling? Homework page 243 #36 40 Handout 2

9 Lesson33.notebook Power Today's goal: I can explain the concept of power and successfully complete calculations with respect to real world situations. Power is often confused with work. The difference is: Work: The amount of energy transferred during an interaction. Power: Is the rate at which work is done. Using the definition of power, the corresponding formula is: Graphically, you can see the relationship between work, time and power. Units Examples: If a 100 kg person was to run up stairs that had resulted in a vertical displacement of 20 m, answer the following: A) How much energy was used to climb the stairs? B) How much power was generated is it took: I) 5 minutes to climb the steps? II) 10 minutes to climb the steps? C) If you took the elevator instead of the steps, discuss how energy and power must be considered in the design. 1

10 Lesson33.notebook Efficiency of Energy Transfers As discussed with the bouncing golf ball, only in ideal worlds you will see 100% energy transfer from one form to another. The law of conservation of energy states that energy cannot be created or destroyed. However, it can be transferred into unexpected or undesired forms (heat the most common). When you look around stores selling common househol items they make claims of efficiency. What is mechanical efficiency? Mechanical Efficiency The percentage of useful energy resulting from the total energy input for the system. Example: What is the efficiency of the 100 kg person to climb the stairs (20 m vertical displacement) if they really used J? Example: A light bulb is 10% efficient (10% of the input power is converted to light A) How much light energy is actually provided to a 100 W light bulb in 60 s? B) What would the "effective" light power be from a 100 W light bulb? Homework: page 242, # page 244, #

11 Lesson34.notebook Thermal Energy Today's goal: I can explain the concept of thermal energy and use it to discuss three different types of transfer and complete associated calculations. The Kinetic Molecular Theory is the current theory used to understand atomic theo and related material. The 3 main postulates are: 1) All matter is made up of small, moving particles called atoms. These atoms group together to form molecules. 2) These atoms or molecules exert forces on one another that keep them a c distance from each other by either repulsing or attracting. 3) The distance between molecules and strength of force between them is responsible for the three physical states of matter: solid, liquid and gas. Diagram and Analogy State Matrix Structure Analogy 1

12 Lesson34.notebook Thermal Energy Transfer There are three types of heat transfer: Conduction: The process of transferring heat by particle collisions. Conductor vs. Insulator Convection: The process of transferring heat by a circulating path of fluid. Room in a house: The sea shore: 2

13 Lesson34.notebook Radiation: The transfer of heat through a wave from of electromagnetic radiant energy. Heat lamps: Microwave ovens Specific Heat Capacity Heat transfer depends on three things: 1) Temperature difference 2) Mass of substance 3) Type of substance Specific heat capacity is the amount of heat energy that is needed to increase the temperature of 1 kg of a particle substance by 1 degree Celsius. E H = mcδt where: = mc(t 2 t 1 ) 3

14 Lesson34.notebook Example: How much heat energy is required to heat a piece of gold from 25 degre Celsius to 50 degrees Celsius? (c = 1.3 x 10 2 J/kgC) Example: A 0.75 kg block of aluminum (c = 9.1 x 10 2 J/kgC) at 90 degrees Celsius is cooled by removing 5 x 10 4 J of heat energy. What is the final temperature of the aluminum? Homework: page 275 # 5, 6, 8, 10, 11,