Describing Energy
Today’s learning goals: Be able to describe the difference between kinetic and potential energy. Be able to calculate kinetic energy. Be able to describe different types of potential energy. Be able to calculate gravitational potential energy.
Describing energy: Energy is the ability to change. When something is able to change its surroundings or itself, it has energy. Without energy, nothing would ever change.
Work transfers energy: Energy can also be described as the ability to do work. Energy can be measured in the same unit as work, joules.
Systems: A system is anything around which you can imagine a boundary. When one system does work on a second system, energy is transferred from the first system to the second system.
Different forms of Energy Energy has many different forms. They include mechanical energy, electrical energy, chemical energy and radiant energy. Automobiles use chemical energy of gasoline to transport cars.
Different forms of Energy Radiant energy from the Sun warms the Earth.
Today’s learning goals: Be able to describe the difference between kinetic and potential energy. Be able to calculate kinetic energy. Be able to describe different types of potential energy. Be able to calculate gravitational potential energy.
Kinetic energy Objects in motion can collide with one another and cause change. Objects that are in motion have kinetic energy. The kinetic energy from an object’s motion depends on that object’s mass and speed.
Kinetic energy equation: Kinetic energy = ½ mass x (speed) 2 (in Joules) ( in kg) (m/s)2 KE = ½ m v2
Practice problem (together) A jogger with a mass of 60.0 kg is moving forward at a speed of 3.0 m/s. What is the jogger’s kinetic energy from this forward motion? KE = ?, M = 60 kg, v = 3 m/s KE = 1/2(60) ( 3 ) 2 KE = 270 J
Practice problem (on your own) A baseball with a mass of 0.15 kg is moving at a speed of 40 m/s. What is the baseball’s kinetic energy from this motion? A) 12 J c)1200J B) 120 J d) 240 J
Practice problem (on your own) A runner with a mass of 75 kg is moving forward with a speed of 4.5 m/s. What is the runner’s kinetic energy from this forward motion? A) 759 J C) 7.59 J B) 75.9 J D) 12656 J
Potential Energy: Energy does not always have to involve motion. Potential energy is energy stored due to interactions between objects like an apple hanging from a tree.
Potential energy: Another example is the stored energy between chemical bonds like those of the foods we eat.
Elastic potential energy: If you stretch a rubber band and let it go, it sails across the room. When it is flying is has kinetic energy that came from due to the elastic forces between the particles that make up a stretched rubber band.
Elastic potential energy: Elastic potential energy is the energy that is stored by compressing or stretching an object.
Chemical potential energy: Chemical potential energy is the energy that is stored due to chemical bonds. Examples are the food you eat and the gasoline in cars. When the object is burned, chemical potential energy becomes thermal energy and radiant energy.
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Gravitational potential energy: Gravitational potential energy is energy due to the gravitational forces between objects. Any system that has objects that are attracted to each other through gravity has gravitational potential energy.
Gravitational potential energy: The gravitational potential energy (GPE) of a system depends on the object’s mass, Earth’s gravity and the object’s height. Recall that Earth’s gravitational constant is 9.8 N/kg.
GPE formula: GPE = mass x gravity x height (J) (kg) (N/kg) (m)
GPE practice (together) A 4.0 kg ceiling fan is placed 2.5 m above the floor. What is the GPE of the Earth-ceiling fan system relative to the floor? GPE = ?, m = 4.0 kg, g = 9.8 n/kg, h = 2.5 m GPE =(4)(9.8)(2.5) = 98J
Practice (on your own) An 8.0 kg textbook is placed on a 1.25 m desk. How large is the GPE of the textbook-Earth system relative to the floor? A) 9.8 J c) 980 J B) 98 J D) 0 J
Practice (on your own) A 0.50 kg apple is 2.0 m above in a tree. What is the GPE of the apple- Earth system? A) 98J c) 980 J B) 9.8 J d) 0 J
Today’s learning goals: Be able to describe the difference between kinetic and potential energy. Be able to calculate kinetic energy. Be able to describe different types of potential energy. Be able to calculate gravitational potential energy.
Review of today’s key ideas: Forms of energy include mechanical, electrical, chemical, thermal and radiant energy. Kinetic energy is the energy that a moving object has because of it motion. Potential energy is stored energy due to the interactions between its objects.
Review of today’s key ideas: Different forms of potential energy include elastic potential energy, chemical potential energy and gravitational potential energy.
Directions for today’s end of class quiz: I will show you the quiz questions on the power point. You will do whatever work you need to do on your study guide. You choose the correct answer. You hold up the correct answer when I ask you to.
Directions for quiz continued: Just like any other quiz, there will be no talking or electronic devices out other than your calculator.
Quiz problem: 1) calculate the Kinetic energy of a jogger with a mass of 30 kg moving forward at a speed of 3.0 m/s. A) 13.5 J c) 1350 J B) 135 J d) 1.35 J
Quiz problem: 2) A baseball with a mass of 0.25 kg is moving at a speed of 40.0 m/s. What is the baseball’s kinetic energy from this motion? A) 100 J c) 200 J B) 10 J D) 20J
Quiz problem: 3. When methane burns, it combines with oxygen to form carbon dioxide and water. In this reaction, what type of energy is released? A) radiant energy c) elastic energy B) chemical energy d) electrical energy
Quiz problem: 4. Calculate the GPE of a 2.0 kg ceiling fan that is placed 2.5 m above the floor. A) 4.9 J c) 49 J B) 490 J d) 25 J
Quiz problem 5. Calculate the GPE of a 0.50 kg ball that is 2.0 m above the floor. A) 0.98 J B) 9.8 J C) 98 J D) 980 J
Answers to quiz: 1. b 2. c 3. b 4. c 5. b
Did you meet the goal? Did you get 4 out of 5 correct? Record your score on your tracking sheet.