Unit Title: Defining Energy and Introduction to Potential and Kinetic Energy

Unit Title: Defining Energy and Introduction to Potential and Kinetic Energy

Philip Draviam

February 9, 2003

Unit Title: Defining energy and introduction to potential and kinetic energy

Instructional Goals:

  1. To introduce the concept of energy.
  2. Explain the relationship between energy and work.
  3. Identify different forms of energy and provide examples of the various forms.

Objectives:

  1. Using the definitions of energy and work, students should be able to explain the relationship between energy and work.
  2. Given the position and mass of an object, students should be able to correctly determine the gravitational potential energy of the object.
  3. Given the velocity and mass of an object, students should be able to correctly determine the kinetic energy of the object.

Rationale:

The concept of energy and its application in the world of science is essential for the inderstanding of specific forms of energy, how energy is trasnsferred and more advanced topics in physics. The concept of work is used to describe how energy is transferered from one form to another. Energy exists in daily life and one should know where it exists, in what form it exists, how it can be transferred and its impact on the world.

Content:

The concepts included are relevant to middle school or early secondary science students. This lesson introduces the concepts of energy and work, and the relationship between the two. Different forms of energy are defined and some examples of where the different forms exist are provided.

Learning Activities:

Defining Energy

Ask students to provide their definitions of energy. After a few responses, ask them more specific questions such as what does energy do? Or why do we need energy?

Now provide a specific format for the concept of energy by writing on the board: “Energy is the ability to do ______”, and ask the students to provide responses for the blank space. After hearing some answers, conclude by stating that in science you can think of energy as the ability to do work.

But what is work?

Work occurs when a force causes an object to move in the direction of the force.

Show teaching transparency 232: (Illustration of tennis player striking a ball)

Identify how work is being done on the racket, next on the tennis ball and possibly how the ball can do work on the net. Each time work is done, energy is passed from one object to another. Work is the transfer of energy from one system to another.

Forms of Energy

Demonstrate forms of energy by doing the following:

  1. Light a match and let it burn.
  2. Wind up a toy and let it run.
  3. Turn on a flashlight.
  4. Knock a ball off a table and allow it to bounce on the floor.

Ask students how energy was involved in each event. Lead the students to conclude that there are many different forms of energy.

Introducing Kinetic Energy

Referring to the example of a tennis player, explain that the ball traveling across the net has kinetic energy. All moving objects have kinetic energy. Examples involving kinetic energy include: A bowling ball traveling down a lane; A hammer in motion towards a nail.

Kinetic energy is also dependent on the mass of the moving object.

KE = (1/2)*mv^2

Kinetic energy is only dependent on the mass and the velocity of an object. Which of the two terms has a greater impact in the kinetic energy? Velocity, since it is squared.

Based on the given situation, which would you expect to have te greater kinetic energy and why? Provide illustrations if needed.

Example: Car (500 kg) and bus (2000 kg) traveling at the same velocity (25 m/s).

Example: Two cars (1000 kg) with different speeds (20 m/s) and (25 m/s).

Introducing Potential Energy

Remind students that objects with zero velocity have zero kinetic energy. Does this mean that objects at rest have no energy? No, this type of energy is sometimes called potential energy and does not depend on the velocity of an object.

Potential energy is the energy an object has because of its position or shape. For example, the stretched bow of an archer has potential energy since work has been done to change its shape. Another example is a stretched rubber band.

When an object is lifted such that it opposes gravitational force, it gains gravitational potential energy. So the higher and object is lifted from the earth’s surface, the more gravitational potential energy it has. It is dependent on the objects’ weight and height.

Gravitational potential energy can be defined as:

PE = mgh = whwhere w = weight (Newtons)h = height (meters)

Have the students perform calculations involving the application of the gravitational potential energy equation. Ask the studentsw to work individually until they have developed an answer. Compare results and go over the solution.

Example: If you lift a 50N watermelon to the top of a 2 m refrigerator, how much gravitational potential energy do you give the watermelon?

Introducing mechanical energy

Now that potential energy and kinetic energy have been defined, under which category would juggling pins fall? Ask the students for their opinions and the reasoning for their choic.

State that the energy at any time is defined by the sum of the kinetic and potential energies. This sum is called mechanical energy and can be written as:

Mechanical energy = Potential energy + Kinetic energy