Energy and Its Conservation

Introduction

• Energy is always present, but never visible!

• Instead, we see the evidence of energy: movement, sound, heat, light

Work-Energy Correlation

• In order for work to be done, a force must be applied to an object and cause it to move in the same direction as the force.

• Work transfers energy from one object to another.

What is energy?

• Energy is the ability of an object to produce a change in itself or the surroundings. It is the ability to do work.

Types of Energy

• Mechanical Energy (ME)-enables an object to do work.

A. Kinetic Energy (KE)B. Potential Energy

(PE)

Kinetic Energy• Kinetic energy is defined as the

energy of a moving object.• KE = 1/2mv2

• m is mass (kg) and v is velocity (m/s)

• The unit for energy is the joule.• Kinetic energy is a scalar quantity.

Practice Problem

• What is the kinetic energy of a 45 kg object moving at 13 m/s?

Example 2

• The kinetic energy of a boat is calculated at 52,000 J. If the boat has a mass of 39,000 kg, with what velocity is it moving?

Potential Energy

• Potential energy is defined as the energy in matter due to its position or the arrangement of its parts.

• Often referred to as stored energy.• There are many types of stored energy:

1) Elastic potential energy2) Chemical potential energy

3) Gravitational potential energy

Work and Energy

• Work and energy are related.• When work is waiting to be done,

we call the energy potential.• When work is being done, we call

the energy kinetic.

Elastic Potential Energy• Occurs when an object resists being stretched

or compressed.• Examples: a stretched rubber band, a spring• The elastic potential energy in a rubber band

can be used to do work-such as a toy plane-A rubber band untwists and causes a propeller to turn.

• If you do more work on the rubber band and pull it back a greater distance (or twist it more), it will have greater potential energy, and as a result, can do more work.

Chemical Potential Energy

• Chemical potential energy is the energy stored in molecules.

• Examples: gasoline, food• When gasoline is burned through

combustion, the bonds of molecules are broken and reformed, and energy is released.

• The released energy is used to do work.

Gravitational Potential Energy• If an object moves away from the earth,

energy is stored in the system as a result of the gravitational force between the object and the Earth.

• Work is done in moving the object to its new location-the energy required to do the work against gravity is now stored in the object as a form of potential energy called gravitational potential energy.

• PE = mgh (m is the object’s mass (kg), g is the acceleration due to gravity (9.8 m/s2), and h is the height (in meters)above the reference level .

Practice Problem

• A 3.8 kg object is lifted to a height of 3 meters. What is the potential energy of the object?

Example 4

• A 30 kg child climbs 15 m up a tree. When he stops to look around, what is the child’s potential energy?

Let’s Review: State whether each of the following has kinetic energy, potential energy, or both.

1. If an object is at rest, it certainly does NOT possess this form of energy.

2. Depends upon object mass and object height.3. The energy an object possesses due to its motion.4. The amount is expressed using the unit joule (abbreviated J).5. The energy stored in an object due to its position (or height).6. The amount depends upon the arbitrarily assigned zero level.7. Depends upon object mass and object speed.8. If an object is at rest on the ground (zero height), it certainly

does not possess this form of energy.