CHAPTER 5 ENERGY

Where is energy taking place in this room?

You reading this screen The lights My computer The wind outside The sun You breathing, moving, writing

What is energy?

Energy is defined as the ability to do work.

Work is done when a force causes something to move.

Energy can also be defined as the ability to cause change.

When work is done and a change occurs energy moves from place to place, or it changes from one form to another.

Can you think of some forms of energy?

Electrical Chemical (can be from food) Radiant (from the sun) Thermal Energy can be stored and moved from place to place.

Energy Analogy

Money is similar to energy. Money can be stored in several forms, cash, bank accounts, coins. And can be transferred from one form to another.

Regardless of the form, money is still money. The same is true for energy.

Kinetic Energy

The energy a moving object has because of its motion.

The kinetic energy of an object depends on its mass and speed.

Kinetic energy= ½ mass (in kg) x (speed (m/s))²

KE= ½ mv²

The SI unit of energy is joule It is abbreviated J

Kinetic Energy Problem

A jogger whose mass is 60 kg is moving at a speed of 3m/s. What is the jogger’s kinetic energy?

The equation for kinetic energy is KE= ½ mv²

KE= ½ (60 kg) (3 m/s)² KE= ½ (60 kg) (9)

KE= 270 J

Practice Kinetic Energy Problem

What is the kinetic energy of a baseball moving at a speed of 40 m/s if the baseball has a mass of 0.15 kg?

KE= ½ mv²KE= ½ (0.15 kg) (40 m/s)²KE= ½ (0.15) (1600)KE= 120 J

Potential Energy

Energy does not have to involve motion, even motionless objects can have energy.

This energy is stored in the object The stored energy has to potential to have

energy, so it is called potential energy. Potential energy= stored energy due to

position. Can you think of some examples of potential energy?

An apple in a tree has to potential to have energy if it falls.

If it does fall the stored energy of position is converted to energy of motion.

Elastic Potential Energy

What happens if you stretch out a rubber band and let is go?

Energy that is stored by something that can stretch or compress like a rubber band or a spring.

Chemical Potential Energy

Energy that is stored in chemical bonds. This can be found in the food you eat,

and gasoline in your car, or natural gas in your house.

Gravitational Potential Energy

Energy stored by objects due to their position above the earth’s surface.

This depends on the objects mass, and their height above the ground.

Gravitational Potential Energy (J)= mass (kg)x acceleration due to gravity (m/s²) x height (m)

GPE= mgh

Practice Problems

What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg, and is 4 m above the ground?

GPE= mghGPE= (7 kg) x (9.8 m/s²)x (4 m)GPE= 7 x 9.8 x 4GPE= 274 J

Changing Gravitational Potential Energy

Which object on the bookcase has the greatest GPE?

The highest one (the plant)

If there are 2 objects on the same shelf which would have the most GPE?

The one with more mass.

As an object falls does its GPE increase or decrease?

The closer an object is to the ground, the less GPE it has, therefore as an object falls it has less GPE as it gets closer to the ground.

Conservation of Energy

Transforming Electrical Energy:Electrical Energy is converted into light and

thermal energy. Transforming Chemical Energy:On a car the gasoline holds potential energy

that is converted into kinetic energy to make the car move.

In a car

An electrical spark ignites a small amount of fuel.

This causes the chemical energy to turn to thermal energy.

The thermal energy causes the gases to expand and move parts of the engine, producing kinetic energy.

In a plant

Every green plant you see converts light energy from the sun into energy stored in chemical bonds in the plant.

What it this process called? photosynthesis If you eat the plant the chemical potential

energyin the plant is transformed into other forms of energy in your body.

Conversions between Kinetic and Potential energy

Can you think of anything that converts kinetic into potential energy? Bicycles Roller coasters Swings Bow and arrow Rubber band

Mechanical Energy

Mechanical Energy: the total amount of potential and kinetic energy.

Mechanical energy= potential energy + kinetic energy

Energy Transformations in projectile motions

What is projectile motion? When an object moves in a curved path.

Low KEHigh GPE High KE

Low GPE

High KELow GPE

Energy Transformation in a swing

The ride starts with kinetic energy ( a push) As the swing rises, you lose speed but gain

height. Kinetic energy changes from gravitational

potential energy. At the top of the path GPE is the greatest. As the swing accelerates downward,

potential energy changes to kinetic energy. At the bottom of each swing kinetic energy is

greatest, while potential energy is at a minimum.

The Law of Conservation of Energy

Energy can not be created or destroyed, it only changes from one form to another.

On a large scale this means that the entire amount of energy in the universe stays the same.

The effect of friction

On a swing where does the energy go? It is converted to thermal energy to heat

up the hooks that hold the swing in place.

Converting mass into energy

The sun is able to heat the earth from so far away due to the fusion that is taking place.

Fusion is when two nuclei are combined to form one.

This takes a small amount of mass and turns it into a large amount of energy.

Nuclear Fission

Nuclear fission is when one large nucleus is split into two or more smaller ones.

This also uses mass and converts it into energy.

Nuclear fission is used by power plants to create electricity.

What kinds of energy are found in you?

Moving: stored potential energy is converted into kinetic energy.

Keeping temperature constant: chemical potential energy.

Energy Conversions in your body

Your body obeys the law of conservation of energy.

Your body stores energy in the form of fat and other chemical compounds.

The chemical potential energy is used to keep your heart beating, and you digesting the foods you eat.

Your body also converts energy to heat to make your body move.

Food Energy Your body needs the proper ratio of food

taken in to energy burned. The food calorie (C) is a unit of energy

that nutritionists use to find out how much energy your food has.

1 C= 4184 J 1 gram of fat= 9 C Carbohydrates and proteins each have 4

C of energy.

The number of Calories used

Calories in food