Unit 3 Section 2 Unit 3 Section 2

NotesNotesWhat is Energy?

Energy and Work0 Energy can be defined as: the ability to

do work0 Most of the time we can’t see energy

but it is everywhere around us. 0 Energy can never be created or

destroyed…. It can only be STORED or TRANSFERRED

0 SI unit for energy is: Joules

Energy and Work

0All energy is either Potential Energy or Kinetic Energy

All Energy

PotentialEnergy

KineticEnergy

Potential Energy

0PE is the energy stored in an object0“Potential” means the energy has the

ability to do something useful later on.

Examples of Potential Energy

0A stretched rubber band.

0Water at the top of a waterfall.

0A battery.

0A drawn bow and arrow.

Potential Energy0Elastic Potential Energy: the energy

stored in any type of stretched or compressed elastic material.

Potential Energy

0This is an example of Potential Energy: there is stored energy in the elastic bands which will convert to kinetic energy when it is released.

How are these pictures examples of potential energy?

Gravitational Potential Energy

0 Gravitational Potential Energy: energy stored in objects separated by a distance; results because of gravitational attraction between objects. 0 Depends on 2 factors:0 mass0 height

Gravitational Potential Energy

0The higher an object and the more massive, the more gravitational PE it has. 0Gravitational Potential Energy Equation:

PE = mgh0grav. PE = mass x free fall acceleration x height

Gravitational Potential Energy

0A flower pot with a mass of 15 kg is sitting on a window sill 10 m above the ground. How much potential energy does the flower pot contain?

0PEgrav = mgh

0PEgrav = (15 kg)·(10 m)·(9.8 m/s2)

0PEgrav= 1500 J

Potential Energy

Potential Energy

Potential Energy& Chemical Reactions0Chemical Energy: energy stored

within atoms and molecules that can be released when a substance reacts.

Potential Energy& Chemical Reactions0Chemical reactions involve PE. Why? When a chemical reaction takes place, bonds

between atoms break apart and a new substance is formed, which involves changes in energy from the relative positions of atoms in the substance. 0If a reaction releases energy, there is a

decrease in PE. Example: when a match is struck.

Kinetic Energy

0 KE is the energy of moving objects due to their motion0 Depends on 2 factors:0 mass0 Speed

0 “Kinetic” means movement

Kinetic Energy

0Kinetic Energy Equation:

KE = ½ mv2 0KE = ½ x mass x speed squared

Kinetic Energy

0Example: How much kinetic energy does a bicycle with a mass of 14 kg traveling at a velocity of 3.0 m/s east have? 0KE = ½ mv2

0KE = ½ (14 kg)·(3.0 m/s)2

0KE = ½ (14 kg)·(9.0 m2/s2)0KE = 63 J

Examples of Kinetic Energy

Kinetic Energy0Scientific proof for why

car crashes are more dangerous at high speeds: KE depends on speed more than mass because speed is squared; so increase in speed = large increase in KE

0SI unit for KE: Joules

Kinetic Energy

0A water bottle is knocked off of a desk. When does it have the MOST KE?0At the top of the fall.0In the middle of the fall.0At the bottom right before it hits the

ground.

Kinetic Energy

0A water bottle is knocked off of a desk. When does it have the MOST KE?0At the top of the fall.0In the middle of the fall.0At the bottom right before it hits

the ground.

Kinetic Energy

0Atoms and molecules have KE. Why? Because atoms and molecules are constantly in motion, and KE is energy of motion. 0KE increases as objects get hotter and

decreases as objects get cooler.

Why Physics is Important!

0Someone failed their high school physics, loading the cart so much that it tipped backwards. Poor Donkey!

Mechanical Energy

0Mechanical energy: the sum of PE and KE in a system0Example: An apple falling from a tree

has both PE and KE

Mechanical Energy

Nonmechanical energy0In almost every system, there are hidden forms of

energy that are related to the arrangement of atoms that make up the objects in the system. For example, when you eat an apple, it gives you energy.

0Nonmechanical energy: energy that lies at the level of atoms and doesn’t affect motion on a large scale. 0In most cases, nonmechanical forms of energy are

just special forms of either kinetic or potential energy.

Living Things & Energy0 Where do we get the energy needed to live?0 The energy comes from food. When we

eat a meal, we eat plants, animals, or both. Animals also eat plants, other animals or both.

Living Things & Energy0Plants and algae do not need to eat because

they get their energy directly from sunlight. Plants use photosynthesis to turn the energy in sunlight into chemical energy. This energy is stored in sugars and other organic molecules that make up cells in living tissue. Thus, when you eat a meal, you are really eating stored (potential) energy.

Nuclear Reactions

0How does the sun get its energy? From nuclear fusion; (fusion is the process in which light nuclei join to form heavy nuclei with a large energy release).

Nuclear Reactions0Nuclear power plants use nuclear

fission to release energy. In fission, a single large nucleus is split into two or more smaller nuclei with a large release of energy.

Electricity0Electricity: a form of energy that results

from the flow of charged particles in an electric field. 0This is how the lights and appliances in

our homes get their energy.

Lightning, a form of electrical energy, results from moving electrons between the ground and a cloud.

Light Energy0Consider a bright sunny day at the

beach. Where is it hotter: in the sand or under the umbrella?

Light Energy

0The reason its hotter in the sand is that light carries energy. 0Light energy travels from the sun to Earth

across empty space in the form of electromagnetic waves. Electromagnetic waves are made of electric and magnetic fields, so light is another example of energy stored in a field.