Matter & Energy #2 1.Kinetic & Potential Energy 2.Energy Sources & Alternatives.

Matter & Energy #2

1. Kinetic & Potential Energy2. Energy Sources & Alternatives

Energy

• Energy- Energy is the ability to do work, and to do work we need energy.

• Ex. A stretched slingshot has the ability to do work • The units for energy is the Joule (J).• Energy can be present in an object or a

system when nothing is happening at all. (see ex above).

Energy of Motion

• Potential energy- stored energy resulting from the relative position of objects in a system.

Types of Potential Energy• There are two (2) types of PE.

• Elastic PE: the energy stored in any type of stretched or compressed elastic material, such as clock spring or a bungee cord.

Types of Potential Energy cont.

• Gravitational PE: The energy stored in an object due to its position above the ground. − Any system of two or more objects separated

by distance contains gravitational PE.

Gravitational Potential Energy Equation

• Gravitational PE depends on both mass and height.

• Gravitational PE Equation:• gPE = mass X free fall acceleration X height

PE=mgh• Note: m needs to be in kilograms (kg), h needs to be in meters (m) and

g = 9.81m/s2, ALWAYS.

• If we rearrange the PE formula and solve for h, we get: h= PE/mg

• To solve for m, we get: m= PE/gh

gPE Practice Problems

• A 65 kg rock climber ascends a cliff. What is the climber’s gPE at 35 m above the base of the cliff?

• A science student holds a 55g egg out a window. Just before the egg is dropped it has 8J of gPE with respect to the ground. How far is the student’s arm from the ground.

• A diver has 3400J of gPE after stepping up onto a diving platform that is 6.0 m above H2O. What is the diver’s mass?

•PE=m X g X h

Kinetic Energy

• KE is defined as energy of motion.• This is obtained when the PE is

released. Therefore PE changes to KE.

Kinetic Energy Equation• KE depends on mass and speed.• KE Equation:• KE= ½ X mass X speed squared

•• ONLY SPEED (v) is squared!!!

• KE depends on speed more than mass.

KE Practice Problems

• What is the kinetic energy of a 44 kg cheetah running at 31 m/s?

• A 35 kg child has 190 J of kinetic energy after sledding down a hill. What is the child’s speed in meters per second at the bottom of the hill?

• A bowling ball traveling 2.0 m/s has a 16 J of kinetic energy. What is the mass of the bowling ball in kilograms?

Other Forms of Energy

• Mechanical energy: The sum of the PE and KE in a system. This type of energy affects the motion of the object or thing in question.

This sum is simply referred to as the total mechanical energy (abbreviated TME).TME = PE + KE

The TME is = to 50,000 J at every position.

Other Forms of Energy cont.

• Non-mechanical energy: this is energy that lies at the level of atoms and does not affect motion on the large scale.

• Note: Other forms of energy are just special cases of KE or PE. • EX: If you eat food, you will receive energy from it.

Other Forms of Energy cont.

• Atoms and molecules have KE. • Atoms and molecules

are always in constant motion.

• As the temp increase their motion increases, and as the temp decreases their motion decreases.

Other Forms of Energy cont.• Chemical Energy: a type of

PE.• When a chemical rxn occurs,

bonds are broken and new ones form.

• This formation and breaking of bonds involve changes in energy because chemical energy depends on position of the bonds.

Other Forms of Energy cont.• Living things get energy from the sun.• Where do you get the energy you need

to live?• It comes in the form of chemical energy

stored in food. • Plants get their energy from the sun through a

process called photosynthesis. We, in turn, obtain energy through a process called cellular respiration.

Photosynthesis

Cellular Respiration

Other Forms of Energy cont.

• The sun gets energy from nuclear fusion.

• Nuclear fusion- a rxn in which light atomic nuclei combine to form a heavier nucleus. In this process a enormous amount of energy is released.

Other Forms of Energy cont.• Electricity is a form of

energy.

• Light can carry energy across empty space.