Energy as a tool in Energy as a tool in physicsphysics

• Energy is a very abstract notion, but Energy is a very abstract notion, but it is a very useful and it is a very useful and quantifiablequantifiable notionnotion

• We use the law of We use the law of conservation of conservation of energyenergy to predict behavior to predict behavior– We rely on the fact that energy is not created out of We rely on the fact that energy is not created out of

nowherenowhere

• Where did the energy we see around Where did the energy we see around us us come fromcome from??– most of what we use derives from the sunmost of what we use derives from the sun– some derives from other, exploded stars (nuclear some derives from other, exploded stars (nuclear

fission)fission)– ultimately, all of it was donated in the Big Bang!ultimately, all of it was donated in the Big Bang!

Of conservation of energy

Energy cannot be created or destroyed

Types of EnergyTypes of EnergyAll energy can be broken down into 2

categories

Kinetic EnergyKinetic EnergyPotential EnergyPotential Energy

Kinetic energy

An objects kinetic energy is equal to……

K=½ mv2

Which has a bigger affect????

kinetic energy = energy due to motion

If this car had a velocity of 24 m/s and a mass of 1000 kg what was its kinetic energy when it entered your living room?

1) 288000 joules

2) 15000 joules

3) 49000 joules

4) 391000 joules

Potential Energy

An objects Gravitational Potential energy is equal to……

Ug = mgh

Which has a bigger affect????

1644 m staircase

Potential energy = stored energy in earth

Calculate Spidey’s Calculate Spidey’s gravitational potential gravitational potential energy if he is 150 m energy if he is 150 m up a building and his up a building and his

mass is 89 kg!!!mass is 89 kg!!!

1) 288000 joules

2) 4388 joules

3) 130830 joules

4) 94224 joules

Dissipated EnergyAn objects Thermal

energy is equal to……

This is energy that is hard to reclaim from the system. Even though the energy is not lost, it is hard to convert into another form

of energy.

An object gives off thermal energy in the form of heat! This is where %75 of your

energy goes!!

kinetic energy = energy due to motion of molecules, but it is extreme hard to reclaim

Elastic and spring Potential Energy

An objects elastic potential is equal to……

Relax! Right now you need to understand that it is another form of potential energy!

Think bow and arrow, rubber bands, springs, catapults, sling shots, etc.

Potential energy = stored energy in the spring

Mechanical vs. Non-mechanical energy

Mechanical energy – Potential and Kinetic Energy

The total mechanical energy of a system will be the sum of the potential and kinetic

energy

Non-Mechanical energy – everything else! Light, Sound, Chemical, thermal.

Of conservation of energy

Energy cannot be created or destroyed!!

So what happens to an object’s potential and

kinetic energy as it moves?!?

So does an object lose or gain energy as it

moves?!?

The point is:Energy is never lost… just

transferred!

Doing “Doing “WorkWork” on objects” on objects

• Work in physics is when energy is transferred into, out of, with in the system.

• Work in the system can result in a change in Kinetic Energy OR a change in Potential Energy.

• Can be positive (net gain in energy) or negative (net loss in energy)

• Work is accomplished by applying a FORCE through a DISTANCE.

Work = Force x distance W = Fd

Anytime there is a change in energy, you do work!

Lets say this fat kid pulls his wagon with a force of 300 N. How much work does he do on wagon if he pulls it for a distance of 35 meters?

A. 8.57 joules

B. 7,777 joules

C. 4,322 joules

D. 10,500 joules

W = Fd

UnitsWork & Energy are scalar quantities.

Joule (J) – SI unit for Work & EnergyJames Prescott Joule (1818-1889) – English physicist; studied heat and work. This led to the theory of conservation of energy. Developed Kelvin scale with Lord Kelvin.