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!

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

categories

Kinetic EnergyKinetic Energy

Potential 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

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

Chemical Energy

An objects chemical energy is equal to……

Relax! That’s really more for chemistry class. But you still need to understand that

it is another form of potential energy!

Just know that chemical energy is potential

energy that is stored in chemical bonds within

molecules!Potential energy = stored energy

Thermal EnergyAn objects Thermal

energy is equal to……

Relax! That’s really more for latter this year. But you still need to understand that

it is another form of kinetic 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

Elastic and spring Potential Energy

An objects elastic potential is equal to……

Ue = ½ kx2

k = spring constant Think bow and arrow, rubber bands, springs, catapults, sling shots, etc.

Potential energy = stored energy

If a spring has a spring constant of 400 N/m, how much work is required to compress the

spring .250 m from its undisturbed position?

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, and they are reusable

Non-Mechanical energy – everything else, and non reusable!

Doing “Doing “WorkWork” on objects” on objects

• Work in physics is when energy is transferred.

• This can result in a change in Kinetic Energy OR a change in Potential Energy or losted by heat.

• 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 parallel distance OR W = Fd cos θ

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

Ex) A 20.0-N force is used to push a 2.00-kg cart a distance of 5.00 m. Work done on cart =

W = Fd cos (0)W = (20.0 N)(5.00 m) (1)W = 100 J

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 cos θ

This really strong kid can lift the barbell that weighs 888 N a distance of 0.5 meters. How much work does the strong kid do on the barbell?1) 444 joules

2) 4,444 joules

3) 1,776 joules

4) 0 joules

The same kid then carries the barbell that weighs 888 N a distance of 0.5 meters to the right. How much work does the strong kid do on the barbell?1) 444 joules

2) 4,444 joules

3) 1,776 joules

4) 0 joules

C3P0 gets mad and pushes R2D2 a distance of 15 meters and exerts 6,675 J of energy (work). How much force did C3P0 push with?

A. 9,999 N

B. 0 N

C. 100,125 N

D. 445 N

W = Fd cos θ

•Power: the rate at which work is performed or energy is converted.

•P = w/t

•Power is measured in Watts

•1 W = 1 J/s = 1 kgm2/s3 = 1 Nm/s

How much power does it take to do 150 joules of work for 5

seconds?

If the motor in my car has a maximum power rating of 950 Watts, how much work can it

do in 3.5 seconds?

If the motor in my car has a maximum power rating of 950 Watts, how much work can it

do in 3.5 seconds?

A car exerts a force of 10 N at a speed of 2 m/s. How much power does the car

use?

Of conservation of energy

Energy cannot be created or destroyed

Mechanical energy…

• Even though the potential and kinetic will change, the total energy of a system will never change!

• This is because we are going to ignore friction…

• For most systems, how will we find the total energy?

Remember…

• The law of conservation of energy says that energy is never created or destroyed, but changes from one form to another.

• The total mechanical energy of a system is the sum of the potential and kinetic energy.

• K = (½)mv2

• U = mgh

Conservation of Energy Formula

• Mechanical Energy – sum of kinetic energy and all other forms of Potential Energy.

E = K + Ug

E = E’

K + Ug = K + Ug

½ mvi² + mghi = ½ mvf² + mghf

ExampleA kid on a bicycle coasts up a 4m hill. What is the kid’s velocity at the top of the hill if the velocity is 10 m/s when he has already gone 1m?