ENERGY, WORK and POWER

How can we measure it?How does energy change when an object

is in motion or changes position?

The ability to do work

“Stored” energy due to position or shape

Energy of motion

Joules (J)

Pull of gravity is 9.8 m/s2

(mass x gravity) = weight

mass x gravity x height

Gravitational PE = m x g x h

KE = ½ m x v2

Jumper at top of ski jump (gravitational PE)

Molecules of sugar in a granola bar (chemical PE)

Stretched rubber band (elastic PE)

Speed skater racing around the track

Slalom skier on the downhill

Curling stone gliding across the ice

Definition: Work is the product of _________________ and ________________.

Work requires: __________________________ (no work is being done without it!)

For a force to do work, it must act in the _____________________ direction as the

motion of the object.

Work = ____________________________ x ______________________________

Work is measured in: _________________________ We abbreviate it: _______

force distancemotion

same

force distanceJoules J

James Prescott Joule

Definition: Power is_______________________________________

In other words, __________________________________________

You can increase power by doing two things:

1. _____________________________________________________

2. _____________________________________________________

Power = _____________________ ÷ ________________________

Power is measured in: _______________We abbreviate it: _______

the rate of doing work

how quickly you apply force and move something

MORE work done in a specific amount of time

Do a specific amount of work in LESS time

timework

Watts W

James Watt

Calculating KE and PE

How can we know how much energy is in an object?

• How can we compare the relative kinetic energy (in words) of a downhill skier moving at different speeds?

• Rank them from most KE to least KE.Skier moving at 10 mph ____________________

Skier moving at 30 mph ____________________

Skier moving at 60 mph ____________________

First, an easy one…

Least KE

medium KE

most KE

Calculating KE• Kinetic energy is the energy associated with

objects that are MOVING.• We can calculate how much KE an object has

by using the following formula:– KE = ½ mv2

• m = mass• v = velocity• make sure to follow order of operations!!!

– Please Excuse My Dear Aunt Sally

Calculating KE• For each problem you are trying to solve, it helps to

organize and always follow a set of steps. This way, you can receive partial credit (possibly) for the work that is shown.

First:

write the

formula

Then:

plug in the

numbers

Finally:

write the answer (with

units!!!)

Examples of KE

Two bobsledders and their sleigh are travelling at 36 m/s south and have a mass of 390 kg. Calculate the kinetic energy of the sledders and their sleigh.

write the formula plug in the numbers write the answer (with units!!!)

KE = ½ mv2 KE = ½ x 390kg x (36m/s)2 252,720 J

Meters per second

Examples of KE

Determine the kinetic energy of a 100 kg winger skating toward the goal in a hockey game at a speed of 3.5 m/s.

KE = ½ mv2 KE = ½ x 100kg x (3.5m/s)2 612.5 J

write the formula plug in the numbers write the answer (with units!!!)

Potential Energy

• There are three types of potential energy:- elastic potential energy (what a rubber band

has when it’s stretched out).- gravitational potential energy based upon the

height of the object from Earth.- chemical potential energy (the energy stored

in the bonds between atoms, like in sugar.)

Calculating GPE• We can calculate how much gravitational PE an

object has by using the following formula:– GPE = mg x h

• m = mass• g = the acceleration due to gravity (always 9.8m/s2)• h = height• make sure to follow order of operations!!!

– Please Excuse My Dear Aunt Sally

Calculating GPE

• If you are given a mass (the units will be kilograms), then you will need to multiply this mass by 9.8 m/s2 (the acceleration due to gravity) and also multiply by the height above the ground.

Examples of PEA 100 kg skier jumper is at the top of a ramp 50 meters offthe ground. What is the gravitational potentialenergy of the snowboarder?

GPE = mg x h GPE = 100kg x 9.8m/s2 x 50m 49,000 J

write the formula plug in the numbers write the answer (with units!!!)

Examples of PE

What is the gravitational potential energy of a 45 kilogram snowboarder who does tricks 5 meters above the surface of the halfpipe?

GPE = mg x h GPE = 45kg x 9.8m/s2 x 5m 2205 J

write the formula plug in the numbers write the answer (with units!!!)