Energy Ch. 7

Energy is Everything!!!

Work

Work• Remember force x time? This was an amount

of force for a set time - or impulse

• Now we are talking force over a set distance. So work = force x distance.

• Work is the effort exerted on something that will change its energy!

Work• When you lift a load of gravel against Earth’s

gravity, work is done. – A set amount of force is exerted– The gravel is moved a set distance above Earth

• The heavier the load or the higher we lift the load, the more work is done

WorkTwo things enter the picture whenever work is

done: 1) Application of a force2) The movement of something by that force

Work = force x distanceW=Fd

Work• If we lift a load 2

stories instead of one story, we do 2x as much work because the distance is 2x as great!

• W=Fxd

Work• If we lift 2 loads of

gravel one story up, we do 2x as much work as lifting one load the same distance

• BECAUSE the force needed to lift 2x the weight is 2x as much

• W=Fxd

Work• A weightlifter who holds

a barbell weighing 1000 N overhead does no work on the barbell.

• Think of the formula!!!! Why is this true?

When the weightlifter raises the barbell from the floor he does work! There has

to be movement (a change in distance)!!!

Work Falls Into 2 Categories:

Work done against another force

• When an archer stretches her bowstring, she is doing work against the elastic forces of the bow.

• When you do push ups you move against your own weight

Work done to change the speed of an object

• Bringing an automobile up to speed or slowing it down

• A club hits a stationary golf ball and gets it moving.

In both cases there is a transfer of energy

Units for Work• The combination of force

(N) and distance (m) is the newton-meter (N*m)

• We call this a joule (J) – One J of work is done

when a force of 1N is exerted over a distance of 1m.

– Kilojoules (kJ) and megajoules (MJ) can be used for larger amounts

Mini Quiz!Remember – it’s open notes

1) What is the DEFINITION for work in relation to energy (not the formula)

2) Explain how and what in the formula for work would change if I asked you to calculate work of you lifting 2 textbooks instead of 1.

3) A joule is what two units combined?

4) What do we call these two units combined from question 3? (NOT a joule)

1) What is the DEFINITION for work in relation to energy? (not the formula)

2) Explain how and what in the formula for work would change if I asked you to calculate work of you lifting 2 textbooks instead of 1.

3) A joule is what two units combined?

4) What do we call these two units combined from question 3? (NOT a joule)

Work is the effort exerted on something that will change its energy

W=Fxd The force will double, and therefore the work

Newtons and meters

Newton-meter

Work Practice • Pg. 117 Review Question #2-4 • Pg. 119 Exercises #1-9

Power