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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!
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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
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