Energy & Work

Before we can register for LCC’s Physics 201, everyone needs to have their LCC “L” number. Who doesn’t have their number?

“Work” in Physics

You've all got some pre-existing ideas about the word “work”, right?

After all, you've heard: “(your name here), get to work!”

In physics, work is very different. It has a lot to do with, and in common with, energy.

Def'n: the transfer of energy via a force. So, a force can do work! .

How Work is Done by a Force

When a force is applied on an object, often “work is done by that force” How much?

Work = Force x Displacement Example: Alex pushes his desk, with FA=150N,

for 0.5 m across the floor. Work done is F x ∆d = 150N x 0.5m = 75 N-m

“Newton Meters” .

Energy

Energy is the ability to do work. Energy also has the units Joules. .

More on Work

What are the units for work? Again, since Work = Force x Displacement, the units are Newtons - meters. Scientists simply call this combination of units “Joules” (for English physicist, James Prescott Joule)

Joules are also the metric unit for energy! Thus, Work and Energy are “the same kind of

animal”! .

More on Work

Now, to do work, the Force has to go, even a little bit, in the direction of the displacement

If you pushed straight sideways on the driver’s door of a car, while it slowly rolled forward. The displacement vector is horizontal to the left (say), and your force is perpendicular to that (so it has zero component in the direction of motion). Thus, zero work is done.

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Do These Forces Do Work?

You push on the whiteboard, with 50N force, for ten seconds.

Answer: There’s no work done! ∆D=0m Multiply your force by zero, and get zero!

You grab the back of a child's tricycle, pulling backward, and slow it down.

Answer: Work done is your Pulling Force x ∆ d. (Force will be negative here, so the work done has a negative value).

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Do These Forces Do Work?

A student pushes a giant crate of dark chocolate up a ramp, toward his truck bed.

Answer: Yes, Work is done by your applied force. And, yes, the force of gravity also does some work, but we didn't ask about gravity.

Gravity pulls the bowling ball (downward) as it rolls across the alley toward the pins. Is work done by gravity?

Answer: No, gravity acts downward, perpendicular to the motion (displacement) of the ball. Since there’s no component of gravity’s force in the direction of motion, zero work is done.

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& if force isn't pointing in the direction of the displacement?

If a waiter, carries a plate to the table, his force is upward, and the (constant speed) motion is horizontal (to the right, say). Thus, zero work is done.

Now, here’s the more general formula: Work = F|| x ∆ d. So, you find the component of force in the direction of the displacement. (See definition in our text, section 6-1)

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Work it Out

If you push a crate up a 7m-long ramp, with FA=400N. How much work is done?

W = FA x ∆d = 400N x 7m = 2800 J (Joules)

Do these forces do Work?

See some from HW.

Energy

Potential Energy = energy stored (in some way, any way)

Examples of PE: energy can be stored in a:

Spring or bungee: called Elastic PE (EPE) Explosive material: called Chemical PE (CPE) Putting something up high: Gravitational PE (GPE)

Gravitational Potential Energy

Gravitational PE = E stored “by gravity” You've done work to lift it up there in earth's

gravitational field (it's got “stored energy”). Now it has the ability to do work do work (like turn a

generator) as it falls back down) It's calculated this way: GPE = mgh

Mass, gravity (9.8m/s2), and height of object Let’s see an example…

Work it Out

If we lift a 6kg bowling ball up, onto a 2.2m high shelf, how much work is done?

The lifting force, FA is as strong as gravity (otherwise it would fall). We write this as: FA=mg. Now, W = mg x ∆d = 6kg (9.8)(2.2m) = 129.4 J

This suggests a simpler formula: When lifting an object straight up, it gains Gravitational Potential Energy GPE. How much? GPE = mgh (units are Joules!)

Follow-up question: How much energy would be turned into Kinetic Energy if that bowling ball ball were to fall?

Answer: all 129.4 Joules would become KE by the time the ball fell to near the floor.

Kinetic Energy

Finally, if you want to know the Kinetic Energy (energy of motion) of an object, it’s just:

KE = 1/2 m v2

Practice Time

Handout: Digger The Dog problems (we'll trade papers and correct these in about 10 minutes).

Public Service Announcement

SPLAT! Day is only 2 weeks from now. Have you begun building your craft, drawing a diagram of craft, or writing a “why you designed it the way you did”? Start Now!

Your next challenge: test drop your craft at similar height to SPLAT! Day (6 to 8 meters). Your write up will include: how the craft and egg fared during the test. If any problems, exactly what you changed and what happened with next test. Do this right away!

(End w/this slide for Day 1, Mon. May 3)

Let's view a short video on the concept of "Introduction to Work and Energy" (we ran out of time, did not watch this:) http://www.youtube.com/watch?v=2WS1sG9fhOk

At home tonight, tour the online tools provided by the publisher of our Giancoli text: The Online Companion to Giancoli Physics is here: http://cwx.prenhall.com/giancoli/chapter6/deluxe.html

HW: Read 6-2 and 6-3, taking notes. Text pg. 174 # 1-3. Also, go online, follow instructions there.

Tuesday, May 4th

Reminder to self: images w/conceptual Qs and several work-it-out problems to be done each day.

Will watch “Work & Energy Part 2” http://www.youtube.com/watch?v=3mier94pbnU&

feature=PlayList&p=AD5B880806EBE0A4&playnext_from=PL

Weds will watch http://www.youtube.com/watch?v=kw_4Loo1HR4&

feature=PlayList&p=AD5B880806EBE0A4&playnext_from=PL

Upcoming...

Observation that PE + KE stays the same for all heights. This sum is called Mechanical Energy (ME)

Showing where GPE = mgh comes from a the formula for calculating work, by lifting an object at constant speed upward.