What is work?

How would you define it?

Expanding our energy model.

When we began energy we played with the idea

of money being conserved.

The person had a job and that brought money in

he had expenses and that took money out

In physics the process of bringing energy into or out of a system is work.

If work is done on the system, W > 0 and it is brought in

If work is done by the system, W < 0 and is taken out.

Also our little parable, only had one type of energy, Mechanical

He transferred his savings (potential) to liquid (kinetic) energy.

Mechanical isn't the only type of energy.

There is an internal energy of the motion of each atom, this is a

thermal energy, it has to do with temperature.

Thus we define energy of a system now as:

With this addition of thermal energy we can transfer our total

energy around a bit more.

We can explain why a book comes to rest.

The initial kinetic energy, got turned into thermal energy.

It heated up the atoms.

As we consider our basic model of energy we now can discuss decreases to our system, Not just straight conservation inside the system.

Looking at this. We can say energy is transferred in

to our system if W is positive. (out if negative)

and

Energy is transformed in our system if W is zero.

Think back to the text book on table.

Tell me what happens to each of these terms and

what does that mean for my system.

Can you think of a similar example with Work being

the opposite sign?

Work and Kinetic Energy

Your book gives you 7 great definitions

1 Labor, task

2 job

3 task duty

4 work of art

5 Steel works

6 works (how things operate)

7 Transfer of energy to a body by application of a force

When we derived conservation of kinetic and potential energy of gravity and a spring we played around with Newton's 2nd law, and partial differentials. We'll do the same here.

This last term we will use as a definition

Specifically the one that answers

'how much energy does work transfer'

From these two equations together we can get a definition of work.

Note this was just one force in one direction

If we add up all forces and directions we get the following

Not a lot of change, but important none the less. This is the

Work-Kinetic Energy theorem.

One of the questions that we might have thought about

as we began this section on energy was:

"how does a system gain or lose energy"

Answer: One answer at least:

A system gains or loses kinetic energy when work transfers

energy between our system and the environment.

Now there is a really cool analogy

that can be made here.

We have two definitions of Work:

This looks similar to Impulse

momentum theory

Looking at them close we see that a force acting on a system will change the state of the particle.

If the force is done over a time, it changes the momentum,

if it is done over a distance it changes the kinetic energy.

This does not mean that the force either does one or the other.

Either changing the momentum or the energy.

It actually does both, the question is how we look at it.

Consider a car race

Same force

a= F/m

B will accelerate more

After Some amount of

Time = t1

B will be winning

But their momentum's

will be the same.

When they both finally finish the race

B will have gotten there first

But over the same distance their Kinetic

energies will be the same

If this idea is true, there must then be some

way of comparing momentum to energy.

When the Kinetic energies

are the Same

Smaller mass smaller Momentum

When both Momentum's

are the same

Smaller mass Larger Energy

**Also note, the integral-area concept**

**All right.**

now that we have had some fun comparing

these two theorems

Let us get back to work.

now that we have had some fun comparing

these two theorems

Let us get back to work.

**Example**

How much work is done to slow a

1500 kg car from an initial speed of 35 m/s

to rest?

How much work is done to slow a

1500 kg car from an initial speed of 35 m/s

to rest?

**919 KJ**

Evaluate the dot product A*B if

a. A = 4i - 2j B = -2i - 3j

b. A = -4i +2j B = 2i +4j

What is the angle between the vectors A and B for both parts a. and b. ?

-2

0

90

97

"why does the normal force exert zero work?"

"How can we use work to show how much energy is lost in a system?"

"Can we have a little review about the differences between momentum, energy and work? "

"Can we do an example of a dot product?"

"What is the difference between power and work?"

"Could you explain net work (Wnet)?"