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(Chelsea Tafoya) What does work mean to you?
Doing chores around the house?
Sitting at a computer and studying for an exam?
Or writing a paper?
There is a lot of work going on.
So what does that have to do with physics?
Well, in physics,
work has a more specific meaning,
and it does include some of the things I just mentioned
and a lot more.
In physics, work happens
when a force causes an object to move,
or be displaced.
An object must accelerate for work to be done.
If it doesn't move,
or it moves at a constant speed,
no work is done.
Work also relates to the amount of force
we apply on an object,
like when we're mowing the lawn
or hitting a tennis ball.
To figure out how much work, you need to know the force
that's moving the object, F.
And the distance the object moves,
it's displacement, d.
The relationship between these terms is force
times displacement equals work, W.
If it takes 15 newtons of force to move an object 4 meters,
then 15 newtons times 4 meters
equals 60 newton meters.
The combination of newtons and meters
is called a joule.
Capital J stands for joule.
So it's 60 joules of work.
Because a joule combines newtons and meters
to make a new unit, it's a derived unit.
Work is a scalar quantity.
Even though it has no direction,
it can be positive, negative, or zero,
so if I apply a force in a certain direction,
say, I lift this box up, for example.
I'm applying force upward,
and the box is traveling upward, too.
This is positive work because force and displacement
are in the same direction.
If I just hold this box and don't move it,
there is zero work going on.
Work is negative when the force goes in one direction
and the object moves in the opposite direction,
so if you're looking at the force of gravity,
and I drop this box,
the work done by the gravitational force
on the box is positive.
It's moving in the direction of the force,
down.
If I lift this box up,
the work done by the gravitational force on the box
is negative.
It's displacement is in the opposite direction
to gravity,
and there's no work done when the force and movement
are perpendicular to each other.
If I pick up the box, I'm doing positive work.
And then, when I walk,
I'm doing zero work on the box,
since I'm pushing it upward, but it's moving to the side.
When I set the box down, I'm doing negative work
because my applied force is up
but the box's displacement is down.
So here's another question.
When I pick up the box, is work done
in the X or Y direction?
It's a vertical movement,
so the work is in the Y direction.
If I push the box across the desk,
where's the work being done?
X or Y axis?
The direction of the motion in the box is horizontal,
so the work is done on the X axis.
So now that we know what work is in physics,
think of all the work there is.
A dancer being lifted up by another dancer: work.
A weight lifter, picking up a weight:
that's work, too.
Where is the work when someone sits at a computer?
Just sitting there isn't work.
It is work if you're typing,
putting force on the keys.
You might argue that the body is burning energy,
and that's work. You're correct.
But that's a separate system,
a chemical energy system.
What we're addressing here is mechanical energy.
So force and movement in the same direction
is 100% positive work.
Let's say the football player on the left
is carrying the ball to the right,
and he runs in to the defender on the right.
The defender will slow down the motion of the carrier.
The ball carrier is slowing down
and losing energy due to the force
exerted on him by the defender.
And the total amount of work done on the ball carrier
is negative because he is losing energy.
That's exactly how friction works.
In physics, the work of friction
is exactly the work of the defender.
Friction is slowing the forward motion
of the ball carrier,
so the work done by friction is negative.
How much work is done when you do a sit up?
Because you are lifting up, that's positive work,
and then going back down, that's negative work,
so the total work done is zero
because the total displacement from the starting and end point
is zero.
Okay, here's another one.
Five football players push a 300.0 kilogram tackle sled
across the field.
The sled is accelerating at 0.25 meters per second squared.
With negligible friction,
what is the work done on the sled
by the football players over 15.0 meters of pushing?
To solve this, draw a free body diagram.
Figure out all of the forces acting on the sled.
Add arrows to see if any of the forces
have the same or opposite directions.
There's gravity pulling it down
and the normal force acting up.
In the horizontal direction,
the players are applying a force to the sled,
causing it to accelerate.
To find the force of the football players,
the formula is force, F,
equals mass times acceleration.
We can combine the force and work equations.
Substitute the ma for F in the work equation,
and see that work is equal to mass
times acceleration times displacement,
or mad.
So that's the applied work.
W sub app,
or the work exerted on the sled
by the football players.
Remember, the mass of the sled is exactly 300 kilograms,
and the acceleration, a,
is 0.25 meters per second squared.
And the distance the players are moving the sled
is 15.0 meters,
so it's 300 times 0.25
times 15.0,
which equals 1,125 joules.
Using significant figures,
the answer is 1.13 times 10 to the 3rd power joules.
What about work done by gravity and the normal force?
Remember, because they're perpendicular to the motion,
they do no work.
Now let's add energy to the mix.
You know, there are all kinds of energy.
There's electrical, solar, wind, and hydroenergy.
Without energy, there would be no work.
Work transfers energy from one object to another.
So think of energy as the ability to do work.
According to the Law of Conservation of Energy...
And there are a lot of different types of energy.
Energy, like work, is a scalar quantity.
It can have a negative or positive value,
depending on if its added or removed from a system.
But what is a system?
A system is simply a collection of objects
in a particular area that you want to study.
Let's say it's the human body.
Energy goes into the body, like food,
which is chemical energy,
and energy leaves the body like in the form of heat.
That's thermal energy.
Let's put a box around the system.
In this case, let's make it the human body.
If the system is isolated from the outside world,
it's called a closed system.
If our system interacts with the world outside this box,
it's called an open system.
In this course, assume that a system is closed
unless stated otherwise.
So remember,
a system depends on what you want to study.
Your entire body could be considered a system.
The earth itself, or the entire galaxy,
could be a single system.
So let's summarize what we learned
with this ball as the system.
If I throw the ball in the air and catch it,
we know that gravity does negative work
as the ball rises and positive work
when it falls,
and the net work done on the ball by gravity
is zero.
There are all types of energy
that we'll explore in other segments.
That's it for this segment of "Physics in Motion."
We'll see you next time.
(announcer) For more practice problems,
lab activities, and note taking guides,
check out the Physics in Motion toolkit.