♪♪
(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.
