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There are forces all around us.
Like the wind from this fan, that is quite the force.
And if your job is to build a bridge
or design a spacecraft,
even if you're driving a car,
or flying on a jet,
forces around you have an impact.
And sometimes they're pretty noticeable.
If you've ever been on a bumpy flight,
you kinda get the drift.
In this segment,
lets take a look at some of those forces.
And later, I'll show you how these forces
work within Newton's second law of motion.
Remember, a force is a push or a pull
and there are two basic types of forces.
The first is field forces.
You can't see them, but field forces
are all around us.
They're actions at a distance.
Invisible and powerful. Like...
...gravity. That's a field force.
Ever fall down?
Then you've felt the force of Earth's gravity.
Anything with mass exerts gravity on objects.
Not just large objects.
All of us exert a gravitational force
on the objects around us.
The law of gravitation describes how objects
exert the force of gravity
on all other objects.
The amount of force depends on how far apart
they are from each other
and how much mass they have.
The equation for that is:
the force due to gravity
equals the gravitational constant, capital G,
times the mass of the first object,
multiplied by the mass of the second object,
divided by the distance between them squared.
This is an example of an inverse square law.
That states...
Earth's gravity pulls objects including you and me
towards its center.
So take a sky diver, ignore all the other forces,
and just measure gravity as they're falling.
And let's stop the action right here.
How fast will they accelerate due to gravity?
Well, it's about 9.8 meters per second, every second,
which looks like this.
Then this... Then this.
And finally, this is how it's written most efficiently.
Meters per second squared.
The 9.8 number changes depending on how far you are
from the center of the Earth.
The further from Earth's center,
the force due to gravity becomes smaller.
9.8 is a good approximation for solving our problems
on Earth's surface.
When one object is on the surface
of another very large object like the Earth,
the force due to gravity, written F sub G,
equals the weight of an object,
which is equal to its mass,
written with a small M,
times the acceleration due to gravity,
small G.
Which on Earth is 9.8 meters per second squared.
So, what if I weigh 75 kilograms here on Earth?
My mass won't change no matter where I am,
but my weight will change.
To figure it out on Earth,
you take acceleration due to gravity--
9.8 meters per second squared,
times the mass, 75 kilograms.
So my weight on Earth is 735 Newtons.
On the moon, which has only
one-sixth the amount of gravity as Earth's,
or 1.6 meters per second squared,
you will multiply that by 75 kilograms.
On the moon, I weigh 120 Newtons.
And on Jupiter,
where gravity is a lot more than Earth's
because the planet is huge,
I'd weigh 1,838 Newtons.
So what is a Newton?
A newton is a unit of measurement for force,
written with a capital N.
More specifically,
a Newton is the amount of force required
to give a one kilogram mass an acceleration
of one meter per second
for every second that goes by,
or one meter per second squared.
Along with gravity, there are other field forces around us.
There's the electromagnetic force
and strong and weak nuclear forces.
We'll talk about those in another segment.
So, as we just learned,
one type of force is the field force.
The second type of force is a contact force.
A contact force...
(crashing)
...just like it sounds,
occurs when there's contact between two objects.
And it's not just head-on collisions.
They just have to touch.
One type of contact force is tension.
That's a pulling force.
Kind of like the one this rope exerts on the bucket.
Tension force is written F sub T.
Another type of contact force is friction.
That's when an object rubs against another object.
Friction is always in the opposite direction
as the motion of the object.
It's written F sub F.
We're going to have some fun with friction
in the next section.
Okay, let's pull this spring.
What happens here?
It springs back. That's another force.
Spring force is when an object is pulled by another object,
or by me,
and has elasticity to bounce back
to its original form.
We write that as F sub spring.
Then there's applied force.
That's basically when forces are applied to an object
by a person or another object.
I'm pushing this box, applying force.
Applied forces are contact forces written F applied.
Then there's a normal force.
That's the force exerted by the surface of the table
that is pushing up on this box.
Normal force is written F sub N.
It's always perpendicular to the surface
that the object is resting on.
Okay, so how do these forces come into play
with Newton's second law?
An object will accelerate in the direction
of the net force.
To figure out the force on an object,
you want to first account for all the forces on the object
and determine the net force.
The net force is equal to the mass of the object in kilograms,
multiplied by its acceleration,
which is F equals M times A.
If all the forces are balanced,
there's no acceleration.
Okay, now let's put what we've learned into play.
As you know, forces are happening all around us.
But to keep it simple,
let's look at one dimension at a time.
The forces that go vertically or in the Y direction,
or forces that go horizontally in the X direction.
So remember, force is a push or a pull with a direction.
A vector quantity pointing in one direction or another.
"F" is for force.
"M" is for mass.
And "A" is for acceleration.
Forces are measured in Newtons,
and mass is measured in kilograms.
Net force is when you add up all the forces
in a given direction and you write it as F sub net.
Even though it's not moving,
what forces are acting on this box?
Well, you've got the weight of the box-- F sub G,
which equals its mass times the acceleration from gravity,
which is the force pulling down on the object.
In this case, it's equal to the normal force--
the force pushing up.
Because the box is at rest,
it isn't accelerating
up from the table or falling through it.
That's because the forces acting on it are equal,
so the net force is zero.
That's happening in the vertical,
or the Y direction.
And this is how you would communicate that.
You write net force F sub net equals zero.
In another segment, we play a game of tug of war
to explain unbalanced and balanced forces.
Now, it will help us explain Newton's second law of motion,
that an object accelerates
in the direction of the net force.
If each side pulls with a force of four Newtons,
the force is balanced.
It's also in equilibrium.
So what's the net force?
It's zero.
If one side pulls four Newtons to the right
and the other pulls two Newtons to the left,
the forces are unbalanced.
They're not in equilibrium.
So, what's the net force this time?
It's two Newtons to the right.
And following Newton's law,
the object accelerates to the right
in the direction of the net force.
Now that you know some of the forces
and how they interact around us,
let's see if you can figure this out.
What is the magnitude of the normal force
acting on a 50 kilogram box
that is resting on a surface.
Remember from the free body diagram segment,
first you draw the box and put a dot in the middle.
Now, what forces are you dealing with in the vertical direction?
As always, there's gravity
acting on the box.
So draw an arrow pointing downwards.
And there's a normal force.
And for that, draw an arrow
pointing upwards.
Next, pick a positive direction.
In this problem,
let's make our positive direction
up for the Y direction
and to the right for the X direction.
That's standard practice
for these types of problems.
If a force is acting in the positive direction
then it has a positive value.
If it's acting in the negative direction,
it will have a negative value.
For example, if you have two Newtons
and add two more Newtons
acting in a positive direction,
that would equal four Newtons.
If they were acting in the opposite direction,
you would subtract them
and the net force would be zero.
Next, look at all the forces acting
in a given direction separately.
Look at the forces in the positive and negative Y direction,
then look at all the forces acting in the positive
and negative X direction.
Based on the information in the problem,
you should be able to figure out
whether or not they are accelerating
in either direction.
So again, the positive direction
is up, that's your normal force.
Gravity is acting in a negative direction,
which is downwards.
Add both of these forces,
and what do you get?
You have to subtract gravity
because it's acting in the opposite direction
to the normal force.
F sub N minus F sub G
is equal to zero.
If you add gravity to both sides,
we can see how the two relate.
And you see that in this case,
the normal force
is equal to the force of gravity.
So F sub N is equal to F sub G.
Returning to our original problem
of a 50 kilogram box resting on a surface,
F sub G equals 50 kilograms
times the acceleration of gravity,
which is 9.8 meters per second squared.
Our equation for the force due to gravity,
F of G, is equal to M G.
M is the mass of our box in kilograms
and G is the acceleration due to gravity on Earth,
which is 9.8 meters per second squared.
If the force of gravity is equal to the normal force,
then the normal force is also mass times
the acceleration due to gravity.
Plug in the numbers and you get a normal force of 419 Newtons,
which should be written with significant figures
in scientific notation which looks like this.
For a more in-depth look at how to solve these types of problems,
see the closer look on Newton's second law.
That's it for this segment of "Physics in Motion"
and we'll see you next time.
For more practice problems, lab activities,
and note-taking guides, check out
the "Physics in Motion" toolkit.