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- Welcome back.

So now we know if a net
force is acting on a particle

then it will accelerate in that direction.

By how much will it accelerate?

To answer the question of
how force and acceleration

are related, Newton observed
that if you increase

the net force by, say, a factor of two,

then the acceleration
increases by that same factor.

This means that force and acceleration

are proportional to one another.

But that's not all that matters.

Next, let's consider the
mass of our particle.

Imagine we have two
particles floating in space,

which are the same size
but have different masses,

like if one is a ping pong ball

and the other is made of lead.

If we applied an equal force, like wind,

to both particles, what would happen?

Both particles would
experience the same net force

in the direction of the wind,

but they wouldn't
accelerate at the same rate.

The less massive particle,
the ping pong ball,

would accelerate faster
than the one made of lead.

So less mass results in more acceleration

and more mass results
in less acceleration,

meaning that mass and acceleration

are inversely proportional to one another.

And we already know that acceleration

is proportional to force.

Putting these together we see that

acceleration depends on
the magnitude of net force,

which is proportional to acceleration,

and the mass of the object,

which is inversely
proportional to acceleration.

This gives us a is
proportional to f divided by m.

Multiplying both sides by m gives

m times a is proportional to f.

And if we flip this, we get f
is proportional to m times a.

Newton found that f isn't
just proportional to ma,

it's in fact equal to ma.

This is Newton's second law, f equals ma.

To recap, f is the net force
acting on the particle,

m is the mass of the particle,

and a is the acceleration of the particle.

Now let's consider the force of gravity.

You made have heard of the famous story

about Galileo's experiment in 1589,

where he dropped two balls
off the Leaning Tower of Pisa.

One was made of a light material,

the other a heavy material.

You might be surprised
to know that he observed

that the two balls accelerated
at exactly the same rate.

That blew everyone away.

At the time, everybody, starting
with the ancient Greeks,

just assumed that heavier objects

fell faster than lighter objects.

So unlike wind, the force of gravity

seems to be independent of mass.

The interesting question is why.

Newton gave us the answer.

His first law of gravity said
that more massive objects

experience greater gravitational force

and his second law says

that mass is a resistance to acceleration.

These two competing trends,
one encouraging acceleration

and one resisting it,
cancel each other out.

To see why this happens mathematically,

Newton theorized that force
due to gravity, call it big F,

is proportional to the
mass of the particle.

Big F is proportional to ma.

Think of gravity as an
acceleration vector, call it g,

such that big F is equal to mg.

So we have two equations.

Newton's second law,
little f is equal to ma

where little f is the net force

and Newton's law of gravity
where big F is equal to mg.

For a particle being
acted on by only gravity,

the net force little f is big F.

Little f is equal to mg is
equal to big F is equal to ma.

Or more simply, mg is equal to ma.

Notice the m cancels,
leaving just g is equal to a.

That is, the acceleration of a particle,

when acted on only by gravity,

is independent of the
mass of the particle.

This is why objects of different
mass fall at the same rate.

An equation like this one, that allows us

to compute the acceleration of particles,

is called an equation of motion.

We've covered a bunch of new

and important concepts in this video.

So let's stop here for some practice,

using the next exercise.