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Why is it when a car suddenly veers right
around a curve,
our bodies seem to lean in the opposite direction?
Or, when the car suddenly stops, our bodies keep going.
Thank goodness for seat belts.
Why does this happen?
In a previous video, we learned how things move.
Now let's head full-throttle
into Sir Isaac Newton's Three Laws of Motion.
That will help us understand why things move.
Sir Isaac Newton was born in England in 1642.
He's considered one of the most influential physicists
of all time.
He developed these three laws of motion.
Engineers need to know these laws to design
better safety features in cars.
For example...
animators and game designers need to know these laws
to draw believable cartoons and better games,
and you do not want an engineer
to build a rocket to launch into space
without knowing about Newton's three laws of motion.
Before we launch into those three laws,
let me explain a little about forces,
because forces are why things move.
Say, for example, I push Summer here.
She moves! I can pull her too.
Forces like a push or a pull can cause us to move.
Force is a vector quantity.
That means it has magnitude and direction.
Okay, so, we're gonna arm wrestle now.
Okay, we're gonna arm wrestle.
It's a great way to show you the difference
between balanced and unbalanced forces.
If there's an equal push and pull, that's balanced.
The forces cancel each other out.
You push on my arm, I push on hers with the same force,
there's no movement.
That's balanced.
If one is greater than the other...
Summer's arm pushes more than my arm does...
the forces are unbalanced,
the object accelerates in the direction of the net force,
and Summer wins.
Same in a tug of war.
If both sides are pulling with equal force,
there's no movement.
That's balanced.
But if one side pulls harder, it's unbalanced,
which means there's a greater force on one side
than the other.
The difference between the greater and weaker force
is the net force.
I win.
Now that you know about balanced and unbalanced forces,
let's speed right into Newton's 1st Law of Motion.
It states...
In this case, my car is stopped.
There are forces acting on me, but they're balanced.
That's why I'm not moving.
We'll explain more about those forces a little later.
Now once the car takes off,
I feel like I'm pinned to the back of my seat
because my body at rest wants to stay at rest,
and resists that forward motion.
Newton's 1st law goes on to say that once...
Newton calls this the Law of Inertia.
Inertia is an object's resistance to change.
So hit the breaks and my body wants to keep going forward,
and the more massive the object,
the more inertia it has and the more resistance to change.
Let's look at an example.
What do you think will happen when a larger person
and a smaller person push each other
while both are on skateboards?
Who moves more?
The larger person hardly moves
compared to the smaller person.
Why is that?
Because the larger person has more inertia.
He's more resistant to moving.
On the other hand, the smaller person
is less likely to stay at rest
because he has less inertia and is less resistant to change,
so he moves more.
So, remember, objects at rest stay at rest.
And objects in motion stay in motion because of inertia,
which means they're resistant to the forces acting on them.
Okay, so let's say you're in deep space
and there are no forces like friction or gravity
acting on an object, like this bicycle wheel.
How long do you think this wheel will spin?
It will spin forever at a constant velocity
because of inertia,
and because there are no outside forces
causing it to stop.
Which brings us to Newton's second law.
Remember these three words.
Mass, force, acceleration.
How do these three variables interact?
Let's use this truck.
If I pull the truck, it accelerates.
If I pull the truck with twice the amount of force,
it will accelerate twice as much.
Because force and acceleration
are proportional to each other,
if you increase the force, then you increase acceleration.
The acceleration is always in the direction of the net force.
The net force is moving the truck to the right,
causing it to also accelerate to the right.
Okay, let's double the mass of the truck
by adding this weight.
How does that affect acceleration?
When I double the weight of the truck
and pull it with the same amount of force,
it will accelerate half as much.
Given the same amount of force,
the heavier truck accelerates slower.
Now, mass and acceleration are inversely proportional
given the same amount of force.
You increase the mass and decrease the acceleration
when the force stays the same.
Okay, here's something about forces you should know.
So when I hit the gas on the car,
the force from the tires pushes on the track
causing an unbalanced force on the cart,
which causes the cart to accelerate.
If the cart is already moving and I push on the gas
just enough to move at a constant velocity,
then all the forces in the cart are balanced.
Forces that could slow the cart
include things like air resistance,
friction between the tires and the track,
along with the friction of all the moving parts
of the cart's engine and drive train.
So if I give it just enough gas to counter those forces,
I'll move at a constant velocity
because the forces are balanced.
If the cart is at rest,
then all of the forces acting on it are balanced.
So, remember, net force equals mass times acceleration.
Written F sub net equals small M times small A.
Okay, so lets move on to Newton's Third Law of Motion,
which states...
That means there are forces at work all around us
that we often don't even think about
and may even seem counterintuitive.
Newton realized that forces work opposite of each other.
For example, I walk and my foot pushes on the sidewalk.
The sidewalk is pushing back up on my foot.
If the sidewalk didn't push back on my foot,
my foot would go right through the sidewalk.
The tires on this go-kart push against the track
at the same time the track pushes against the tires,
causing it to move forward.
These examples are called action-reaction pairs.
You'll find there's still a lot we're learning everyday,
and a lot still yet to explore.
So get to know Newton's three laws,
because they set in motion all kinds of discoveries,
like making safer, faster, and more efficient vehicles.
Take some time to really understand these three concepts.
Inertia...
net force equals mass times acceleration,
and action-reaction pairs.
It will definitely help you now and on down the road.
And that's it for this segment of "Physics in Motion",
we'll see you guys next time.
For more practice problems,
lab activities, and note taking guides,
check out the Physics in Motion toolkit.