Oh! Welcome back, fellow scientists! Justin here. I am, uh, on my way to the library right now. I
want to check out a book on a cool property of matter I just found out about called "inertia."
Oh, here's my stop. [screech] W-W-Woah!
I think I just experienced the effects of inertia.
Have you ever been traveling in a car or a bus and you've just been thrown around everywhere? Well,
today we're gonna... Oh, hold on, hold on, that's my stop. That's my stop! Ah! Hey, wait!
By the end of today's lesson, you will be able to relate inertia to forces and changes
in motion and explain how inertia impacts real-world scenarios. Let's get into it!
What is inertia? Well, whether or not you've actually heard of it before, you've certainly
experienced it. Inertia is the tendency for objects to resist changes to their velocity.
Uh, what does that mean? Remember in our Forces in Motion lesson, we learned that velocity is
the speed and direction of an object's motion. So, inertia is an object's tendency to resist
changes to its speed or changes to the direction it's traveling in. Inertia is not a force. Rather,
it's just how objects behave in the absence of outside forces. So, really,
inertia just says that an object that's moving will keep moving, and an object that's sitting
still wants to stay still, and that's not going to change unless an outside force comes in.
Huh. Are you getting déjà vu right now? You should! Inertia is Newton's first law of motion:
an object at rest will stay at rest, and an object in motion will stay in motion unless
acted upon by an outside force. Newton's first law of motion is also known as the law of inertia.
Let's take a look at what that means in practice. Imagine a ball sitting on
the ground. Until an unbalanced force comes along, the ball is going to continue sitting
still. That's inertia. Now imagine the ball being thrown through the air. No matter how
hard it was thrown or how fast it's going, what will eventually happen to the ball?
Right! It'll slow down and eventually fall back down to the ground.
A common mistake here is to think that the ball stopped moving because it ran out of energy,
but that's not the case. We know the ball has inertia, so unless an outside force interferes
with it, it's just gonna keep moving in the same direction at the same speed forever.
However, there are outside forces at play here. Both air resistance and the pull of
Earth's gravity slow the ball down and stop its motion. But, if the ball were instead
thrown in the vacuum of space where there's no air resistance or gravity, the ball would continue to
travel forever and ever and ever and ever and ever and ever and ever and ever and ever and ever...
So, inertia is an object's resistance to changes in its velocity. Everything in our universe has
inertia, but not everything's inertia is the same. An object's inertia is dependent on its mass.
Objects with large amounts of mass have larger amounts of inertia, meaning that a much greater
force is required to stop it when it's in motion, and a much greater force is required
to move it when it's at rest. For instance, have you ever tried to move a train? These things are
massive. [laughs] There is so much mass in there, so if you've tried to [strains] get it to move...
Ah. [laughs] Yeah, you're not gonna do it. It's just gonna take too much force. And,
um, when it starts moving, it's going to take a lot of force to get it to stop...
A toy train, on the other hand, has much less mass. With just a small push,
you can start it moving along its track, and you can stop its motion just as easily.
Less mass means less inertia. What things around you right now have large amounts
of inertia? Pause the video here and write down your observations in your guided notes.
So, everything in the universe has inertia, and we are constantly experiencing its effects. Although,
you might not have realized why this was happening. For instance, think back to the
last time you were riding in a car or on a bus, like how I was earlier. Have you ever felt like
you were being thrown around whenever the car was turning or whenever the car stopped? Well,
your body is traveling in a certain direction at a certain speed. You have inertia, so your body
wants to maintain its current velocity. But when the car turns, the car changes its direction of
motion, but your body wants to keep going in the direction you were going. As a result [strains] it
really feels like you're being pushed to the side. The car is turning right, but you keep going left.
You are experiencing inertia. A similar thing happens when you're in a car that's stopped
at a light and then the light changes, and the car starts to accelerate. Your body's at rest
and wants to stay at rest, so it can feel like you're –oh – being pushed back into the seat.
And likewise, when the car stops, you're in motion, your body wants to stay in motion,
and it can feel like [screeching brakes] you're getting pushed forward. Inertia is actually the
reason we need seat belts. When a car stops, [screeching brakes] we have to make sure that the passengers stop too.
Cars aren't the only places we experience inertia. Athletes utilize inertia in their sports all the
time. For example, a long jumper will be able to jump farther if they get a running start. Inertia
means that their bodies will maintain the speed they pick up while running once they leap into
the air. But, remember, inertia isn't just about maintaining motion; it also means that an object
at rest wants to stay at rest. So, if you jiggle a tree branch that's got a bunch of snow on it,
the branch will move, but the snow doesn't want to. The snow's inertia, resisting the
change in motion, is what causes it to not stick to the branch and fall off it instead,
giving you quite the shower. Can you think of any other examples of how you've experienced
the effects of inertia? Pause the video here and write down your thoughts in your guided notes.
Let's review everything we've learned about inertia today. Inertia is the
tendency for an object to resist changes in its velocity, or its speed and direction.
Newton's first law of motion describes the concept of inertia. The amount of inertia an object has
depends on its mass. The more mass an object has, the more difficult it will be to change
its velocity. And, the effects of inertia constantly surround us in our daily lives.
To learn more about inertia, be sure to check out the activities
and practice questions for this lesson. As for me, I've got all my books checked out.
[strains] Oh, wow, why do I pick the books with the most inertia to take home? [laughs]
Until next time, remember: Science is all around us. See you next time!