Name: Is Gravity An Illusion? | Space Time | PBS Digital Studios
Uploaded: 2016-11-30T08:04:35.000Z
Duration: 12 min 33 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Transition words & phrases

because a gravitational force accelerates it

toward the ground, but what if it's really

the ground accelerating up to meet the apple?

can be considered at rest, Earth applies a gravitational force

to the apple, and that force causes the apple

of the apple as stationary and the ground--

along with everything on the ground-- as accelerating upward

Now what I just said sounds preposterous and maybe

and today I'm going to clarify what exactly this point of view

and how it planted the seeds for what would eventually

need to begin with some Physics 101 and Newton's

That's just some X-Y-Z axes to label points in space

watch this super awesome 1960s black and white video from MIT

It's amazing and I promise you won't be disappointed.

whether a frame of reference is really at rest

can tell you whether your frame of reference

Here's how that works-- take an object with no forces on it

If it stays right where it is, then your frame of reference

is not accelerating and we call it an inertial frame.

Now in Newtonian physics, inertial frames

are special because Newton's second law, F equals ma,

In other words, the net force on an object

will equal that object's mass times its acceleration

only if you're measuring that acceleration

For example, suppose that you're in a train

will accelerate backward, even though you can't identify

So inside the train car, F decidedly does not equal ma

and the train car's frame of reference is not inertial.

In contrast, a frame attached to the tracks

because relative to that frame, you don't accelerate at all.

Instead, the train car accelerates forward

accelerates relative to an inertial frame

Inertial frame and non-accelerating frame

In fact, you can think of inertial frames

as the standard against which you measure true acceleration.

And from the perspective of inertial frames,

motion obeys a simple rule-- F equals ma.

All right, let's look at things from the train car's

frame of reference though a little more carefully.

not only does everything accelerate backward

lurch toward the back of the car with the same acceleration.

Remember, from the preferred point of view

of the inertial frame that's attached to the tracks,

you, the book, and the elephant are all stationary

and it's only the train car that actually accelerates forward

Subtitles ListPlay Video

Is Gravity An Illusion? | Space Time | PBS Digital Studios

perspective

illusion

constant

straight