How do I understand physics easily?

Think a little bit like Isaac Newton. The easiest way is always to ask yourself "For every action, there's a reaction. So if I suddenly add/subtract different things such as mass/charges/energy/momentum etc. something will happen. What can be affected by these stuff?"

I tend to think much about energy when dealing with problems. Since energy must be conserved. And if something decelerate/accelerates, loses altitude, gains height, is transported and so on, energy must in some way be added or subtracted. So what stuff can steal or give you energy?

If you don't know what energy is, read a couple of comicbooks or watch superheromovies or whatever. That can make you get a simple image about it and from that point, things get clearer.

But the most simple question is ALWAYS: "How does it work?" or "Why does it work?"

Because then you will look it up on the internet. You learn some stuff, but more questions will pop up in your head. You want to learn more about something. More questions. And on and on it goes...

And it never gets boring. Because not only will you appreciate the beautiful things in the world. You will also understand how much work the nature has done to make it that way and how simple, but still complex nature's way is.

How can a planet become a black hole?

                                                               ANS>

Apart from compressing Earth (a planet)   artificially to the Schwarzschild radius,which happens to be 8.7 millimeters (Yes. You will have to compress all of the Earth's mass into a sphere with a radius of less than a centimeter to turn it into a black hole), we can also go in the opposite direction. 

And by opposite direction, I mean keep on adding mass to the planet until it becomes a star. A star massive enough to collapse into a neutron star massive enough to collapse into a black hole.

I did a little research (read: Google) and found that one of the smallest stars ever observed is about 100 times heavier than Jupiter.

And Jupiter is nearly 317 times heavier than Earth.

That basically means you will have to fuse together 317 earths to create a Jupiter (let's assume we are aiming for Jupiter's volume as well.)

Hence, 317Earths=1Jupiter$317Earths=1Jupiter$

Now to turn Jupiter into a star, we need a hundred of them to fuse together.

Hence, (317Earths=1Jupiter)x100=Thesmalleststarobserved$(317Earths=1Jupiter)x100=Thesmalleststarobserved$

But that's not nearly enough. Jupiter's mass is about 0.09% that of Sun.

Hence, (317x100)/0.09Earths=1Sun$(317x100)/0.09Earths=1Sun$

That's nearly 352,223 earths.

By now, we have calculated that we need 352223 earths to create a sun.[1]


But yet again, that's not nearly enough.
We want our newly formed star to collapse into a neutron star.

For that, the mass should be at least 8M (M= Mass of 1 sun).

Hence, 8x352223earths=Astarmassiveenoughtobecomeaneutronstar$8x352223earths=Astarmassiveenoughtobecomeaneutronstar$

That's 2,817,784 earths.

But we want the resulting neutron star to be massive and unstable enough to further collapse into a black hole. According to various theories, the resulting neutron star be at least 2.5 to 3 times the mass of our sun to collapse into a black hole. For that to happen, the original star must be at least 20 times the mass of our sun.

Hence, 20x352223earths=Astarmassiveenoughtobecomeaneutronstarmassiveenoughtocollapseintoablackhole.$20x352223earths=Astarmassiveenoughtobecomeaneutronstarmassiveenoughtocollapseintoablackhole.$

That's  7,044,460 earths.

There you have it.

If you are planning to convert an earth like planet into a black hole, you just need about 7 million of them.

And a little time.

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P.S. Before someone points out in the comments section that it's more about density and less about mass. And that I could've increased the density by reducing the volume while keeping the mass constant, that part has already been acknowledged withSchwarzschild radius.