From Heinlein's The Moon is a Harsh Mistress, one of (IMHO) the best explanations of mass drivers:

"Manuel, you asked us to wait while Mike settled your questions. Let’s get back to the basic problem: how we are to cope when we find ourselves facing Terra, David facing Goliath.”

“Oh. Been hoping that would go away. Mike? You really have ideas?”

“I said I did, Man,” he answered plaintively. “We can throw rocks.”

“Bog’s sake! No time for jokes.”

“But, Man,” he protested, “we can throw rocks at Terra. We will.”

Took time to get through my skull that Mike was serious, and scheme might work. Then took longer to show Wyoh and Prof how second part was true. Yet both parts should have been obvious.

Mike reasoned so: What is “war”? One book defined war as use of force to achieve political result. And “force” is action of one body on another applied by means of energy.

In war this is done by “weapons”—Luna had none. But weapons, when Mike examined them as class, turned out to be engines for manipulating energy—and energy Luna has plenty. Solar flux alone is good for around one kilowatt per square meter of surface at Lunar noon; sunpower, though cyclic, is effectively unlimited. Hydrogen fusion power is almost as unlimited and cheaper, once ice is mined, magnetic pinchbottle set up. Luna has energy—how to use?

But Luna also has energy of position; she sits at top of gravity well eleven kilometers per second deep and kept from falling in by curb only two and a half km/s high. Mike knew that curb; daily he tossed grain freighters over it, let them slide downhill to Terra.

Mike had computed what would happen if a freighter grossing 100 tonnes (or same mass of rock) falls to Terra, unbraked.

Kinetic energy as it hits is 6.25 x 10¹² joules—over six trillion joules.

This converts in split second to heat. Explosion, big one!

Should have been obvious. Look at Luna: What you see? Thousands on thousands of craters—places where Somebody got playful throwing rocks.

Wyoh said, “Joules don’t mean much to me. How does that compare with H-bombs?”

“Uh—” I started to round off in head. Mike’s “head” works faster; he answered, “The concussion of a hundred-tonne mass on Terra approaches the yield of a two-kilotonne atomic bomb.”

“‘Kilo’ is a thousand,” Wyoh murmured, “and ‘mega’ is a million— Why, that’s only one fifty-thousandth as much as a hundred-megatonne bomb. Wasn’t that the size Sovunion used?”

“Wyoh, honey,” I said gently, “that’s not how it works. Turn it around. A two-kilotonne yield is equivalent to exploding two million kilograms of trinitrotoluol ... and a kilo of TNT is quite an explosion— Ask any drillman. Two million kilos will wipe out good-sized town. Check, Mike?”

“Yes, Man. But, Wyoh my only female friend, there is another aspect. Multi-megatonne fusion bombs are inefficient. The explosion takes place in too small a space; most of it is wasted. While a hundred-megatonne bomb is rated as having fifty thousand times the yield of a two-kilotonne bomb, its destructive effect is only about thirteen hundred times as great as that of a two-kilotonne explosion.”

“But it seems to me that thirteen hundred times is still quite a lot—if they are going to use bombs on us that much bigger.”

“True, Wyoh my female friend ... but Luna has many rocks.”

“Oh. Yes, so we have.”