Why did the gate before Minas Tirith break so very easily?

Question

Unless I got something wrong, it took only three (3) bashes with the huge door-breaker for the "mighty" gate before Minas Tirith to burst open.

Isn't it strange? Is that really how a real such gate would have behaved? Naturally, a real gate would've been smaller and weaker, but also the door-breaker and who/what is handling it would also be proportionally smaller/weaker in historic reality. So it "evens out" for this question.

Isn't three "bumps" way too few to open any door which is meant to actually stop serious intruders?

I thought they would be bumping over and over for hours and hours until eventually it could no longer resist the ever repeated heavy forces. That it would be a constant topic of discussion among those fighting, fearing that it will happen at any moment. For all that build-up (admittedly while calling it the natural weak spot of the city), it basically seems useless in practice. It surprises me that the Invisible Skeleton King didn't roll in that door-breaker from the very start.

How can such a proud and well-crafted gate give in so easily, as it was apparently not explained in any way?

Answer 1

The ram, Grond, was something more than just a giant tree; it was magically forged specifically for this purpose:

The drums rolled louder. Fires leaped up. Great engines crawled across the field; and in the midst was a huge ram, great as a forest-tree a hundred feet in length, swinging on mighty chains. Long had it been forging in the dark smithies of Mordor, and its hideous head, founded of black steel, was shaped in the likeness of a ravening wolf; on it spells of ruin lay. Grond they named it, in memory of the Hammer of the Underworld of old. Great beasts drew it, Orcs surrounded it, and behind walked mountain-trolls to wield it.

_{The Return of the King, Book V, Chapter 4, "The Siege of Gondor"}

Plus it undoubtedly had magical assistance from the Witch-king himself in breaching the City Gate:

The drums rolled and rattled. With a vast rush Grond was hurled forward by huge hands. It reached the Gate. It swung. A deep boom rumbled through the City like thunder running in the clouds. But the doors of iron and posts of steel withstood the stroke.

Then the Black Captain rose in his stirrups and cried aloud in a dreadful voice, speaking in some forgotten tongue words of power and terror to rend both heart and stone.

Thrice he cried. Thrice the great ram boomed. And suddenly upon the last stroke the Gate of Gondor broke. As if stricken by some blasting spell it burst asunder: there was a flash of searing lightning, and the doors tumbled in riven fragments to the ground.

_ibid

Answer 2

The screenplay for the film version of Return of the King aligns nicely with the description of Grond in the source novel. In brief, this is an exceptionally large and powerful battering ram and far heavier than was envisioned by the makers of the mighty gate.

ANGLE ON: Out of BLACK SWIRLING BATTLE. SMOKE crawls an IRON MONSTER! A MASSIVE WHEELED BATTERING RAM . . . 60 FEET HIGH and 150 FEET LONG . . . pushed by 20 huge MOUNTAIN TROLLS . . . a MASSIVE IRON HEAD, cast in the likeness of a SNARLING WOLF! ... this is GROND ... the BATTERING RAM FROM HELL! The Ore are chanting as the BEAST is wheeled closer and closer to the GATES ...

ORCS: Grond! Grond! Grond!

CLOSE ON: GANDALF . . . blanching at the sight of this crawling IRON MONSTER!

When it hits the gate, it's hitting so hard that the city walls are trembling under the impact.

GROND thuds against the MINAS TIRITH GATE . . . sending a VIBRATION running through the city!
[...]
BOOOMM The GATE SPLINTERS under GROND'S mighty weight . . .
[...]
SUDDENLY the FEARSOME HEAD protrudes into the MINAS TIRITH!
[...]
Within MOMENTS the GATE is smashed and GIANT CAVE TROLLS enter into the FIRST CIRCLE of MINAS TIRITH, under the archway that no enemy had ever passed!