# Eli5: when two objects colide what decides which breaks?

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Like if I threw an egg at a wall the egg would break, but if the egg is going really fast the wall would break too? Whats the underlying mechanic for this?

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The amount of force involved. The egg needs 1kN of force applied to the shell to break and the wall needs 100kN. Force from the impact is proportional to the speed you throw at and the mass of the thrown object.

Edit: after the egg shatters the force it exerts on the wall quickly drops because it isn’t a single solid object anymore, so the calculations get a bit harder than just speed and mass. But that’s the general idea.

(All the values just estimated without any calculations, actual numbers are probably very different).

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Given that in an impact both objects experience the same force, what decides which object breaks is the hardness of the material. Softer materials, ergo, the egg, will break when hitting stronger surfaces. Obviously, the force of impact adds to it, but very little.

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F=ma. Force = Mass x acceleration. You can make a lot of force with a small amount of mass by increasing the acceleration by a lot, granted the lighter something is the harder it is to actually throw it hard but it’s not impossible. But this is why a relatively soft bullet can damage a much stronger piece of metal.