If the force of the egg striking the floor orground is greater than the force that the eggshell can exert on itself to keep its shape, theegg will break. Some eggs are stronger thanothers, depending on how thick the shell is.Certain chemicals, such as the pesticide DDT, areknown to cause egg shells to be thinner and easierto break. I don"t know what is "normal" fordifferent species of birds.
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The ground pushes on the egg and breaks itsshell. It"s like when you hit an egg against aplate or something and its shell breaks from theforce and work of the hitting. Students of allages do experiments where they try to protect anegg so it won"t break when they drop it. Thephysics students at my university drop their eggsfrom the 6th floor of the physics building.
An egg, and anything else for that matter, willbreak ("fail") when the force applied is greaterthan that required to cause a loss ofload-bearing capacity. Often thiscorresponds to the formation and growth of cracks.(Hopefully this is intuitive - once the materialseparates into two pieces (cracks), load cannot betransferred from one to the other.) Eggshells failin a brittle manner (more or less), so the forceto cause the formation of the crack is larger thanthe force to keep it growing: once a crackstarts, it won"t stop even if the load isremoved. (There is much more happening at theatomic level, but I think it is beyond the scopeof this question.) For objects of the samematerial, size matters - a small object can"t holdup the same force as a large one. So to compareloads across size scales, force F isnormalized bythe area it acts on A to give a quantity calledstress, σ=F/A. In these terms, a material(such aseggshell) will break when the stress exceeds thefailure stress (sometimes called strength) of thematerial. Note - it is still force that causesfailure; rewriting it as stress is a way tocompare the relevant forces at different sizes.
The force required to break the egg can beestimated using a bit of physics.
If an egg breaks by dropping it from a heightof 4 ft, the force applied to the egg must begreater than what the eggshell can withstand. Inthis case, the force is from the collision of theegg with the ground. For collisions, forces aregoverned by conservation of a quantity calledmomentum p. The momentum of an object withmass m is given by mass*velocity, m*v. In acollision, the force F on an object of mass m canbe calculated by manipulating Newton"s 2ndLaw, F=m*a, into a slightly different form.Acceleration a is defined as the change invelocity dv that occurs over some timet. So, Newton"s 2nd Law can be rewritten as F=m*(dv/t), or F*t=m*dv. This second form is called theImpulse-Momentum Change equation, becauseF*t is a quantity called impulse, and m*dvis the change in momentum (as follows from m*vbeing momentum). To reach a specific number for F,values for each of these terms need to be given.The mass of an egg m is around 50 g (assuming amedium-large egg in the USA), and the time of thecollision t must be estimated, which I"ll take as0.01 seconds. The change in velocity dv is thedifference between the velocities before and afterimpact.
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After is easy - 0 m/s. The velocity mustbe estimated, which can be done with more physics.
When you drop the egg, it accelerates due togravity. This means that the force ofgravity pulling on the egg will cause the egg togo faster. This means that the eggwill gain momentum (which is a fancy wordfor speed) as it falls. When the egghits the ground after falling the 4 feet, themomentum very suddenly goes to zerosince the egg stops moving. This causes the egg toexperience a very large forcefrom the ground. This large force causes thedelicate egg shell to break. Whydoes the sudden change in momentum cause a largeforce? Well momentum andforce are related by a quantity that physicistscall impulse!
Basically if somethingexperiences a very sudden change in momentum, thatcorresponds to a largeforce. If something experiences a gradual changein momentum, the force issmaller. Think of how if you are in a car and itstops quickly you might feel likethere is a big force pulling on your seat belt. Onthe other hand, if the car stopsslowly you might not feel too much. It is the sameidea!