  # Help with physics

<p>Okay, suppose you drop an egg from the ceiling to the "air bag" on the floor. </p>

<p>1) How does the air bag converts energy in order to stop the egg? </p>

<p>2) How do you find the kinetic energy and the final velocity of the egg? The mass of the egg is .06 g and the height is 2 m.</p>

<p>Thanks!</p>

<p>ans 2) according to the law of conservation of energy, energy can neither be created nor destroyed. it can only be converted from one from to another.</p>

<p>When the egg is at the ceiling it has potential energy. when it is dropped, and almost near the ground...all the PE converts to Kinetic energy</p>

<p>Now</p>

<p>Potential energy=kinetic energy
mgh= 1/2mv2 (half) (m)(v square)</p>

<p>put the values and get the answer
:)</p>

<p>g=10m/s2</p>

<p>thank you so much!</p>

<p>just remember to convert the mass into kilograms....(thats the SI unit)</p>

<p>yeah that's right.</p>

<p>i'm still stuck on the first question. How does the air bag convert energy to stop the egg? It absorbs kinetic energy from the egg, helping control the velocity at which the egg may tend to be dropped from 2 meters, and thus lessening the severity of any damages?</p>

<p>When the egg hits the bag, it causes parts of the bag to move, it may stretch or deform the material that makes up the bag, it compresses the air inside the bag (thus heating it up a little), and it probably also produces some sound. All of these things take energy.</p>

<p>What? Hairpotty: an air bag is all about conservation of momentum. Just the way that if you jump down from ten feet on concrete you break your legs and nothing happens when you jump on a trampoline, the longer your collision time is, the less force the surface exerts.
The equation is force * time = mass * velocity.
(Impulse equals change in momentum.)</p>

<p>As far as energy goes, you need potential energy = kinetic energy to find the velocity for your momentum value.
Chances are that you're starting at height h and landing at h=0, so
mass * g * height = (1/2) mass * velocity ^2, then you use that velocity in the impulse equation.</p>

<p>The airbag spreads the force of the impact not only in time but also in space. The force applied to the egg by the collision occurs over far more of the surface area of the egg when an airbag is involved rather than a flat, hard surface like a floor. Cracking eggs is all about sheer. Eggs can withstand a fair amount of pressure applied uniformly provided there is not a pre-existing weak spot, but a relatively small pressure gradient will cause a crack.</p>

<p>BassDad: That sounds more like chemistry... I don't think first year physics deals with the nature of materials. You're right, of course, but I think that Hairypotter's problem is about simpler ideas.</p>

<p>Actually, I wasn't trying to help him solve the problem at that point. He already had what he needed to do that. I was referring to his post #6 wherein he asks whether "controlling the velocity" lessens the damage to the egg. I think he meant reducing the amount of acceleration (hence the force, hence the pressure) involved in slowing the egg from its maximum velocity (just before striking the airbag) to zero. This is very much along the lines of what you were getting at with the trampoline.</p>

<p>I think the increase in area over which the force is applied is probably the more important reason the egg is less likely to crack. You have probably seen the experiment where you cradle an egg in your hands then start squeezing it. As long as the egg is perfectly sound and you squeeze it evenly, you can exert a lot of pressure without crushing it. Given a tiny crack in the shell, you will wind up with messy hands in short order.</p>

<p>Strength of materials is indeed one of the areas where chemistry and physics overlap. As you point out, it is unlikely to be something that a first year physics students would see much of. I just thought it might be an interesting observation since the problem involved eggs rather than ball bearings or bowling balls.</p>

<p>Got it. It's amazing how much one little egg can do when enough science nerds analyze it, isn't it? :)</p>