# Eraser

You could substitute just about any action movie here. Anything starring Arnold Schwarzenegger, Sylvester Stallone, Bruce Willis, Vin Diesel, etc. should do fine. They all have the same cliches and the same bad examples of physics. The two most widely violated physical principles in movies such as “Eraser” are the Law of Conservation of Momentum and the Law of Conservation of Energy. Mind you these are two of the most immutable laws of the universe for us mere mortals, yet somehow they don’t seem to apply in Hollywood action films. (You could add the Law of Conservation of Bullets to this list, too.) One of the most egregious examples of the violiation of the Law of Conservation of Momentum comes across this way: the hero can fire away on his/her gun with practically no recoil whatsoever, yet any bad guy hit by a bullet from said gun will be violently launched off their feet and into whatever structure happens to be behind them (often a glass wall or window). It just can’t happen that way!

Assignment: Analyze in detail any of the scenes where someone fires the rail gun and hits a victim, sending them flying. Your analysis should be centered around the application of the law of conservation of momentum (which we’ll cover in class this week). This will require calculating the momentum of each of the objects involved (shooter, victim, bullet, and gun) and then appropriately applying the law of conservation of momentum. Momentum is equal to the mass of an object times its velocity, so these are the quantities you need to estimate from the movie. Keep in mind that you may need to invoke conservation of momentum more than once to fully analyze the problem. Also remember that objects can exchange momentum with other objects, provided the total momentum remains constant. If you have trouble, help each other out or ask me for more input.

You will do this week’s assignment on paper and turn it in at the start of class on Monday, 11 Sep.

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