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Unformatted text preview: Applying the momentum principle 1. Chose your system, everything else belongs in the surroundings. 2. Draw a force diagram 1. All forces must be do to interactions with surroundings 3. Chose your time interval 1. Given or estimated 4. Substitute known values into the equation and solve for the unknowns 5. Check units and reasonableness of answer Physics 2211: Matter and Interactions Dr. Ed Greco Example: The colliding students Miles and Mary are going to be late for class and break into a full run so they don't miss any of Dr. Greco's lecture. They turn a corner at the same time and suddenly collide; coming to a complete stop. What force did they exert on each other? p Miles p Mary Physics 2211: Matter and Interactions Dr. Ed Greco Example: Cont. (1) Choose Miles as our System (2) Draw a force diagram on Miles (3) Initial time is just before collision and final time is when Miles has zero velocity. (4) Apply momentum principle (1) Assume that the motion takes place all in the xhat direction (2) Assume friction is negligible Physics 2211: Matter and Interactions Dr. Ed Greco F coll f air F g F ground f ground Example: Cont. F coll =− mv i t x To get a number we have to make some estimations: Mass equals about 150 pounds or 68 kg Velocity is about 5 m/s (half an Olympic sprinter) Time is harder to estimate.... Physics 2211: Matter and Interactions Dr. Ed Greco p f = p i F net t = mv i x F coll t Example: Cont. What can we say about time? Only that the collision was fast (less than a second) If we assume that during the collision the bodies of Miles and Mary only compress a few cm, we can use the definition of the average velocity to back out a good time estimate. t = x v avg t = 0.025 m 5 m / s 2 = 0.01 s Physics 2211: Matter and Interactions Dr. Ed Greco Example: Cont. Substituting all of these values into the momentum equation gives us an estimate for the collision force F coll =− 68 kg × 5 m / s 0.01 s x =− 34,000 x N This simple estimate tells us a lot about collisions They are very large (about 50 times larger than Miles's weight) They happen over short times for reasonably hard objects (like the heads of undergraduates) A simple model can give us much of the physics of a problem & tells us where to refine our model Physics 2211: Matter and Interactions Dr. Ed Greco PRS: One student exerts a force of magnitude F on the other student. Suppose we choose BOTH students as the “system” to which to apply the momentum principle. What is the net force acting on this system?...
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 Fall '08
 UZER
 Physics, Force, Mass, Momentum, momentum principle, Dr. Ed Greco

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