HW 7 Tips FINAL

# HW 7 Tips FINAL - HW 7 Tips PHY 131 Sect 10 11 15 Fall 2010...

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1 HW 7 Tips; PHY 131 Sect. 10, 11, 15; Fall 2010 Prof. Pascuzzi Information you may need; m/s 2 Two-body p -conservation for collisions; For several 2-body HW 7 problems, you must realize that for elastic collisions ( KE and p are both conserved), you must solve for the final impact speeds of each of two objects ( and ). Do this as follows; p- conservation; Equation 1 KE conservation; Equation 2

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2 Notice in Equation 2 that the term is a FOIL of the binomials and . Thus, Equation 2 can be rewritten as follows; New Equation 2 The trick is to now divide the New Equation 2 by Equation 1 so that all one simpler equation can be found which will help determine the final speeds of A and B ; New Eq. 2 divided by Eq. 1; Equation 3 Nearly done! Now, the next trick is to substitute Equation 3 back into Equation 1 to first eliminate so you can obtain the equation for . After that, rearrange Equation 3, solving it for so that you can then insert it back into Equation 1 once again, but this time to eliminate to solve for . Once you’ve done this, you will now have the two velocities of both objects right after they collide, again, assuming a perfectly elastic collision. Important steps in solving Momentum Conservation problems; 1) Just like energy conservation, momentum conservation means that the total momentum of the closed system remains unchanged, and is the same numerical value at all points in the problem. As shown above, write it out as the total momentum before the collision or explosion equals the total momentum after the collision or explosion. 2) Get into the habit of immediately writing down the law of p -conservation before and after the collision/explosion, which will eventually look like this; This is often for an explosion too…
3 3) Next, decide if this collision/explosion is elastic or inelastic. If it is elastic, you must also apply kinetic energy conservation (see full description above) to determine the final (new) speeds of each object in the collision (most common is the two-body problem). 4) Solve for any unknown/requested variable in the problem. First Problem “One Dimensional Inelastic Collision”

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HW 7 Tips FINAL - HW 7 Tips PHY 131 Sect 10 11 15 Fall 2010...

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