Posts Tagged ‘Acceleration then Stop’

January 13, 2014 · by physics · Acceleration, Collisions, Momentum

Popeye, of mass 70kg has just downed a can of spinach. He accelerates quickly and stops Bluto, of mass 700kg (Bluto is very dense), who is charging in at 10 m/s. What was Popeye’s speed?

a. 10 m/s
b. 31 m/s
c. 50 m/s
d. 100 m/s

Raw Transcript

Hello this is Tom from every step physics dot com going to an actual physics problem from a test final
and I’m gonna do number nine here on this sheet, I’ll show you the sheet let’s do it I’m going to scroll down to collisions and this next one in the problem it says that he stops Bluto so that word is a key one whether it’s a momentum or collision problem. Now this is a special collision problem because when it says stop that means that both of the well after they hit they stick together and stop so that means that
the final velocities are 0 for both of them so it’s a special momentum problem maybe a trick problem
a lot of trick problems in physics as if physics wasn’t hard enough and we’re gonna find what are we gonna find we’re gonna find Popeyes initial…um velocity so we’re going to choose Vi1…we’re going to enter the mass Alpha is 0:00 kilograms, you have to press alpha to enter anything into my entry lines here and Bluto is 700 kilograms initial velocity of Bluto is 10…Bluto is 10 meters per second oops, press Alpha first and he’s going to the left, Popeyes going to the right so we choose -x axes and you can see if it’s correct, which it is, you can change it if you you wanted and so the momentum of Pluto is mass times velocity which is his mass is 700 plus that which is minus 7000 kilograms meters meters per second and since one of the laws of physics is that p1 plus p2 equals zero, so we enter that in the equation and then we solve for p1 with becomes a positive 7,000 that’s what the momentum of Popeye is and so we do the calculations here we come up with 100 meters per second for Popeye which is on the test you just circle that one and get a hundred percent on problems like that, pretty neat huh? every step physics dot com.