Posts Tagged ‘Object at Rest’

January 6, 2014 · by physics · Collisions, Physics Test Questions

A 20-kg object sitting at rest is struck elastically in a head-on collision with a 10-kg object initially moving at +3.0 m/s. Find the final velocity of the 20-kg object after the collision.

a. -1.0 m/s
b. -2.0 m/s
c. +1.5 m/s
d. +2.0 m/s

Raw Video Transcript

Okay this is Tom from every step physics dot com we’re gonna work an actual physics problem here problem number 8 on my sheet which I’ll shall show you and let’s do it get to my menu you know it’s a collision or momentum problem..ah because the words and so you know scroll down here to collision
and this decide which from other words in the problem with what they’re talking about in this case number eight they’re talking about a head-on collision so we’re gonna choose 2 which is collides and we’re going to do head on which is 7 I show you the formula for the final velocity of mass 1 and mass 2 final velocity this is the actual formula you can see CR here whic is the coefficient of restitution that’s the bounce ability of a substance and sometimes they give you that to enter into the problem or not and if they do then you’re in good shape here but they find this out by experimentation and the lists of substances about what they’re coefficient of restitution is for instance a super ball which is the most bouncy thing on earth is .78 coefficient of restitution doesn’t come up to one because if it came up to one that would mean that if you drop it from a hundred feet it would bounce right back up in your hand which gravity won’t let it do it so there is no possibility of one clay as an example the coefficient of restitution of zero and that defines whether it’s a perfect elastic collision if it has a coefficient of one or a perfectly inelastic collision which is sticking to mass is sticking and not bouncing off at all so let’s continue here um here’s a note here, if an object is at rest that’s the m2 mass to enter here so we have to press alpha first gonna press alpha and put in the mass that’s not at rest which is 10 you choose kilograms that’s what it says the second mass alpha twenty also kilograms initial velocity is coming at it at what does it say 3 meters per second we choose number three here and it’s going in the easterly direction or the plus X axis direction choose that and we have the initial velocity of the the mass 2 which is at rest if its at rest we enter 0 Alpha 0 and coefficient of restitution if it’s not given just enter one alpha 1 I show you what you’ve entered you can change it if you want looks pretty good to me were going to say okay and I give you both final velocities of both because they’re both gonna bounce off somewhat go somewhere and so the velocity final velocity mass 1 was minus point one notices its going off to the left there with the minus sign after they hit and here’s the final velocity mass 2 and 2 is positive so it’s going off to the right two meters per second so that’s the calculations notice on the test they do have plus 2 is meters per second and so you circle that one and get a hundred percent on it pretty neat huh..every step physics dot com and because you know its a collision problem our momentum problem you’re going to scroll down to a collision if you didn’t know it was that