<p>Could someone please explain this to me? It's probably really simple but i'm having a brain fart.</p>
<p>A 4-kilogram mass has a speed of 6 meters per second on a horizontal frictionless surface, as shown above. The mass collides head-on and elastically with an identical 4-kilogram mass initially at rest. The second 3-kilogram mass then collides head-on and sticks to a third 4-kilogram mass initially at rest.</p>
<p>The final speed of the first 4-kilogram mass is
a) 0
b) 2
c) 3
d) 4
e) 6</p>
<p>The answer is a.</p>
<p>i dont even think the second sentence is necessary to solve this one.</p>
<p>if the first mass and the second mass are colliding elastically, this means that the Kinetic energy (KE) from the first mass is all transferred to the second mass (because they are identical masses). so the first one looses all kinetic energy, and the second now is at 6 m/s</p>
<p>(4)(6) + 4(0) = (4)(0) + (4)(6)</p>
<p>it's like one of these thingers:</p>
<p><a href="http://physics.ucsc.edu/lecturedemonstrations/images/mechan/P1010035.JPG%5B/url%5D">http://physics.ucsc.edu/lecturedemonstrations/images/mechan/P1010035.JPG</a></p>
<p>the first mass transfers all its KE to the second mass and so on, until the final mass moves.</p>
<p>yea that last sentence of the question is confusing</p>
<p>ohh well there was another part to that question...
but thanks, it makes sense now!</p>
<p>The 2nd sentence is there because there are two questions relating to this situation. The 2nd question asks the speed of the two that stick together, which is 3 m/s of course =P.</p>
<p>I know because I have seen the question before =o.</p>
<p>EDIT: beat me to it =[</p>
