So basically I have a homework question that says:
Block 1 (m1, Vo) hits block 2 (m2, Vf) at rest. Block 1 rebounds at Vf1, and block 2 moves forward at a speed not given.
So m1, Vo, m2, and Vf1 are given, but Vf2 must be calculated. My question is how is it that in the end energy is lost?
Final equation: delta KE= (m1•Vf1^2/2+m2•Vf2^2/2)-m1•Vo^2/2
And Vf2=(m1•Vo-m1•Vf)/m2
So if all numbers are calculated why is there energy loss?
In: 0
Most is lost through heat and sound. Anything that produces sound or heat is losing energy as it moves into the environment.
If energy loss is ignored then it’s not. How are you getting that energy loss equation? Is you derive it and plug in numbers and see it’s wrong? What explicitly is the problem asking for. Are you just trying to find vf2 and noticing that the energy is wrong? Because the equation for vf2 is wrong. You can’t just cancel or add/subtract squares like that, it’s non linear. I think it’s that.
Edit: also, collisions in general are momentum problems, not energy problems. Does the problem say to use energy?
There isn’t any energy loss. It’s conserved. There is a *kinetic* energy loss because, in most collisions, there’s heat, sound, and (often) distortion of the colliding objects. The energy changed forms to some that aren’t kinetic, so naturally you’ve got less kinetic energy after than before.
Your second equation (Vf2=…) is just conservation of momentum. This doesn’t looks like it’s an energy question.
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Are we assuming a perfectly elastic collision here? Probably, since it’s not stated otherwise.
Kinetic energy should be conserved in this problem. If you are getting a change in kinetic energy then you must be making an error.
I’m guessing you need to reverse the sign of Vf1. Your book probably provides you with speeds, but velocity is also about direction. If block 1 is rebounding then it should be moving in the negative direction after the collision and Vf1 should be negative.
When we talk about elastic collisions energy is conserved such as momentum: p1_1 + p2_1 = p1_2 + p2_2. So for energy E1_1 + E2_1=E1_2 + E2_2. But in reality energy doesn’t convert with 100% efficiency. Some of it will be lost to heat/light and sound.
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