Did i do this Kinetic Energy lost Collision Problem Right?

[Lori]

Homework Statement

Homework Equations

Total Ke initial - Total Ke final = lost KE
P= m1v1 + m2v2

The Attempt at a Solution

Initial P : sqrt((0.08*50)^2 + (0.06+50)^2)
Final P : (m1+m2)vf = 140vf
Initial P = Final P and solve for vf , vf = 0.0357

KE initially : .5(0.080)(50^2) + .5(0.060)(50^2)
Ke final: .5(140)(0.0357)^2

Lost KE = KE initial - KE final = ~174964 KJ = 175 J

 

[Charles Link]

(Editing: This first paragraph you can skip)..This one is somewhat complicated because their initial momentums are each at right angles to each other. You need to first compute the complete vector (having an $x$ and $y$ component), for the combined initial momentum, and also compute the amplitude of that vector. You then need to set the initial momentum equal to the final momentum,(because of conservation of momentum), from which you can determine a final velocity vector for the objects that are stuck together. After that, you can work on finding any changes in kinetic energy. $\\$ For starters, can you write out the initial total momentum vector $\vec{p}$? $\\$ Editing: I see you did some of that already=it was a little hard to read your solution=let me have a second look at it... $\\$ I spotted one error: Your final $m_{total}=.14$ kg, (not 140 because you need to stay in M.K.S.) $v_f$ will thereby be much larger. (Instinctively, $v_f$ should be in the 50 m/s range=maybe 35 m/sec, but somewhere in that ballpark). In writing out the final momentum, and also the final kinetic energy, you incorrectly used $m=140$ in both cases. $\\$ Additional item: Please show your computed results for $K.E._{initial}$ and $K.E._{final}$. It will help in checking the arithmetic.

 

[Lori]

(Editing: This first paragraph you can skip)..This one is somewhat complicated because their initial momentums are each at right angles to each other. You need to first compute the complete vector (having an $x$ and $y$ component), for the combined initial momentum, and also compute the amplitude of that vector. You then need to set the initial momentum equal to the final momentum,(because of conservation of momentum), from which you can determine a final velocity vector for the objects that are stuck together. After that, you can work on finding any changes in kinetic energy. $\\$ For starters, can you write out the initial total momentum vector $\vec{p}$? $\\$ Editing: I see you did some of that already=it was a little hard to read your solution=let me have a second look at it... $\\$ I spotted one error: Your final $m_{total}=.14$ kg, (not 140 because you need to stay in M.K.S.) $v_f$ will thereby be much larger. (Instinctively, $v_f$ should be in the 50 m/s range=maybe 35 m/sec, but somewhere in that ballpark). In writing out the final momentum, and also the final kinetic energy, you incorrectly used $m=140$ in both cases. $\\$ Additional item: Please show your computed results for $K.E._{initial}$ and $K.E._{final}$. It will help in checking the arithmetic.

Thank you. I always forget that they give us the mass in grams. I did the problem again and calculated a new vf. So the lost in KE is 85.7. I used .140 grams instead of 140 grams and got vf= 35.7