Question

A 5.0 kg block with a speed of 2.9 m/s collides with a 10 kg block that has a speed of 2.1 m/s in the same direction. After the collision, the 10 kg block is observed to be traveling in the original direction with a speed of 2.6 m/s.

Suppose, instead, that the 10 kg block ends up with a speed of 4.0 m/s. What then is the change in the total kinetic energy?

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I used m1v1 + m2v2 = m1v1f + m2v2f
to find v1, which I found to be -.9

Then I used 1/2mv^2 in place of each m1v1 term and found the change in KE to be (80-2.025 = 77.975) = (21.025+24.2 = 45.225), or 32.75J, which is incorrect.

Answer 1

huh?

.5*5 *2.9^2+.5*10*2.1^2 = 43.1 Joules

.5*5 *0.9^2 + .5*10* 4^2 = 81.1

81.1-43.1 = 38 Joules more

Something exploded :)

Answer 2

It seems 38J is incorrect too. Thanks though :)

Answer 3

Pay attention to PrfDamon's note: something exploded, which means, energy was added to the system during the collision.

Answer 4

In other words unless there was energy input from a explosion on impact or something, no way the big block took off at 4.0 m/s