Block A in the figure has mass 1.00 kg, and block B has mass 3.00 kg. The blocks

Question

Block A in the figure has mass 1.00 kg, and block B has mass 3.00 kg. The blocks are forced together, compressing a spring between them; then the system is released from rest on a level, frictionless surface. The spring, which has negligible mass, is not fastened to either block and drops to the surface after it has expanded. Block B acquires a speed of 1.60 m/s.

http://session.masteringphysics.com/problemAsset/1037813/5/YF-08-35.jpg


A- What is the final speed of block A ?

B- How much potential energy was stored in the compressed spring?

Explanation / Answer

Momentum is conserved;

m1v1 = m2v2

1 x v = 3 x 1.6

(a) v1 = 4.8 m/s

(b) Energy is also conserved.

0.5m1v12 + 0.5m2v22 = PE of spring

PE of spring = (0.5)(1)(4.8^2) + (0.5)(3*1.6^2) = 15.36 J

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