Homework Help: physics

Posted by Sarah on Thursday, March 29, 2012 at 5:57pm.

A bullet of mass 12.4 g is fired into an initially stationary block and comes to rest in the block. The block, of mass 1.02 kg, is subject to no horizontal external forces during the collision with the bullet. After the collision, the block is observed to move at a speed of 4.80 m/s.
(a) Find the initial speed of the bullet.
(b) How much kinetic energy is lost?

physics - Elena, Thursday, March 29, 2012 at 6:24pm
The law of conservation of linear momentum for inelastic collision
m•v =(m +M) •u
v =(m +M) •u/m =(0.0124+1.02) •4.8/0.0124 = 399.6 m/s
KE1 =m•v^2/2 =0.0124•(399.6)^2/2 =990 J
KE2 = (m+M) •u^2/2=11.9 J
KE1 –KE2 =990 -11.9 = 978.1J

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