Question

A bullet traveling at 100 meter per second just Pierce's a Target 4cm think. What is the velocity required so that it can just Pierce's a Target 9cm thick? Please help!

Answer 1

Use the formula v²-u²=2aS.
The final velocity is 0 and when that happens the equation reduces to -u²=2aS
Now for this case: Given initial velocity u=10m/s and it is lost after travelling S=1 planks unit.
So with initial velocity u=u m/s which is lost after travelling S=10 planks unit. 
So dividing the above two conditions: 10²/u² = 1/10 or u = 10√10 m/s