A shot travelling at the rate of `100m//s` is just able to pierce a plant 4 cm thick. What velocity is required to just pierce a plank 9cm thick ?

Explanation

To solve the problem of determining the velocity required to pierce a 9 cm thick plank, given that a shot traveling at 100 m/s can pierce a 4 cm thick plank, we can follow these steps:

### Step 1: Understand the Problem
We know that a shot with an initial velocity \( U_1 = 100 \, \text{m/s} \) can pierce through a plank of thickness \( S_1 = 4 \, \text{cm} \) (which is \( 0.04 \, \text{m} \)). We need to find the velocity \( U_2 \) required to pierce a thicker plank of thickness \( S_2 = 9 \, \text{cm} \) (which is \( 0.09 \, \text{m} \)).

### Step 2: Apply the Work-Energy Principle
According to the work-energy theorem, the change in kinetic energy is equal to the work done against the resistance force of the plank.

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