A bullet of mass 0.01 kg is fired horizontal into a 4 kg wooden block block at rest , on a horizontal surface. The coefficient of kinetic friction between the block and surface is 0.25 the combination moves 20 m before coming to rest . With what speed did the bullet strike the block ?

Explanation

To solve the problem, we will follow these steps:

### Step 1: Understand the Problem
We have a bullet of mass \( m = 0.01 \, \text{kg} \) that strikes a wooden block of mass \( M = 4 \, \text{kg} \) at rest. The bullet embeds itself in the block, and together they move a distance of \( s = 20 \, \text{m} \) before coming to rest due to friction. The coefficient of kinetic friction \( \mu_k = 0.25 \).

### Step 2: Calculate the Frictional Force
The frictional force \( f_k \) acting on the block and bullet system can be calculated using:
\[
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