I gotcha

Split the question up into 3 parts.

** Part 1: **The dart is going through the tube.

**Part 2:** The dart is moving and collides with the block.

**Part 3:** The dart and block system move together

**For part 1:**
Recall the formula of Work done and write it out for this question which is trivial:

**Part 2:**
For this part remember it is an

**inelastic** collision between the dart and the block which means

** momentum is conserved but energy isn't**. Using this you can find out an expression for the start/initial velocity of the dart & block system as it begins to move.

**Part3:**
Now for this part you get an expression for the friction force being:

Now again recall the formula for Work done and

**find an expression for D in terms of m,v,u_k and g.**
Finally take a look at

**part 1** again and

**substitute appropriately into the expression you obtain in part 3 for D** and that gives you the final answer!