Sample Problem 9-7
A firecracker placed inside a coconut of mass 
, initially at rest on a frictionless floor, blows the coconut into three pieces that slide across the floor. An overhead view is shown in Fig. 9-10a. Piece C, with mass 0.30 , has final speed 
= 5.0 m/s.
(a) What is the speed of piece B, with mass 0.20 ?
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SOLUTION: A Key Idea here is to see whether linear momentum is conserved. We note that (1) the coconut and its pieces form a closed system, (2) the explosion forces are internal to that system, and (3) no net external force acts on the system. Therefore, the linear momentum of the system is conserved.
To get started, we superimpose an xy coordinate system as shown in Fig. 9-l0b, with the negative direction of the x axis coinciding with the direction of 
. The x axis is at 80° with the 
and 50° with the direction of 
.
A second Key Idea is that linear momentum is conserved separately along -x and у axes. Let's use the у axis and write
where subscript i refers to the initial value (before the explosion), and subscript у refers to the у component of 
or 
.
The component 
of the initial linear momentum is zero, because the coconut is initially at rest. To get an expression for 
, we find the у component of the final linear momentum of each
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