**Goal:** Problem solving

**Source:** UMPERG-ctqpe88

Two blocks, M=2m, sit on a horizontal frictionless surface with a

compressed massless spring between them. After the spring is released

M has velocity v. The total energy initially stored in the spring was:

- mv
^{2} - 2mv
^{2} - 3mv
^{2} - 4mv
^{2} - 5mv
^{2} - None of the above.
- Cannot be determined

## Commentary:

## Answer

(3) The big mass has kinetic energy mv

^{2}and the small masshas energy 2mv

^{2}. Some students may answer (6) because theyhave confused M and m. It is important to determine the reasons that

any student might select (7). They might be unwilling to assume that

the system is initially at rest. Students taking this perspective

should not be disconfirmed but congratulated for making a critical

interpretation of the wording.