Suppose that instead of the two moving blocks in Figure \(9.3 a\), just one block is used to compress the spring while the other end of the spring is held against a wall.

(a) Is the system comprising the block and the spring closed?

(b) When the system is defined as being only the spring, is the work done by the block on the spring positive, negative, or zero? How can you tell?

(c) Is the work done by the wall on the spring positive, negative, or zero?

Transcribed Image Text:

Figure 9.3 Only forces that are external to a system can do work on the system and change the system's energy. System spring+ blocks Forces are internal to system, so they do no work on system, and system's energy does not change. System = spring Forces are external to system, so work they do on system changes system's energy. (a) Moving blocks compress spring (b) Spring expands, pushing blocks apart Fix Fi 2 I 2 F F E F 1 Force and force displacement in same direction: work done on system is positive, system's energy increases. Force and force displacement in opposite directions: work done on system is negative. system's energy decreases.