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--- 183_notes:system_choice [2014/10/29 22:06] – pwirving
+++ 183_notes:system_choice [2021/04/15 17:19] (current) – [Lifting a box] stumptyl
@@ Line 1: / Line 1: @@
+Section 7.7, 7.8 and 7.9 in Matter and Interactions (4th edition)
 ===== Choosing a System Matters =====
-Earlier, [[183_notes:energy_cons#multi-particle_systems|you read about a puzzle involving a ball falling to the Earth]]. We resolved this puzzle by introducing [[183_notes:potential_energy|the concept of potential energy]]. In these notes, you will look an example of a person lifting a box using different choices of system. The goal here is to demonstrate that the choice of a system matters.
+Earlier, [[183_notes:energy_cons#multi-particle_systems|you read about a puzzle involving a ball falling to the Earth]]. We resolved this puzzle by introducing [[183_notes:potential_energy|the concept of potential energy]]. **In these notes, you will look an example of a person lifting a box using different choices of system**. The goal here is to demonstrate that the choice of a system matters.
-==== Lifting a box ===
+===== Lifting a box ====
-A person is lifting a box (of mass  $m$ ) from the ground to a height  $h$ . At that height, the box is moving with an upward speed  $v$ . To lift the box, we assume the person applies a force  $F$  upward. Let's consider three different choices of system to demonstrate what can be learned from each choice.
+A person is lifting a box (of mass  $m$ ) from the ground to a height  $h$ . At that height, the box is moving with an upward speed  $v$ . To lift the box, we assume the person applies a force  $F$  upward. Let's consider three different choices of the system to demonstrate what can be learned from each choice.
-=== System 1: Box, Earth, and Person ===
+==== System 1: Box, Earth, and Person ====
 While lifting the box, the person's internal energy will change, so we will keep track of that change with  $E_{person}$ .
   * System: Box, Earth, and Person; Surroundings: Nothing
-  * Initial State: Box on ground with no velocity; Box at height  $h$  with velocity  $v$
+  * Initial State: Box on the ground with no velocity; Box at height  $h$  with velocity  $v$
  $\Delta E_{sys} = W_{surr}$
@@ Line 22: / Line 24: @@
 Notice that in this system, the person does no work on the box because the box and person are in the system. Instead, we are accounting for the change in energy of the person.
+\\
-=== System 2: Box only ===
+==== System 2: Box only ====
 Because the system is only a single particle, there can be no potential energy. There is work done by both the person and the Earth, but they have opposite signs because the applied force and the gravitational force are in different directions.
@@ Line 39: / Line 41: @@
 Notice also that because the person is outside the system, we no longer consider the internal energy change of the person, but the work the person does on the box while lifting it.
+\\
-=== System 3: Box and Earth ===
+==== System 3: Box and Earth ====
 Finally, this system has potential energy, but there is work done by the person.