183_notes:pp_vs_real

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183_notes:pp_vs_real [2021/05/08 19:09] – [Energy in the Real System] stumptyl183_notes:pp_vs_real [2021/06/02 22:36] (current) – [Energy in the Point Particle System] stumptyl
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 These deformable systems can be quite complicated because these systems have some structure and size, they are not [[183_notes:point_particle|point objects]]. **External forces can be applied to different locations and the resulting motion can produce vibrations, rotations, and temperature changes, as well as translations.** However, you can simplify the system significantly by modeling it as a point particle located at its [[183_notes:center_of_mass|center of mass]]. Doing so, allows you to determine the translation kinetic energy change of the system, but nothing else. A [[183_notes:point_particle|point particle has no internal structure]], so it cannot vibrate, rotate, or experience any temperature change. This is because the forces acting on the different parts of the system add to produce the net force, which (in this model) acts at the center of mass. **The work done by this force over the displacement of the center of mass precisely determines the translation kinetic energy of the system.** These deformable systems can be quite complicated because these systems have some structure and size, they are not [[183_notes:point_particle|point objects]]. **External forces can be applied to different locations and the resulting motion can produce vibrations, rotations, and temperature changes, as well as translations.** However, you can simplify the system significantly by modeling it as a point particle located at its [[183_notes:center_of_mass|center of mass]]. Doing so, allows you to determine the translation kinetic energy change of the system, but nothing else. A [[183_notes:point_particle|point particle has no internal structure]], so it cannot vibrate, rotate, or experience any temperature change. This is because the forces acting on the different parts of the system add to produce the net force, which (in this model) acts at the center of mass. **The work done by this force over the displacement of the center of mass precisely determines the translation kinetic energy of the system.**
  
-[{{ 183_notes:pp_vs_real.007.png?450|Modeling a deformable, extended system as a point particle means tracking the motion of the center of mass.}}]+[{{ 183_notes:week11_realsystem.png?450|Modeling a deformable, extended system as a point particle means tracking the motion of the center of mass.}}]
  
  
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 In the point particle system, you need information about the net force and the center of mass; the point of application of the net force is the center of mass and it acts through the displacement of the center of mass. From the point particle system, you gain information about the translational kinetic energy of the system only. In the point particle system, you need information about the net force and the center of mass; the point of application of the net force is the center of mass and it acts through the displacement of the center of mass. From the point particle system, you gain information about the translational kinetic energy of the system only.
  
-==== An application ====+===== An Application =====
  
-This might all seem abstract right now, but consider two pucks with same mass ($m$) that are pulled across an icy surface. Puck 1 has the rope attached to its center and Puck 2 has the rope wound about its edge (see below).+This might all seem abstract right now, but consider two pucks with the same mass ($m$) that are pulled across an icy surface. Puck 1 has the rope attached to its center and Puck 2 has the rope wound about its edge (see below).
  
 {{ 183_notes:hand_puck.png }} {{ 183_notes:hand_puck.png }}
  
-=== Both pucks travel the same distance ===+==== Both Pucks Travel The Same Distance ====
  
 Both pucks start from rest and are pulled with the same force ($F_T$) over the same amount of time. But, Puck 2 will rotate as the rope is unwound from the edge of the puck. Because both pucks experience the same force over the same time, both will have the same final momentum, Both pucks start from rest and are pulled with the same force ($F_T$) over the same amount of time. But, Puck 2 will rotate as the rope is unwound from the edge of the puck. Because both pucks experience the same force over the same time, both will have the same final momentum,
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 $$\Delta K_{trans} = \dfrac{1}{2}m\,v_{cm}^2 = W_{cm} = F_T\,d$$ $$\Delta K_{trans} = \dfrac{1}{2}m\,v_{cm}^2 = W_{cm} = F_T\,d$$
  
-=== Where does the rotational energy come from? ===+==== Where does the rotational energy come from? ====
  
 But why does Puck 2 rotate? You know there's [[183_notes:rot_ke|energy associated with rotation]]. If both pucks experience the same work in the point particle system, why does Puck 2 rotate? But why does Puck 2 rotate? You know there's [[183_notes:rot_ke|energy associated with rotation]]. If both pucks experience the same work in the point particle system, why does Puck 2 rotate?
  
-The point particle system can only describe the translation kinetic energy of a system. For Puck 1, the point particle and real system are exactly the same. Puck 1 only has translational kinetic energy.+The point particle system can only describe the translation kinetic energy of a system. For Puck 1, the point particle and real system are exactly the same. **Puck 1 only has translational kinetic energy.**
  
-But Puck 2 has rotational kinetic energy also. So, you must analyze the energy in the real system. For now, let's assume there's only changes in kinetic energy (translation and rotation) and no thermal energy changes (and no Q).+**But Puck 2 has rotational kinetic energy also.** So, you must analyze the energy in the real system. For now, let's assume there's only changes in kinetic energy (translation and rotation) and no thermal energy changes (and no Q).
  
 $$\Delta K_{tot} = \Delta K_{trans} + \Delta K_{rot} = W_{surr}$$ $$\Delta K_{tot} = \Delta K_{trans} + \Delta K_{rot} = W_{surr}$$
  • 183_notes/pp_vs_real.1620500957.txt.gz
  • Last modified: 2021/05/08 19:09
  • by stumptyl