E The total energy of the system does not change No heat goes out the exhaust

# E the total energy of the system does not change no

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E. The total energy of the system does not change. No, heat goes out the exhaust pipe (from burning fuel)…. Clicker Question Discussion

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The force of friction from the road on the tires is M a cm . The center of mass of this system is accelerating. Hence, M a cm must be equal to the sum of all external forces. Friction is pushing the car! (Does friction always oppose the motion?) F uv net , ext =M total r a cm = d r P total dt Further Discussion
F uv net , ext =M total r a cm = d r P total dt This result says that the center of mass of a complex system moves as if all forces were applied to the entire mass at that point. Throw a baseball, and you see parabolic motion. What happens if you throw a baton? 1D-11 Trajectory of the Center of Mass Throw a Baton

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Which puck will move faster? Both pucks move at same speed! HOWEVER: Hand #2 has to pull the string farther: W 2 > W 1 . Where does this energy go? Rotational energy. The bottom spool is spinning. d r P tot dt ! M r a cm = r F net , surr Center of Mass Motion - Review
Point Particle vs Real Systems When we treat a real system as a point particle system: •The net external force acts on the Center of Mass •The change in the point particle’s translational kinetic energy is equal to the work done by the net external force in moving the CM through a displacement Δ r, ie, ! K trans = r F ext " d r r cm i f # •The point particle system has no internal degrees of freedom. Therefore it cannot undergo a change of its internal energy - of any sort!

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Point Particle vs Real Systems In contrast, a real multiparticle system has internal degrees of freedom that may undergo a change of energy when the system is subjected to exertnal forces. The net external force does Work on the system given by: W ext = r F ext ! d r r i f " Note that the displacement in the above expression is not that of the CM but that through which the net force acts. If an external force does not act through a displacement, it does no work on the real system! (Friction, for example.)

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