EK 1001 physics #331

[2010premed]

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Why isn't the following an example of work being done?

The moon orbits the earth

The original questions is: In which of the following examples is work done?
A. A man rubs his hands together and they get warm
B. A 50kg girl leans against a wall at a 30degree angle
C. A pot of water sits over a gire until it boils
D. The moon orbits the earth
Answer:A

 

[lorenzomicron]

well, work technically is defined as: w=f*r*costtheta, where f and r are in the same direction, this simplies to f*r. thus, work can only be done by a force is the direction it is in.

in B, there is no r (displacement), so no work is done.

in C, once again, there is no displacement.

in D, there is a displacement of the moon, but this displacement is perpendicular to the force of the earth. this is the definition of uniform circular motion. thus, no work is done.

in A, there force of moving the hand is horizontal and the displacement if horizontal. this makes it perform work. from a themodynamic point of view, also, this work energy is dissipated as heat.

 

[2010premed]

thanks! I sorta forgot that the displacement is perpendicular to the force in UCM never again though!