Physics 221
posted by K .
A 20g particle is moving to the left at 30 m/s. How much net work must be done on the particle to cause it to move to the right at 30 m/s.
The answer is 0J of work, and i understand the method of equations, but i don't understand the logic behind it.
Thanks for any help.

You are not changing the kinetic energy and you do not have to change the potential energy to change direction. Therefore no work is required, if the reversal is done in a frictionless manner.
You could let its direction be reversed by a spring, or by a Ushaped skateboard ramp, for example. 
,jguyk
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