Homework Help: Physics

Posted by Hadeel on Thursday, April 7, 2011 at 1:14am.

A 65kg boy and his 40kg sister, both wearing roller blades, face each other at rest. The girl pushes the boy hard, sending him backwards with a velocity of 2.9m/s towards the west. Ignore friction. What is the minimum amount of chemical energy is converted into mechanical energy in the girl’s muscles?

Physics - drwls, Thursday, April 7, 2011 at 3:05am
It is easy to calculate how much kinetic energy must be given to both brother and sister. The girl acquires an equal momentum in the opposite direction, with velocity 2.9*64/40 = 4.64 m/s.

Calculate and add the kinetic energies of both brother and sister.

Chemical (food) energy cannot be converted to mechanical energy with 100% efficiency. The efficiency varies from person to person, but is around 20%. They may want you to assume 100% efficiency for the minimum food energy required, but the real number is much higher.

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