# physics

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A 68.0 kg boy and his 45.0 kg sister, both wearing roller blades face each other at rest. The girl pushes the boy hard, sending him backward with a velocity 3.70 m/s toward the west. Ignore friction.
How much chemical energy is converted into mechanical energy in the girl's muscles?
whenever i figure the answer i get 720 but htis is not right.

• physics -

What you want is the total kinetic energy of BOTH of the skaters after she pushes him. Because they are on skates, and total momentum is conserved, she scquires the same momentum as her brother, but in the opposite direction.

Her own velocity will be 68*3.7/45 = 5.59 m/s after the push, in order for her to have the same momentum. Add her kinetic energy to his.

His is 465.5 J
Hers is 703.1 J

Add them. I don't see how you got 720.

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