Homework Help: physics

Posted by s on Monday, November 21, 2011 at 6:29pm.

A 54.5 kg. skateboarder starts out with a speed of 1.90 m/s. He does +80.0 J of work on himself by pushing with his feet against the ground. In addition, friction does -265 J of work on him. In both cases, the forces doing the work are nonconservative. The final speed of the skateboarder is 5.90 m/s.
(a) Calculate the change (ΔPE = PEf - PE0) in the gravitational potential energy.

physics - Damon, Monday, November 21, 2011 at 7:01pm
initial Ke = (1/2)(54.5)(1.9)^2 = 98.4 J
final Ke = (1/2)(54.5)(5.9)^2 = 948.6 J

increase in Ke = 85.2 J

PEf + Ke f = PEi + 80 - 265 + Ke i

PEf - PEi = 80 - 265 + 98.4 - 948.6

= - 1035 J

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