Nonconservative work

1. Jul 21, 2009

pttest

1. The problem statement, all variables and given/known data

At a playground, a 21kg child plays on a slide that drops through a height of 2.4m . The child starts at rest at the top of the slide. On the way down, the slide does a nonconservative work of -376J on the child. What is the child's speed at the bottom of the slide?

2. Relevant equations

Change in Kinetic energy + change in potential energy = nonconservative work

3. The attempt at a solution
[0.5(21)(v^2 - 0)] + [(21) (9.8) (0 - 2.4^2)] = -376
Therefore v = 8.77m/s
But the answer is wrong. Could someone help ??/

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jul 21, 2009

cepheid

Staff Emeritus
Your equation is wrong. It should be:

change in potential energy - nonconservative work = change in kinetic energy.

Can you see why? If there were no friction, ALL of the gravitational potential energy would be converted into kinetic energy. Since there is some friction, not all of the potential energy is converted. Some of it is wasted.

3. Jul 21, 2009

JazzFusion

You also have your calculations for 'change' wrong -

Should not be an ^2 in the PE term.

One more suggestion: Try doing the calculation once assuming there is no friction at all - then ALL the PE would be changed into KE. Now think: should the speed you calculate with friction be more or less than this value? This should help bracket your answer.