Find the maximum vertical height inc of the ball

[moondawg]

Homework Statement

A chilld rolls a .6kg bball up a long ramp. The bball can be considered a hollow sphere. When the child releases the bball at the bottom of the ramp, it has a speed of 8m/s. Assume the work done by friction on the bball is the same when the ball moves up or down the ramp and that the bball rolls without slipping. Find the maximum vertical height inc of the ball as it rolls up the ramp.

Homework Equations

The Attempt at a Solution

Originally i though i should use energy to find my height by doing kinetic translational + kinetic rotational= ggravitational potential. But when i did that my number was too high when compared to the answer in the back of the book. so i took kinetic rotational away and got the correct answer of 3.26m but i don't understand why rotational kinetic would not be used to find this because the ball is rolling with friction. Explain?!

 

[Delphi51]

Friction is very small for a rolling object; it makes sense to ignore that.

There certainly is rotational kinetic energy when the ball is rolling. What isn't clear in the question is whether or not the ball is rolling when the child releases it. The first line, "A child rolls a .6kg bball up a long ramp." is very unclear.

You know, this is a wonderful opportunity for you to talk to your prof. You may well impress him or her, and get the question fixed for next year.