What is the Velocity and Energy of a Child on a Swing?

[physicsquest]

Homework Statement

A 25.0kg child on a 2.00m long swing is released from rest when the ropes of the swing make an angle of 30 degrees with the vertical. (a) Neglecting friction, find the child's speed at the lowest position. (b) If the actual speed of the child at the lowest position is 2.00 m/s, what is the mechanical energy lost due to friction?

Homework Equations

(1/2)mvf^2 - (1/2)mvi^2

The Attempt at a Solution

Do I need to use an energy equation to solve for velocity?

 

[Nebozilla]

Yep energy is a good idea :) Start by finding the potential energy of the system at rest. You know its at 30 degrees and its 2m long so by using simple trig you can find h, the height. With conservation of energy you know the bottom of the swing, its all kinetic energy so the potential energy at rest is equal to the kinetic energy. The 2nd part is pretty simple but please respond if its not clear to you.

 

[physicsquest]

I don't understand how to solve the 2nd part without given the coefficient of friction.

 

[Nebozilla]

You don't actually need to know k since you already know the velocity of the kid at the bottom with friction involved. You also know the kinetic energy of the kid at the bottom without friction.