# Homework Help: Energy and Work

1. Nov 3, 2004

### Soaring Crane

A ball is attached to a horizontal cord of length L whose other end is fixed.

a. If the ball is released what will be its speed at the lowest point of its path?

(1/2)mv^2 = mgr, or v^2 = 2gr, v = sqrt(2gr)???

b. A peg is located a distance h directly below the point of attachment of the cord. If h = 0.80 L, what will be the speed of the ball when it reaches the top of its circular path about the peg?

pic:

O_____L________o
+----------------|
-+---------------|
--+--------------|h
---+-------------|----+
----+------------*peg--+
------+----------|----+
---------+----+-O--+

+ = path of circular motion
O = ball

Any help to start is appreciated.

2. Nov 3, 2004

### Leong

The first one is correct.(r=L).
When the ball reaches its top circular path, it has both kinetic energy and potential energy.
Initial energy,E(initial) = mgL
Total energy when the ball reaches its top circular path,E(final) = $$\frac{1}{2}mv^2 + mg*2(L-h).$$
Law of conservation of energy : E(initial) = E(final)

Last edited: Nov 4, 2004