- #1
emmy
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Homework Statement
In the Figure 8-36, the string is L = 220 cm long, has a ball attached to one end, and is fixed at its other end. A fixed peg is at point P. Released from rest, the ball swings down until the string catches on the peg; then the ball swings up, around the peg. If the ball is to swing completely around the peg, what value must distance d exceed? (Hint: The ball must still be moving at the top of its swing. Do you see why?)
http://edugen.wiley.com/edugen/courses/crs4957/art/qb/qu/c08/fig08_38.gif
2. The attempt at a solution
I thought I had this one, and it ended up being wrong...
L=2.2m
Before pendulum is released:
Ug0=mgL
Ug0=KEf=.5mv2=mg(2r)
where KEf is the kinetic energy at the lowest point, and r is the radius of the circle the pendulum makes around the second peg.
mgL=mg2r
L=2r
L/2=r
2.2m/2=1.1m=r
so d (being the remaining cord length) is
L-r=d
d=2.2m-1.1m=1.1m
Where did I go wrong