PURPOSE: To show the tension in the string exerted by a swinging pendulum.

DESCRIPTION: The spring scale reads the tension in the string as the pendulum swings, about 25 Newtons at the center and 19 Newtons at the ends of the swing with a bob mass of 1.1 kG. In this setup:

F (center) = mg + mrw^2 = mg (3 - 2 cos a)

F(end) = mg cos a

where by conservation of energy

m v^2 /2 = m r^2 w^2 /2 = mgr (1 - cos a)

m r w^2 = 2mg ( 1 - cos a)

SUGGESTIONS: See Question of the Week #200 for suggestions on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Pendulum with bifilar support, 40 N spring scale, 22 N (5 lb) weight.

SETUP TIME: 5 min.