EXP 9 - PRELAB
PRE-LAB Instructions:
Print out these pages. Feel free to refer to the lab instructions and other materials, your
physics textbook, other students, etc. to help you to ponder, understand, and work out
answers to the following question(s). Show your work & answers in the space(s)
provided.
PRE-LAB Questions:
Questions 1, 2, & 3: A meter-long ruler (meterstick) has a mass of 200 g giving it a
weight of 1.96 N (nearly 1/2 pound). A pencil lying flat on a tabletop serves as a fulcrum,
and if the ruler is placed with the 50-cm mark above the pencil the ruler is balanced –
showing that the center of mass is at the center of the ruler.
1) Draw a diagram showing the ruler placed horizontally on the pencil with the ruler's 35cm mark above the pencil. Draw vectors showing the two forces that act on the ruler
(weight and normal force) at the INSTANT it is released (we realize, of course, that the
ruler will quickly be rotationally accelerated by the net torque).
2) As you have drawn it above, is the torque acting on the pencil clockwise (CW) or
counterclockwise (CCW)?
3) What is the magnitude of the torque acting on the ruler?
Questions 4 – 7: If a 100 gram mass is correctly placed on the above ruler, the ruler +
100g system will remain balanced (with the 35-cm mark above the pencil and the ruler
horizontal). Such a system is in equilibrium since the forces and torques are balanced and
the acceleration is zero.
4) Draw a diagram of the balanced system described above - showing the forces that are
acting on the ruler (label your vectors).
5) According to your diagram, what is the CW torque acting on the ruler?
6) According to your diagram, what is the CCW torque acting on the ruler?
7) Where is the 100 gram mass placed (above what ruler mark) to achieve this
equilibrium?
8) A meter stick is supported by a pivot at its center of mass. Assume that the meter stick
is uniform and that the center of mass is at the 50 cm mark.
a) If a mass m1 = 80 g is suspended at the 30 cm mark, at which cm mark would
a mass m2 = 110 g need to be suspended for the system to be in equilibrium?
b) If a mass m1 = 80 g is suspended at the 25 cm mark, and a mass m2 = 110 g is
suspended at the 60 cm mark, from what cm mark would a mass m3 = 45 g
need to be suspended for the system to be in equilibrium?
9) A uniform meter stick of mass ms = 100 g is supported by a pivot at its 35 cm mark.
When an unknown mass is suspended from the 10 cm mark, the system is in equilibrium.
What is the value of the unknown mass?