# Keeping In Balance

Document Sample

```					                                      Keeping In Balance

Objective: To use the principle of balanced torques to find the value of an unknown
mass.

Procedure:

Part I: Determining Unknown Mass of Block:

1. Determine the position of the center of mass of the meterstick by determining the
position at which it balances on the fulcrum. Keep the fulcrum located at this position.

2. Using a paper clip, attach the block of unknown mass to the 90 cm mark on the
meterstick.

3. Place 200 g of additional mass on a 5 g mass hanger. Attach the hanger to the
meterstick and locate the position (to the nearest mm) at which it must be attached to
balance the unknown mass.

4. Using the principle of balanced torques, calculate the mass of the block.

5. Measure the actual mass of the block on a balance and compute the % error.

Part II: Determining Unknown Mass of Meterstick:

1. Place the fulcrum at the 80 cm mark on the meterstick.

2. Add 250 g of mass to the hanger for a total mass of 255 g. Attach the hanger to the
meterstick such that it balances, and record the position of the hanger (to the nearest
mm).

3. Using the principle of balanced torques, calculate the mass of the meterstick.

4. Measure the actual mass of the meterstick (do not include the fulcrum!) on a balance
and compute the % error.

Part III: Finding the Center of Gravity of an Irregular Object:

1. Using masking tape, attach a cylinder to the meterstick somewhere between the 0 and
50 cm mark.

2. Place the fulcrum at the 60 cm mark of the meterstick.

3. Add 100 g of mass (or more if necessary) to the hanger and determine the total mass.
Attach the hanger to the meterstick such that it balances, and record the position of the
hanger (to the nearest mm).

4. Remove the fulcrum from the meterstick and measure the mass of the
meterstick/cylinder combination on a balance.

5. Using the principle of balanced torques, determine the location of the center of gravity
of the meterstick/cylinder combination.

6. Replace the fulcrum at the location of your determined center of gravity. Check to see
whether or not the meterstick/cylinder combination balances. If it does not, move the
fulcrum to the position at which it does balance and record this position.

Data:

Part I: Determining Unknown Mass of Block:

o n es C
s o tt G
i
Po M k ( )
t f ec
i   ri  cm
n ag
o s)
ws
KMn (
o n n ac
s o o sm
i
t f ws
i   n (
Po K M )
o n n n sm
s o k M)
i
t f n a
i   o
Po Uwsc   (
e ron ac
v m ws m
e fK
r      ns
LA r oM )  (
e ronw sm
v m n M)
e fU
r      o a
LA r knsc   (
ou n n s)
m k M
p  e n a
d o
C t Uwsg  (
e r n n s)
a dk M
sue n a
M Uwsgo   (
%r
Eror

Part II: Determining Unknown Mass of Meterstick:

nM
o s
ws
n (
K a) g
P n nM
o o o am
s f ws )
i
t K s
io   n (c
P n et M
o o t i am
s f ec s )
i
t M
io   s
rk s c
(
eA K s )
v m ws
e f nM
r o n (
L r ro am c
eA M c
v m ecm
e f ei
r o rks
L r rt t ( )
o ea M g
m s
pu s ti )
d o ec
f rk
Ct M et (s
er a M g
s
M M et (
u s
e  f rk
s
o ec
E
%rrr
o

Part III: Finding the Center of Gravity of an Irregular Object:

nM
o s
ws
n (
K a)  g
P n nM
o o o am
s f ws )
i
t K s
io    n (c
a Ca g
s
s mn
o bo)
f
Mon ( iti
eA K s )
v m ws
e f nM
r o n (
L r ro am   c
o eeA C( )
m vm b
pu e f o c
d r o .m
Ct L r rm
o eo o m(
m s f b c
p
Ct P n o C )
u i
dt C G
i
o    . m
A on o C )
c P o m(
tl s f b c
u i
a t C G
i
o    .   m
E
%rrr
o

Interpretations:

1. Where is the mass of an ordinary meterstick effectively located?

2. How did your calculated masses for the unknown block and meterstick compare with
the actual values? What were the % errors?

3. In part III, did the meterstick balance at your predicted location? If not, how far was
the actual location away from your predicted position for the center of gravity?

4. Does the principle of balanced torques appear to be a good method for determining an
unknown mass? Why or why not?

5. In part I of the experiment, why didn't the mass of the meterstick itself need to be
taken into account?