EFFICIENCY OF A PULLEY SYSTEM LAB
EQUATIONS:
F
MA = r
Fe
IMA =
r = resistance force (output)
de
dr
WOut = Fr d r
WIn = Fe d e
efficiency (%) =
efficiency (%) =
Wo
x100
Wi
MA
x100
IMA
e = effort force (input)
PROCEDURE:
1. Lay the two triple-pulleys on the table as shown above. The pulleys are labeled in the drawing above
to help you with the stringing process.
2. Attach one end of a 3 m long piece of string to the hook next to pulley #4.
3. Wrap the string around pulley #3, then pulley #4, then #2, then #5, then #1 and then #6.
4. Let the string hang away from the set-up after going around #6. The string should go on each of the
6 pulleys!
5. CAREFULLY hang the whole set-up from the hook next to pulley #6. If the string falls off you can
put them back on the pulleys – you will not have to restring it.
6. HOLD the end of the string that comes off of pulley #6 so that the whole set-up doesn’t fall.
7. Use the 1000 g mass for Fr, and record Fr as 1000 g. Technically the 1 kg mass should weigh 9.8 N,
but it may not in actuality. Check the weight using a spring scale and record its actual weight in the
data table.
8. Choose weights for Fe such that a slight push downward will cause the
weights to move downward at constant speed (this means that you have
exactly balanced the friction force).
9. Check the weight of Fe (which you found above) using a spring scale and
record its actual weight in the data table.
10. Once you have found the proper weights, adjust the system so that the tops of
the effort and resistance weights are even with each other. Carefully note their height from the floor.
11. Move the effort weights (Fe) down 50.0 cm from its starting position, and note that the resistance
weight simultaneously moves up. Carefully note the new height of the resistance weight, and use
subtraction to determine how far it moved.
12. The effort distance is 50.0 cm, and the resistance distance is the distance the resistance weight
moved upward. Record both distances to nearest 0.1 cm and then convert to nearest 0.001 m.
13. Repeat Steps #7-12 with a 700 g resistance force (Fr).
14. Count the number of strings supporting the load and record.
EFFICIENCY OF A PULLEY SYSTEM LAB
Name:
Date:
Per:
DATA TABLE:
Fr
Trial #1
Trial #2
dr
g
N
g
N
Wout
cm
m
cm
m
Fe
J
J
de
g
N
g
N
Win
cm
m
cm
m
# of strings supporting load:
CALCULATIONS: For each calculation, show all of the Four Step Method:
1. % Efficiency (using work values)
Trial #1:
Trial #2:
4. Mechanical Advantage
Trial #1:
Trial #2:
5. Ideal Mechanical Advantage
Trial #1:
Trial #2:
6. % Efficiency (using mechanical advantage values)
Trial #1:
J
J
Trial #2:
QUESTIONS:
1. How can you quickly tell the IMA of a pulley system by inspection?
2. If output work in a pulley system is always less than input work, why are pulleys used?
3. (a) If there were no friction in the pulley system, how would IMA compare to MA?
(b) Which one is always larger in real pulley systems with friction?
4. (a) If there were no friction in the pulley system, how would Wout compare to Win?
(b) Which one is always larger in real pulley systems with friction?
5. Which ONE of the following quantities is affected by friction? Circle one: Fr dr Fe de
Does friction make the quantity larger or smaller?
6. On the basis of your answers to #3, 4, and 5, explain why MA is always smaller than IMA, and Win is
always larger than Wout in complete sentences.
7. Is it possible to get 100% out of this pulley lab set-up? Explain.