Physics
Sensors:
Loggers:
Force
Any EASYSENSE
Logging time: Meters
Teacher’s notes
37 Pulleys
Read
In this investigation they will make up combinations of pulleys to lift a standard load and use a Force
sensor to measure how much force is needed to lift the load. The information collected will allow the
students to calculate the mechanical advantage, and investigate how the MA is related to the
configuration of the pulley system.
The students will collect and record the information into a results chart for later analysis.
A pulley is wheel with groove cut around its circumference to accept a rope. Winding ropes around
pulleys creates a very powerful but simple machine. As the number of pulleys the rope winds around
increases, the effort to move the load decreases, but the distance the rope has to be moved
increases.
In a pulley system the load is shared by all the suspending ropes. In practice, for convenience,
collections of pulleys are made into a ‘block’.
Pulley systems are very important on sailing ships. On military sailing vessels, a well designed block and
pulley system could significantly alter the manoeuvrability of the vessel and alter the vessels strategic
significance. Pulleys are still in use the modern world in heavy lifting equipment, winching devices, etc.
Apparatus
1.
2.
3.
4.
5.
6.
An EASYSENSE logger.
A Smart Q Force sensor
500 g mass on a hanger or with a hook attachment.
Spoked pulley or a single pulley
A double pulley block OR two more Spoked pulleys OR two more single pulleys
Pillar from the Dynamics system, large brackets, hex M6 bolts and two wing nuts
OR retorts stand with appropriate bosses and clamps.
7. Thin string.
Set up of the software
Recording method
Display
Meters
Numeric, Full size, Decimals
Physics L2
T37 - 1 (V2)
Notes
Care should be taken to ensure the Force sensor is not overstressed. Refer to the sensor booklet prior
to the experiment to check on use and limits of the sensor.
The Spoked pulleys were not originally designed for use in a block and tackle. They appear to work well
with loads up to 1 kg. However the investigation works well with the suggested load of 0.5 kg. This
reduces the risk of damage due to weights dropping on to benches and/or feet.
It is advisable to have some thick cloth or cardboard protecting the bench, in case the weights should
fall.
If using a load mass of 0.5 kg it is quite acceptable to state that the load is 5 N. This is within 2% of
the actual value, and well within experimental error in this investigation. It is certainly good practice
to measure the weight of the load with the Force sensor, but we suggest that the extra time needed is
not really worthwhile.
The pulley systems can be difficult to set up if the user is not familiar with them. There may be
advantage in having the pulley systems set up at several stations around the room and get the students
to visit the appropriate station to collect the results. In this case the strings can be tied with loops at
the correct place to connect to the load and the Force sensor.
The free pulley should have two ‘v’ shaped notches cut into the frame, level with the groove on the
wheel, to locate the loop the load is hung on. As shown below.
V notch cut into pulley to locate
string supporting the load. You
need one each side.
The notch should be in line with
the groove of the pulley wheel
If you are not using the Data Harvest Dynamics System apparatus, it can be effective to have the
pulley system mounted against a board rather than hanging in space, this prevents the pulleys from
swinging around too much.
In the diagrams that follow the pulley schematic is followed by the diagram of how to construct the
pulley system using components from the Data Harvest Dynamics system.
In all cases the string used is tied at one end to a Force sensor and to the other end to the support
bracket.
An alternative would be to have the string tied to a fixed point and a “free end” tied to a Force sensor
which is then pulled to lift or suspend the attached mass.
Note: With pulley systems the method of connection to the fixed points will depend on the
manufacture of the pulley / block. Many have a hook top and bottom, others have a load hook and fixed
connection points for tying off the ends of the rope or fixing the whole system to a solid surface. In
sailing the pulley blocks will often have more of a floating arrangement to allow for movement of the
rigging and flexing of the ropes.
Physics L2
T37 - 2 (V2)
Fixed A fixed or class 1 pulley has a fixed axle. That is, the axle is "fixed" or anchored in place. A
fixed pulley is used to redirect the force in a rope (called a belt when it goes in a full circle). A fixed
pulley has a mechanical advantage of 1.
Movable A movable or class 2 pulley has a free axle. That is, the axle is "free" to move in space. A
movable pulley is used to transform forces. A movable pulley has a mechanical advantage of 2. That is,
if one end of the rope is anchored, pulling on the other end of the rope will apply a doubled force to
the object attached to the pulley.
Compound A compound pulley is a combination fixed and movable pulley system.
Block and tackle - A block and tackle is a compound pulley where several pulleys are mounted on each
axle, further increasing the mechanical advantage.
Acknowledgements to Wikipedia
String goes over one pulley, masses attached to one end tied to Force sensor at the other end
Basic pulley system with the single pulley fixed to a surface e.g. a ceiling beam.
Physics L2
T37 - 3 (V2)
Single pulley system, with pulley floating
Floating pulley with
masses suspended
from it
String is attached to Force sensor, passes over pulley and the other end is attached to hanging
bracket.
Physics L2
T37 - 4 (V2)
Two pulley system, one pulley attached one floating.
Fixed pulley
Floating pulley with
masses suspended from it
String is attached to Force sensor at one end, passes over a fixed pulley and a floating pulley. Tied to
support bracket at the other end
Physics L2
T37 - 5 (V2)
Three pulley system, normally a double pulley (block) is attached and a single pulley is floating. In this
schematic the double pulley has been shown as two separate single pulleys.
.
Fixed pulleys
Suspended
mass
Floating pulley
String is attached to Force sensor, passes over a fixed pulley, floating pulley, fixed pulley and then
tied to the support bracket.
Physics L2
T37 - 6 (V2)
A more complex 2 x double system using double pulley blocks.
Results and analysis
This an actual set of results using a load of mass 500 g.
Pulley system
Load L in
Newtons (N)
Effort E in
Newtons (N)
Number of
supporting ropes
1
5
5.1
1
2
5
2.6
2
3
5
2.5
2
2
4
5
1.6
3
3.1
MA =
L
E
1
1.9
The results show, very reliably, the relationship between the number of supporting ropes and the MA.
Extension
Hold the Force sensor in your fingers and measure the force needed to keep the load moving steadily
upwards. Calculate the ‘moving’ mechanical advantage for each pulley system. Explain why this value of
the MA is different from the one you measured above.
Physics L2
T37 - 7 (V2)