2.2.3 Simple Machines: Pulleys
NAME:
1
DATE:
PARTNERS:
Purpose:
To investigate pulleys, block and tackle, and mechanical advantage.
Materials:
1.
2.
3.
4.
5.
6.
7.
One single pulley (optional)
Two double pulleys
20N spring scale
Weight
Heavy string
Scissors
Retort stand, or another form of pulley support
Diagrams:
Part 1 – The Single Pulley
1.
2.
3.
4.
Attach a weight to the bottom of the spring gauge. Record the weight in Newtons: ______________
Suspend one pulley (either single or double) as shown in Figure 1.
Pass a length of string over the pulley (if using the double pulley, pass the string over one wheel only)
Tie the weight from step 1 to one length of the string, and hook the spring scale to the other end of the
string.
5. Lift the weight off the floor or desk and record the reading on the spring scale, in Newtons: _________
6. Measure how far the weight moves up: _______ cm, and measure how far the spring scale moves
down: ______ cm
2.2.3 Simple Machines: Pulleys
2
Does this single pulley system magnify the applied force, multiply the speed of raising the weight, or
change the direction of the applied force? Compare the distance moved by the weight to the distance you
move the spring scale down-- are these the same or different?
List a couple of advantages of using this kind of pulley to lift a heavy load.
Part 2 – The Block and Tackle
Now you and your group can experiment with different types of block and tackle systems. A block and
tackle is simply two or more pulley linked together by loops of rope. With your group construct at least two
of the pulley systems shown in Figures 2, 3, and 4. Feel free to try a pulley system of your own design,
too! With each system, lift the weight off the desk or floor, and record the reading on the spring scale (col
2). Also record the distance the weight moves up (col 4) when you move the spring scale down a certain
distance (col 3), and the ratio of the spring scale distance by the weight distance (col 5=col 3/col 4).
Record your observations in the table below.
Note: if you used a pulley system of your own design, sketch it in the Observation section
Observations
Known weight from Part 1 (in Newtons): __________
Spring scale
measurement
(Newtons)
Figure 2
Figure 3
Figure 4
Other
Distance spring scale
moves down
(cm)
Distance weight
moves up
(cm)
Spring scale distance
/
Weight distance
2.2.3 Simple Machines: Pulleys
3
Questions
1. Mechanical advantage is defined as the original weight (from Part 1) divided by the applied weight
(the spring scale measurement after you hook up the pulley system). What is the mechanical
advantage for each kind of pulley system you tried?
2. How does the mechanical advantage you measured depend on the arrangement and number of the
ropes?
3. In theory, the number of supporting strands supporting a block and tackle equals its mechanical
advantage. Brainstorm with your group some reasons why you might not have observed this exact
one-to-one ratio. Note: the strand on which you are pulling – the one with the spring scale – is not
a supporting strand.
2.2.3 Simple Machines: Pulleys
4
4. If you ignore friction, how many supporting ropes would be needed in a pulley system to lift a 500N
weight with a 125N pull?
5. How could you test the amount of friction in a pulley system?
6. You may have noticed that as you add more supporting ropes to a block and tackle, you benefit by
needing to apply less force to lift a weight, but you need to pull the rope a longer distance for each
supporting rope you add. Compare the ratio of the spring scale/weight distances in Col 5 of the
table with the mechanical advantage you calculated earlier for each block & tackle arrangement.
Can you make any connections between the pulley system, and other simple machines (i.e. levers,
inclined planes)?
References:
Cunningham, James and Norman Herr. Hands-On Physics Activities With Real-Life Applications, (West
Nyack: The Center for Applied Research in Education, 1994), p.336.
Lorbeer, George C. Science Activities for Elementary Students, (Toronto: McGraw Hill, 2000), p. 117.
2.2.3 Simple Machines: Pulleys
5
Summary and Suggestions for the Future:
(Please attach extra pages if necessary, to fully answer these questions)
1. What were the important concepts of physics/science that you learned from this activity? What else did you learn?
2. Can you think of alternative hands­on ways in which these concepts could be demonstrated?
3. What changes (if any) would you make to teach these activities in a Grade 7­8 classroom? What difficulties can you foresee?