Physical Science Lab #7
Compound Machines:
Galaxy Gym Field Trip
WARNING: The machines in the gym are not toys and can be pretty
dangerous if you put your hands or fingers in the wrong places. Proceed with caution and put the machines on
their lowest weight settings when taking measurements.
Theory
Also see lab 5 theory
section
A compound machine is a machine composed of
two or more simple machines. Galaxy Gym
has many compound machines made of a combination of pulleys and levers. The net ideal mechanical advantage of such a
machine is calculated by multiplying the mechanical advantages of all the simple
machines that it is composed of, or by measuring the effort and resistance
distances at the opposite “ends” of the machine and calculating it as in the
previous labs.
Procedure
You will work in groups of two or three. Each group needs a tape measure. Try
doing the following for at least three machines.
1. Choose a machine that has at least
one lever and one pulley. Draw a picture of your chosen machine leaving space
for adding measurements and calculations.
Label the effort and resistance forces using the variable names used
previously.
2. Try to calculate the net ideal mechanical
advantage of your machine by measuring the various effort and resistance arms
of the levers in it and counting the number of cables attached via pulleys. If your machine allows you to choose
settings or hand/leg placements that alter the effort arms try to measure the
range of values and find the range of mechanical advantages for your machine.
3. Using the tape measure take
measurement of the effort and resistance distances to find the net ideal mechanical
advantage of your machine with another method.
If your machine allows you to choose settings or hand/leg placements
that alter the effort distance try to measure the range of values and find the
range of mechanical advantages for your machine. How close are the net ideal mechanical advantages that you found
in 2 and 3?
4. Calculate the force needed to lift a
mass of 50kg using your machine (assuming no friction, so Ama=Ima)
and write the values in for the effort and resistance forces on the diagram you
have drawn.
1. Why do you think they use these
compound machines in the gym? Is it to
make lifting the weights easier? (i.e. does it significantly lower the effort
force needed – is the mechanical advantage much bigger than 1?)
2. On the machines where you can choose
settings and hand/leg placements that affect the mechanical advantage, who is going
to find it easier (i.e. who needs less effort force to lift a given weight) a
bigger or a smaller person?
3. Try to think of some compound
machines outside of the gym. List them
and say what simple machines you will find in each.