8.3. Simple Machines http://www.ck12.org
Review
Questions
- Is it possible to get more work out of a machine than you put in?
- A worker uses a pulley system to raise a 225 N carton 16.5 m. A force of 129 N is exerted and the rope is
pulled 33.0 m.
(a) What is the IMA of the system?
(b) What is the AMA of the system? - A boy exerts a force of 225 N on a lever to raise a 1250 N rock a distance of 0.13 m. If the lever is frictionless,
how far did the boy have to move his end of the lever? - How can you increase the ideal mechanical advantage of an inclined plane?
- Diana raises a 1000. N piano a distance of 5.00 m using a set of pulleys. She pulls in 20.0 m of rope.
(a) How much effort force did Diana apply if this was an ideal machine?
(b) What force was used to overcome friction if the actual effort force was 300. N?
(c) What was the work output?
(d) What was the ideal mechanical advantage?
(e) What was the actual mechanical advantage, if the input force was 300N? - A mover’s dolly is used to pull a 115 kg refrigerator up a ramp into a house. The ramp is 2.10 m long and
rises 0.850 m. The mover exerts a force of 496 N up the ramp.
(a) How much work does the mover do?
(b) How much work is spent overcoming friction? - What is the ideal mechanical advantage of a screw whose head has a diameter of 0.812 cm and whose thread
width is 0.318 cm?
- simple machine:A mechanical device that changes the direction or magnitude of a force.
- ideal machine:An ideal simple machine in one in which the work input equals the work output.
- effort force:The force used to move an object over a distance.
- resistance force:The force which an effort force must overcome in order to do work on an object via a simple
machine. - ideal mechanical advantage:The factor by which a mechanism multiplies the force put into it. The mechan-
ical advantage is called the ideal mechanical advantage if there is no friction or if friction is ignored.
Generally, the mechanical advantage is calculated as follows: MA=distance over which the load is moveddistance over which effort is applied. - actual mechanical advantage:The mechanical advantage of a real machine. Actual mechanical advantage
takes into consideration real world factors such as energy lost in friction. In this way, it differs from the
ideal mechanical advantage, which, is a sort of ’theoretical limit’ to the efficiency. The AMA of a machine is
calculated with the following formula: AMA=resistance forceeffort force. - compound machine:A combination of two or more simple machines.
Work, measured in Joules, is the measurement of the force exerted on an object in the direction it moves multiplied
by the distance the object moved. Power, measured in Joules/second, is the amount of work done divided by the
time it took. Machines are devices that transform input work into equivalent amounts of output work in a different
form; a small force over a large distance may become a large force over a small distance. The six simple machines
discussed in this chapter are the building blocks of all machines.