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What is a machine?A machine may be thought of as a device that makes work easier .
How do machines make work easier?
Machines make work easier by:multiplying the size of the force you exert (such as in this pulley system).
Machines also make work easier by: changing the direction of that force (such as in the gear, roller & rope assembly in these mini-blinds)
A pulley lifting a mass or a pulley on a flagpole.
A machine can also do both: multiplying the amount of the force, as well as changing the direction of the force. (such as this pry bar)
Other things machines do:Machines transfer force from one place to another. Example: A bicycle chain transfers force to the pedals.
belt driven
chain driven
feet driven
Machines can convert energy from one form to another. Example: This electric generator converts chemical energy (gasoline) to mechanical energy to electrical energy. – Not a simple machine.
Machines can multiply speed or distance.During one revolution of the bicycle’s pedals, the rim of the wheel travels faster than the pedals.
When you use a pry bar to remove a lid, you are working against friction----the friction between the nails in the lid and the crate.
Applying Force & Doing Work• Two forces are involved when a machine is
used to do work.
First: Effort force (Fe ) – the force applied to the machine
• The pry bar is used to multiply the effort force and open the crate. However, to gain force the effort must push through a greater effort distance than the resistance distance.
Law of Work and the Ideal Machine
• In an “Ideal Machine” is frictionless ---therefore there is no energy loss from friction and heat.
• For an ideal machine, the work input is equal to the work output.
• This is called the “Law of Work.”
So in order that work input equals work output, the effort force must travel a greater distance than the resistance force.
Law of Work Calculation -Ideal Machines• A 500 N barrel is rolled up a 4.00 m ramp to a
platform 1.20m above the ground. What force must be applied to the barrel.
How much force would be used to lift the barrel to then platform without a ramp?
Since the barrel has a weight of 500 N, the effort force required to lift the barrel would be 500N. The work done on the barrel would be W = Fd (500N x 1.20m = 600N).
500 N Effort = 112 lbs
150 N Effort= 34 lbs
THE MECHANICAL ADVANTAGE:The Number of times a machine multiplies the effort force is the Mechanical Advantage (MA) of the machine.
Mechanical Advantage Calculation –for Ideal Machines
• For the barrel-ramp problem:
The ramp, therefore multiplies the effort put into moving the barrel to the platform over three times.
What about a less than ideal machine?
• In the real world, friction acts against a machine’s work output, whether it is a pulley system or a ramp.
The less friction the more efficient the machine, and the greater the work output.