Machines Chapter 15

What is a machine? A device that makes work easier. 2 types of machines (simple and compound) 2 types of force used in machines: effort force – force applied to a machine. resistance force – force that opposes the effort force. When you use a machine you often sacrifice distance to multiply your effort force. Machines make work easier but they do not change the amount of work you do. Do Now 1. List 5 items in the classroom that are machines. 2. Choose 1 item from your list and tell the person next to you how it works as a machine. Most machines are made up of two or more simple machines.

Simple Machines Lever Inclined plane Pulley Screw Wedge Wheel and axle – two different sized wheels that turn together around the same point

Compound Machines a machine that combines two simple machines or more Examples: bicycles, scissors, clocks, telephones, cars, computers

Effort Force and Resistance Force The force applied to a wrench is the effort force. The nut is the resistance force you are trying to overcome when you turn the handle. The wrench acts as a wheel and axle machine to let you use a small force to overcome a large force.

Mechanical Advantage (MA) Number of times a machine multiplies the effort force. MA = resistance force / effort force If you apply a force of 100 N to a machine to move a 1500 N object, what is the MA? Ideal Mechanical Advantage (IMA) Mechanical advantage a machine would have with no friction. If IMA of a pulley is 2, then the pulley should multiply your effort force by 2. But it does not because of friction. So the actual MA of the pulley will be less than 2.

Do Now What are the six simple machines? What makes a wheel and axle a simple machine? What is the IMA of a machine?

Work Output Efficiency of a Machine efficiency – ratio of work output to work input Percentage efficiency = (work output / work input) x 100 Efficiency can be increased by decreasing the resistance force or reducing the force of friction. Work done by a machine Work output = resistance force x resistance distance Work Input Work done on a machine Work input = effort force x effort distance Work output of a machine is never greater than the work input. Efficiency of a machine is always less than 100 percent.