Work, Power, and Simple Machines

•    Work

•    Power

•    Machines

•    Simple and Compound Machines

Work

•    Work is defined as a force acting through a distance.

•    Work=Force x Distance

•    W=fd=Newtons x meters=N-m=Joule

•    A Joule (J) is an amount of work or energy that is equivalent to applying a 1 Newton force to an object for a distance of 1 meter.

•    Energy is the ability to do work.

Work (continued)

Power

•    Power is the rate at which work is done.

•    Power=Work/Time

•    P=W/T=Joule/sec=Watt

•    1 Kilowatt (kW)= 1000 watts

Machines

•    A machine is a device that makes work easier.

•    Work input is the work that is put into the machine (the input distance times the input or effort force).

•    Work output is the work that a machine does (the output distance times the output or resistant force).

Machines (continued)

•    Machines make work easier by changing the input force or distance or by changing the direction of input force.

•    Work is conserved.

•    Theoretically, the work put into a machine must equal the work that comes out of a machine. 

•    In practice this never happens due to energy being lost to friction.

Machines (continued)

•    A comparison of input work to output work is called efficiency.

•    The efficiency of a machine is equal to work output/work input.

•    Due to friction, no machine is 100% efficient.

•    Machines with the smallest amount of friction are the most efficient.

Machines (continued)

•    The mechanical advantage of a machine is also important to know.

•    Mechanical advantage tells how many times the input force is multiplied.

•    The greater the mechanical advantage, the less force is required to do the work.

•    This comes at the cost of distance, however.

Simple Machines

•    All machines are either one or more simple machine.

•    There are 6 simple machines:

•    Inclined plane, wedge, screw, lever, pulley, and wheel and axle.

•    An inclined plane is a flat, sloped surface like a ramp.

•    The longer the inclined plane is, the less force is needed to raise the object because of the decreased slope.

•    Therefore, longer inclined planes with less slope yield greater mechanical advantages.

•    An wedge is a moving inclined plane.

•    Many wedges are made from two inclined planes stuck together like an ax.

•    The longer and thinner the wedge, the greater its mechanical advantage.

•    An screw is an inclined plane wrapped around a central bar to form a spiral.

•    A screw gets its mechanical advantage from the length of the inclined plane wrapped around it.

•    The closer together the threads on a screw, the greater its mechanical advantage.

•    A lever is a rigid bar that is free to pivot about a fixed point called a fulcrum.

•    A good example is the see-saw.

  •    There are three classes of levers.

•    A first class lever has a fulcrum in the middle.

•    A second class lever has the resistance in the middle.

•    A third class lever has the effort in the middle.

•    Just remember FRE.

 •    Notice that different classes of levers change the direction of the input force.

•    This can sometimes make work easier.

•    The mechanical advantage of a lever is generated by changing the length of the force or effort arm relative to the resistant arm of the lever.

 •    A pulley is a rope, belt, or chain wrapped around a wheel.

•    A fixed pulley an change the direction of input force.

•    Pulleys can be combined in systems.

•    The mechanical advantage of a moveable pulley system is equal to the number of pulleys or lifting lines.

Simple Machines (continued)

•    A wheel and axle is a simple machine made from two circular objects of different sizes.

•    The wheel is the larger of the two.

•    The mechanical advantage of a wheel is generated by the difference in the radii.

Compound Machines

•    A compound machine is a combination of two or more simple machines.