Work & Power

Simple & Compound Machines

Mechanical & Ideal Mechanical Advantage

EfficiencyBy: Deborah Wang

modified by: S. Ingle

Work Concepts

Work (W) ~ product of the force exerted on an object and distance the object moves in the direction of the force.– W is transfer of energy by mechanical means. – W is done on an object only if it moves in the

direction of the force. – Only the component of the force in the

direction of the motion does work.

Work Formulas

Work = Force x displacement

W = Fd

1 joule (J) = 1 Newton * meter (N * m)

if force is exerted at an angle to direction of motion,

W = (Fcos)d

or W = Fd cos

F

d

F

d

Power

Power (P) ~ rate at which work is done or rate at which energy is transferred. Measured in watts. – Watt (W) ~ one joule of energy transferred in

one second.

Power = Work / time

1 watt = 1 joule / second (J/s)

Simple Machines Machine ~ makes work easier either by

changing the magnitude or the direction of a force, but can not increase the amount of work done.

1. lever ~ bottle opener, seesaw, crowbar

2. pulley ~ rope on flagpole, block & tackle

3. wheel-an-axle ~ wheel barrow, bike pedals

4. inclined plane ~ wheelchair ramp

5. wedge ~ knife, axe, chisel

6. screw ~ bottle cap, nut, bolt

Mechanical Energy

Effort force (Fe) ~ force you exert on a machine.

Resistance force (Fr)force exerted by the machine.

Win= Fede Work input

Wout= Frdr Work output

Ideal Machines

Ideal machines exist only in a frictionless, air resistance-less world.

No energy or work is lost to the system through outside forces

For ideal machines:

Work input = Work out put so:

Fe de = Fr dr

Mechanical Advantage Mechanical Advantage (MA) ~ ratio of

resistance force to effort force.

MA = Fr / Fe

Ideal Mechanical Advantage (IMA) ~ ratio of effort distance to resistance distance.

IMA = de / dr

Efficiency

Efficiency ~ ratio of work output to work input expressed in percent.

efficiency = (Wout / Win) * 100%

or

efficiency = (MA / IMA) * 100%

Compound Machines Compound machine ~ consists of two or

more simple machines linked so that the resistance force of one machine become the effort force of the second. – Ex. The pedal and gear of a bicycle.

To calculate the efficiency of a compound machine you simply multiply the efficiencies of the simple machines that make it together.