Work-Power-Machines Notes
In the realm of science, work is only accomplished if some force is exerted on an object and that object moves. Hence, no movement = no work. The formula for work is: work = force x distance.
Work is measured in units called joules (the equivalent of a newton-meter).
Example: If Julie lifts her 7 newton bookbag 1 meter, she has done 7 joules of work.
W = 7 N x 1m = 7j
Power is closely related to work in that the rate (how fast or slow) you do work is power. The equation for it is: P = W/t P(power) W(work) t(time) or P = f x d/ t [ f x d = W]
The unit for power is the watt (joule/sec).
Example: If Julie lifts her 7 newton bookbag 2 meter in 2 seconds, how much power has she exerted?
P = f x d / t = 7N x 2 m / 2 sec = 14 j/ 2 sec = 7 watts
Both work and power link to simple machines. All machines can be linked to 6 simple machines. The six simple machines are: lever, wheel & axle, inclined plane or ramp, wedge, screw and pulley. Each simple machine helps to reduce the amount of force needed to get a job done. This reduced force is called the mechanical advantage (MA).
Lever- this is a bar with a pivot point or fulcrum. A see-saw is a lever. Other examples include door, light switch, crow bar, hammer, fishing rod.....
There are 3 classes of levers: 1st, 2nd & 3rd. First class levers have the fulcrum positioned between the effort and resistance. Second class levers have the resistance positioned between the fulcrum and effort. Third class levers have the effort positioned between the fulcrum and resistance. Examples: 1st-pliers, 2nd-wheelbarrow, 3rd-broom.
MA = length of effort arm/ length of resistance arm.
Inclined plane- this is exactly what it says a sloping surface. Stairs, roads running up hill, wheel chair ramps... are inclined planes.
MA = length of ramp / height of ramp
Wedge- this is anything that cuts. Examples: knives, parts of can openers, hatchets, saws....
The wedge is 2 inclined planes sandwiched together that moves.
MA = length of wedge/ width of wedge
Screw- This is an inclined plane wrapped around a rod. The MA is determined by the number of threads. The more threads the more the MA. Screws are used to hold many things together and are also found in drill bits, meat grinders, augers...
Wheel & Axle- This is exactly what the name implies. So, any wheel is a wheel & axle. Other examples include door knobs, handles on fishing reels, knobs on stove....
MA = diameter of wheel / diameter of axle
Pulley- The pulley is very closely related to the wheel & axle. A pulley that is attached is called a fixed pulley. One that moves is called a movable pulley. Many pulley connected together is referred to as a block & tackle.
MA = # of supporting ropes or resistance force / effort force
For supporting ropes, any rope that passes over a pulley and comes out the other side is considered a separate rope. If the end rope (the part you pull on) is pulling down to make the pulley work, that segment is not counted as a supporting rope. However, if the end is pulling up it counts in the total.
Efficiency is how well a machine works. The definition is the percentage of the input work that is converted to output work.
efficiency= work output/work input x 100
No machine is 100% efficient. The major reason is that the force of friction acts to slow down and wear out everything.
A compound machine is a machine made up of two or more simple machines. A car, a pencil sharpener, scissors, mechanical pencil, and many many other machines are actually compound machines.