Ch. 5 Simple Machines

Machines Machines: make work easier by changing the size or direction of a force (they do not save work) If they did what law would this violate? A simple machine has few or no moving parts.

Work W= F x D Effort force (Fe) applied to machine Or Input force Fin Resistance force (Fr) Force applied by the machine Or Output force Fout Work done on the machine is work input Win Work done by the machine is work output Wout An ideal machine would not lose any energy Or Work in= work out

Mechanical advantage We use machines to gain mechanical advantage- divides the force up over a greater distance Mechanical Advantage is how much the machine multiplies the effort force. MA= resistance force/effort force Ideal vs. Actual

Stuff to know AMA- actual mechanical advantage AMA=Resistance force/Effort force Work in= Effort force x Effort distance Work out= Resistance force x Resistance distance IMA- Ideal mechanical advantage IMA = different for different machines

practice You use a 50N force on a crowbar to open a crate with a resistance force or 900N. What is the MA. What is the effort force needed to lift a 2000N rock with a jack that has a MA of 12? What does our triangle look like? Do we always use machines to increase mechanical advantage?

Levers The Lever pivots on a fixed point called the fulcrum Effort arm is the piece of the lever where force is applied The resistance arm is the piece of the lever with the resistance or load

Levers have mechanical advantage as well Ideal mechanical advantage assumes no friction For levers IMA = length of effort arm/ length of resistance arm

Lever Practice You use a lever with an 80cm effort arm and a resistance arm of 10cm to lift a 240N object. What is the IMA of the lever? How much force do you need to apply to raise the object?

Effeciency Efficiency is how much of the work put into the machine becomes work put out by the machine. We strive for higher efficiency. The efficiency of a machine can be found in two ways eff = AMA/IMA x 100% eff = work out/work in x 100%  

Problem Example: An inclined plane 4m long is used to lift a piano weighing 1500 n onto the back of a truck 2m off the ground. A force of 900n was needed to push the piano up the inclined plane. (A) What was the work in? W in = 900n x 4 m = 3600 J (B) What was the work out? Wout = 1500 n x 2 m = 3000 J (C) What was the IMA of the inclined plane? IMA = length/height = 4/2=2 (D) What was the AMA of the inclined plane? AMA = R/E = 1500 n/900n = 1.67 (E) What was the efficiency? eff = 1.67/2 = 83.5% or eff = 3000 J/ 3600 J = 83.3 %

Power W= F x d Power is the rate of doing work or how fast the work is done. To calculate power Power = Work/time P = W/t The unit of power is J/s renamed the watt after James Watt.

Power Ex. If a student weighing 70 N runs up a flight of steps 25 m high in 3.5 seconds, what is his power? W = Fd = (70N)(25 m) = 1750 J P = 1750 J/3.5 s = 500 W or .5 kW 1kW = 1000 W