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?

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

• The efficiency of a machine can befound in two ways

• eff = AMA/IMA x 100%

• eff = work out/work in x 100%

•  

• Example: An inclined plane 4m long is used to lift a piano weighing1500 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 inclinedplane? IMA = length/height = 4/2=2

• (D) What was the AMA of theinclined 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