Machines - Ch. 7 II. The Simple Machines (Chapter 8)  Introduction to Machines  Mechanical Advantage  Lever  Pulley  Wheel & Axle

Machines  Machine device that makes work easier changes the size and/or direction of the force

Machines 2 kinds of work involved when a machine is used  force applied to the machine “what you do”  force applied by the machine “what the machine does”

Machines  Work Input (W in ) Work done on a machine You apply a force, called the input force, to the machine and move it through a distance W in = F × d

Machines  Work Output (W out ) work done by a machine The machine applies a force, called the output force, through a distance W out = F × d

Machines can never get more work out than you put in Work output can never be greater than work input W in = W out F × d = F × d

Machines  Machines make work easier because they change the size or direction of the input force.  Using a machine does not mean that you do less work….a machine allows a smaller force to be applied over a longer distance

Machines Force – Distance Trade – Off Force or distance can increase, but not together. When one increases, the other must decrease. ie. When force increases distance decreases

Mechanical Advantage  Mechanical Advantage (MA) number of times a machine increases the force

Mechanical Advantage  A worker use a crowbar to apply an input force of 20 N to open a window with a output force of 500 N. What is the crowbar’s MA? GIVEN: F input = 20 N F output = 500 N MA = ? WORK : MA = F output ÷ F input MA = (500 N) ÷ (20 N) MA = 25 MA F output F input

Lever  Lever a bar that pivots at a fixed point, called a fulcrum “Give me a place to stand and I will move the Earth.” – Archimedes Engraving from Mechanics Magazine, London, 1824 Input force Output force Fulcrum

Lever  Levers are used to apply a force to a load  There are three classes of levers, based on locations of the fulcrum, the load and the input force

Pulley  Pulley grooved wheel that holds a rope or a cable A load is attached to one end of the rope and an input force is applied to the other end LeLe LrLr F

Wheel and Axle  Wheel and Axle two wheels of different sizes that rotate together a pair of “rotating levers” Wheel Axle

Problems  You use a 140 cm plank to lift a large rock. If the rock is 20 cm from the fulcrum, what is the plank’s MA? GIVEN: L output = 20 cm L input = 140 cm MA = ? WORK : MA = L input ÷ L output MA = (140 cm) ÷ (20 cm) MA = 7 MA L input L output 20cm 140cm

Problems  A crank on a pasta maker has a radius of 20 cm. The turning shaft has a radius of 5 cm. What is the MA of this wheel and axle? GIVEN: r input = 20 cm r output = 5 cm MA = ? WORK : MA = r input ÷ r output MA = (20 cm) ÷ (5 cm) MA = 4 MA r input r output 5 cm 20 cm