Simple Machines and Work
Work Work- exerting a force that causes an object to move. Movement must be in same direction as force. If no movement, then NO WORK has been done.
Examples of Work Ex. 1 A body builder lifts 350 pounds above his head. Ex. 2 A father pushes a baby in a carriage. Ex. 3 A mouse pushing a piece of cheese with its nose across the floor.
Work Work= force x distance Measured in Joules ( J ). Ex. How much work is done when a 5 N force moves a block 4 m? Ex. You move a 25 N object 5 m. How much work did you do?
Machines A machine is a device that lets you do work easier. Does not decrease the amount of work, it changes the: Amount of force needed Distance Direction of the force
Mechanical Advantage Mechanical Advantage (MA)- the number of times a machine increases the inputs force. Input is force applied. Output is what the machine does.
Mechanical Efficiency Ideal is 100% - no loss of work due to friction. Simple Machines are NOT 100% efficient because friction converts energy to heat energy.
Simple Machines The six simple machines are: Inclined Plane Wedge Screw Lever Wheel and Axle Pulley
Inclined Plane Inclined Plane – is a flat, slanted surface. Example: Ramp MA – greater the length to height; greater advantage.
Wedge A wedge is a piece of wood or metal that is thick at one end and sloping to a thin edge at the other. Ex: knife, axe, doorstop MA- greater the distance; greater the output.
Screw Screw – is an inclined plane wrapped around a cylinder. Ex: jar lid, screw, faucets, drill bits MA- more tightly wrapped the treads; easier it is to turn
Pulley Pulley - is a grooved wheel with a rope. Fixed and movable pulleys as well as pulley systems. MA- changes direction of force; the more pulleys, the more MA.
Wheel & Axle Made of two circular or parts fastened together. Ex: doorknob, gears, steering wheel, tire on car MA- depends on size of wheel compared to axle. Greater MA if wheel is bigger than axle
Lever Rigid bar that is free to pivot or rotate. 1st class lever: fulcrum in the middle. Ex: see saw & crow bar 2nd class lever: the output (load) is in the middle. Ex: wheelbarrow 3rd class lever: the input force is in the middle. Ex: hammer and baseball bat
Clever Lever worksheet http://www.enchantedlearning.com/physics/ machines/Levers.shtml
Advance Students Power
Calculate Mechanical Advantage MA is measured by: Output / Input Ex. 1 You apply 50 N to a machine. The machine applies 500 N to an object. What is its MA? Ex. 2 You apply 100 N to a machine. The machine applies 300 N to an object. What is its MA?
Mechanical Efficiency Is the measure of the effectiveness with which a system performs. Efficiency – is the ratio of the work output to the work input. To calculate: Efficiency (in percent) = [output work / input work] x 100%
Calculating Efficiency Ex. 1 A lever does 765 J of work and the person using the lever applies 890 J of work. What is the efficiency of the lever? Ex. 2 To pull a nail out of a wood board a carpenter does 1000 J of work. The hammer he uses does 835 J of work. What is the efficiency of the hammer?
Power Power - is how quickly energy is transferred. Power = work/time Expressed in Watts.
Calculating Power Ex. 1 A horse performs 15,000 Joules of work pulling a wagon for 20 seconds. What is the horse's power? Ex. 2 If it takes 50 seconds to lift 10 N of books to a height of 7 meters, calculate the power required. Ex. 3 A set of pulleys lifts an 800 N crate 4 meters in 7 seconds. What power was used?