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Work and Simple Machines. All you need to know about simple machines. Work. Work is when a force is exerted on an object causing it to move. *If an object doesn’t move, NO work is done.
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Work and Simple Machines All you need to know about simple machines
Work • Work is when a force is exerted on an object causing it to move. • *If an object doesn’t move, NO work is done. • *The force must be in the same direction the object moves. (If you carry something it is NOT work because the force is up & the movement is horizontal).
Work (cont.) • Formula :Work = Force X Distance • Measured in Newton meters (Nm) or Joules (J) (Named after physicist James Joule) • 1 Joule = 1 Nm • Sample problem: • If you use 20 N of force to move a desk 10 m how much work is done? • Solution: • Work = F X D • Work = 20 N X 10 m • (D) • Work = 200 Nm or 200 J
What is a machine? • A machine is any device that makes work easier or transfers energy. • A machine makes work easier 3 ways. • Changing force exerted amount • Changing distance you exert force • Changing direction you exert force • *Input force – force you exert on the machine • *Output force (resistance force) – force exerted by the machine.
Mechanical Advantage • Mechanical advantage = output force / input force • If the mechanical advantage is greater than 1 it multiplies force. • If the mechanical advantage is less than 1 it multiplies distance • If the mechanical advantage = 1 it changes direction.
Efficiency • Efficiency is used to see how good a machine is & how much work is wasted overcoming outside forces like friction or tools that stick. • *Efficiency = output work / input work X 100% • Ideal mechanical advantage – how the machine would work without friction. • Actual mechanical advantage – the actual advantage of a machine including friction. • Sample problem: • You go out to cut grass with a non motorized push lawn mower. You put 250,000 J of work into mowing the work done by the mower is 200,000 J what is the mower’s efficiency? • Solution • Efficiency = output work / input work X 100% • Efficiency = 200,000 J / 250,000 J X 100% • .8 X 100% = 80% • The lawn mower is 80% efficient.
Simple Machines • There are 6 basic kinds of simple machines. • Inclined plane • Wedge • Screw • Lever • Wheel & Axle • Pulley
Inclined Planes • Inclined planes – a flat slanted surface like a ramp. It allows you to exert your input force over a longer distance. • *The longer the inclined plane the greater the mechanical advantage if your planes height doesn’t change.
Wedges • Wedge – 1 or 2 moving inclined planes. • An axe head & a zipper are examples of wedges. • It multiplies the force to do more work
Screws • Screw- an inclined plane wrapped around a cylinder • The closer the threads of a screw the greater the mechanical advantage.
Levers • Lever- a rigid bar that moves around a fixed point. • The fixed point is called a fulcrum. • There are 3 classes of levers • 1st class- multiplies forces or distance depending on fulcrum placement. Also changes input force direction. Ex. Scissors, pliers, seesaws. • 2nd class-multiplies forces but don’t change direction of input force. Ex. Wheelbarrows, doors, bottle openers, nutcrackers. • 3rd class - multiplies distance but doesn’t change input force direction. Ex. Fishing poles, baseball bats, shovels. • The mechanical advantage is greater than 1 if the fulcrum is farther from the input force but you can move it a shorter distance.
Wheel & Axle • Wheel & Axle – 2 cylinders stuck together that rotate about a common axis. • The larger (in diameter) cylinder is the wheel • The smaller (in diameter) cylinder is the axle. • Ex. Doorknobs, screwdrivers, vehicles.
Pulley • Pulley – a grooved wheel with a rope, chain, etc. wrapped around it to pull other objects with. • You change the amount & direction of the input force • Pulley types: • Fixed pulleys – change direction but not amount of force. Mechanical advantage (MA)= 1. • Movable pulleys – change amount of input force. MA = 2. • Pulley systems – also called “block & tackle” • The ideal mechanical advantage of a pulley system is = the number of sections that support the weight.
Compound Machines • A compound machine is a machine that uses 2 or more simple machines. • To calculate the MA of a compound machine you must calculate the MA of each simple machine in it & add them together. • Ex. Pencil sharpener (Wheel & axle & screws). • Gears are toothed wheels that fit together to help turn other objects. (They connect wheels & Axles to each other.)
Machines in the Human Body • Many body parts act like simple machines & help the body do work. • Muscles & tendons act like levers. • Teeth act like wedges.
