Work and Simple Machines All you need to

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 – 1 st class- multiplies forces or distance depending on fulcrum placement. Also changes input force direction. Ex. Scissors, pliers, seesaws. – 2 nd class- multiplies forces but don’t change direction of input force. Ex. Wheelbarrows, doors, bottle openers, nutcrackers. – 3 rd 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.