Simple Machines Unit 2 Simple Machines S8P3. Students will investigate the relationship between force, mass, and the motion of objects. c. Demonstrate the effect of simple machines (lever, inclined plane, pulley, wedge, screw, and wheel and axle) on work. Work In science, the word work has a different meaning than you may be familiar with in your everyday life. The scientific definition of work is: • The transfer of energy when a force moves an object over a distance in the same direction of the force. Energy: the ability to do work • If no movement happens, no work is done. • Work = force x distance (W = Fd) • Measured in newton-meters or joules (J) • Examples: pushing a shopping cart, turning a door knob, kicking a soccer ball, lifting a box Work or Not Work A scientist delivers a speech to an audience of his peers. No A bodybuilder lifts a dumbbell above his head. Yes A student pushes against a wall that does not move. No A father pushes a baby in a carriage. Yes A woman carries a grocery bag to her car. No Simple Machines Simple Machine A device that makes work easier by changing the size and/or the direction of the force used to do the work. A simple machine does not help you to do less work. Work with a simple machine = Work without a simple machine No machine can increase both the magnitude of the force and the distance an object travels at the same time. Therefore, there is a tradeoff between force and distance. Work Work Input (Win) work done on a machine Work Output (Wout) work done by a machine Work Law of Conservation of Energy Energy can never be created or destroyed. Energy can be transformed from one form to another. You can never get more work out than what you put in. In an ideal machine... Win = Wout In a real machine... some energy (output force) is given off (“lost”) as friction. Win > Wout Mechanical Advantage (MA): number of times a machine multiplies the effort force 6 Kinds of Simple Machines Inclined Plane Family Inclined Plane Wedge Screw Lever Family Lever Pulley Wheel and Axle Inclined Plane W Fd Inclined Plane A straight, slanted surface used to raise objects because it is higher on one end Example: Ramps, stairs, ladders How does the Simple The Effects on Work Machine Make Work Easier for You? Inclined planes: You use The longer the inclined less effort force over a plane = smaller effort force longer distance. needed = easier work l Mechanical Advantage MA greater than 1 (MA > 1) h Wedge W Fd Wedge A moving inclined plane with 1 or 2 sloping sides Examples: knives, hatchets, ax blade, blades of scissors, nails, teeth, plow, and chisel A wedge transfers force in one direction into force in two directions. Wedges are used to split or cut things apart. W Fd Screw Screw A screw is an inclined plane wrapped around a shaft or cylinder. Examples: a fastener (screw), jar lid, top of jar, drill bit, light bulb, vise The inclined plane allows the screw to move itself when rotated. How does the Simple The Effects on Work Mechanical Machine Make Work Advantage Easier for You? Screws: You use less The closer the threads on MA greater than 1 effort force over a longer the screw = longer the (MA > 1) distance. inclined plane = smaller effort force needed = easier work Lever W Fd Lever a bar that pivots at a fixed point called a fulcrum Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes The 3 Classes of Levers The class of a lever is determined by the location of the effort force, the load, and the fulcrum. Effort force (input force): the force applied to the lever Load (output/resistance force): the object being moved Lever W Fd Mechanical Advantage (MA) Le MA Lr Le Effort arm length (input force) Resistance arm length (output force) must be greater than Lr in order to multiply the force. Le MA Lr Lever W Fd First Class Lever fulcrum is located between the effort force and resistance force (load) can increase force, distance, or neither always changes the direction of force (i.e. a downward effort force on the lever results in an upward movement of the resistance force) Examples: crowbars, scissors, pliers, tin snips, shovels, and seesaws Le MA Lr Lever W Fd Second Class Lever the load (resistance) is located between the fulcrum and the effort force always increases effort force does not change the direction of force effort force moves farther than resistance When the load is located closer to the fulcrum than to the effort force, an increase in force (mechanical advantage) results. Examples: nut crackers, wheel barrows, doors, and bottle openers Le MA Lr Lever W Fd Third Class Lever the effort force is applied between the fulcrum and the resistance force (load) always increases the distance that the effort force travels does not change the direction of force always produce a gain in speed and distance and a corresponding decrease in force Examples: arm, tweezers, hammers, baseball bats, brooms, and rakes Pulley W Fd Pulley grooved wheel with a rope or chain running along the groove a “flexible first-class lever” a load is attached to one end of the rope and a force is applied to the other end How does the Simple Machine Make Work Easier for You? Pulleys: Your effort force changes direction and/or you use less effort force over a longer distance. The Effects on Work Your effort force changes direction = easier work AND/OR More pulleys = smaller effort force needed = easier work Le F Lr Mechanical Advantage MA equal to 1 (MA = 1) MA greater than 1 (MA > 1) Pulley W Fd Mechanical Advantage equal to the number of supporting ropes MA = 0 MA = 1 MA = 2 Pulley W Fd Fixed Pulley MA = 1 does not increase force changes direction of force Pulley W Fd Movable Pulley MA = 2 increases force does not change direction Pulley W Fd Pulley System/Block & Tackle MA = 4 combination of fixed and movable pulleys increases force may or may not change direction W Fd Wheel and Axle Wheel and Axle two wheels of different sizes that rotate together the wheel is always larger than the axle a pair of “rotating levers” Examples: door knob, gears, car axle, pencil sharpener, screw driver, faucet handles Wheel Axle W Fd Wheel and Axle When effort is applied to move the wheel, the axle turns a shorter distance, but moves with more force. The larger the wheel is when compared to the axle, the larger the mechanical advantage. How does the Simple Machine Make Work Easier for You? Wheel and Axles: You use less effort force over a longer distance when turning the wheel. The Effects on Work Mechanical Advantage The larger the MA greater than 1 wheel = smaller (MA > 1) effort force needed = easier work