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: theabilityto dowork • 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 trade-off 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... • In a real machine... • some energy (output force) is given off (“lost”) as friction. Win = Wout 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
h l Inclined Plane • Inclined Plane • A straight, slanted surface used to raise objects because it is higher on one end • Example: Ramps, stairs, ladders
Wedge • 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.
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.
Resistance arm Effort arm Fulcrum Engraving from Mechanics Magazine, London, 1824 “Give me a place to stand and I will move the Earth.” – Archimedes Lever • Lever • a bar that pivots at a fixed point called a fulcrum
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
Effort arm length (input force) Resistance arm length (output force) Lever • Mechanical Advantage (MA) • Le must be greater than Lr in order to multiply the force.
Lever • 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
Lever • 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
Lever • 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 • 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 Le F Lr
Pulley • Mechanical Advantage • equal to the number of supporting ropes MA = 0 MA = 1 MA = 2
Pulley • Fixed Pulley • MA = 1 • does not increase force • changes direction of force
Pulley • Movable Pulley • MA = 2 • increases force • does not change direction
Pulley • Pulley System/Block & Tackle • MA = 4 • combination of fixed and movable pulleys • increases force • may or may not change direction
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
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.