Presentation on theme: "Work and Simple Machines"— Presentation transcript:
1Work and Simple Machines S8P3. Students will investigate relationship between force, mass, and the motion of objects. a. Determine the relationship between velocity and acceleration. b. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction. c. Demonstrate the effect of simple machines (lever, inclined plane, pulley, wedge, screw, and wheel and axle) on work.
2Essential Questions When is work done on an object? How do you determine the work done on an object?What is the relationship between power and work?
12How Machines Do Work – p.412-415 A MACHINE is a device that allows you to do work one of three ways that is easier:It can change the amount of force that you exertIt can change the distance over which you exert your forceIt can change the direction in which you exert your force
13Input and Output forces – p.413 Input force is the force that YOU exert on the machine, causing it to move a certain distance. (Ex: You exerted a force on a shovel)Output force is the force that the MACHINE exerts over another distance. (Ex: the shovel’s force on the dirt)
14Input and Output work – p.413 Input work is the input force times the input distanceOutput work is the output force times the output distanceWhen you use a machine, the amount of output work can never be greater than the amount of input work.
15Mechanical Advantage – p.416-417 A machine’s mechanical advantage is the number of times a machine increases force exerted on it.
16Efficiency of Machines – p.417-419 A machine’s efficiency(expressed as a percent) compares the output work to the input work.Some work is always wasted overcoming the force of friction.Efficiency = Output work x 100%Input work
19Inclined Plane p.423 A flat, sloped surface Ex: ramp Allows you to exert your input force over a longer distance.Ideal mechanical advantage =Length of inclineHeight of incline
20Wedge p.424A device that is thick at one end and tapers to a thin edge at the other end.2 inclined planes back to back that can move.Ex: ax, knife, zipperAllows the output force at a 90o angle to the slopeIdeal mechanical advantage =Length of wedgeWidth of wedge
21Screw p.425 An inclined plane wrapped around a cylinder. Ex: lids of jarsThreads of screw increase the distance over which you exert the input force. As threads of screw turn, they exert an output force, holding the object in place.Ideal mechanical advantage =Length around the threadsLength of screw
22Levers pA rigid bar that is free to pivot, or rotate on a fixed point, called the fulcrum.Ex: wheelbarrow, hockey stick, crowbar3 types (see next slide)Ideal mechanical advantage =Distance from fulcrum to input forceDistance from fulcrum to output force
231st class levers p. 427 Change the direction of the input force. If the fulcrum is closer to the output force, these levers also increase force.If the fulcrum is closer to the input force, these levers also increase distance.Examples: scissors, pliers, seesaws
242nd class levers p. 427 Change the direction of the input force. If the fulcrum is closer to the output force, these levers also increase force.If the fulcrum is closer to the input force, these levers also increase distance.Examples: scissors, pliers, seesaws, wheelbarrow
253rd class levers p. 427Increase distance, but do not change the direction of the input force.Examples: fishing poles, shovels, baseball bats, hockey sticks, brooms
26Wheel and Axle pTwo circular or cylindrical objects fastened together that rotate about a common axis.Ex: screwdriver, doorknob, steering wheelYou apply an input force to turn the handle, or wheel. Because the wheel is larger than the axle, the axle rotates and exerts a large output force. The W&A increases your force, but you must exert your force over a long distance.Ideal mechanical advantage =Radius of wheelRadius of axle
27Pulley pA grooved wheel with a rope or cable wrapped around it.Ex: flagpole, crane, weight machine, blinds/drapesYou pull on one end of a rope (input force), while the output force pulls the object you want to move. Makes work easier in 2 ways (see next slide)Ideal mechanical advantage =number of sections of rope that support the object.
28Simple Machines in the Body p.432 Your front teeth (incisors) are wedges!Most of the machines in your body are levers that consist of bones and muscles