“Work, Power, and Simple Machines” Chapter 13

Work Work is the quantity that measures the effects of a force acting over a distance. WORK IS ONLY DONE WHEN SOMETHING PHYSICALLY CHANGES POSITION. vs.

The SI Unit for Work is Joules (J) W = F x d Work = Force x distance (J) = (N) x (m) The SI Unit for Work is Joules (J) W F x d

Work A car has run out of gas. Fortunately, there is a gas station nearby. You must exert a force of 715 N on the car in order to move it. By the time you reach the station, you have done 27,200 J of work. How far have you pushed the car? W = F= d = 27,200 J d = W / F W F * d d = 27200 J / 715 N 715 N d = 38 m ?

Power Power is defined as the rate at which work is done (how much work is done in a certain amount of time).

The SI unit for Power is Watts W Power = work / time P = W / t (W) = (J) / (s) The SI unit for Power is Watts W W P x t

Power The world’s most powerful tugboats, which are built in Finland, are capable of providing 8,170,000 W of power. How much work does one of these tugboats do in 12.0 s? P = W= t = 8,170,000 W W = P * t W P * t W = 8,170,000 W x 12 s ? W = 98,040,000 J 12 s

WORK POWER AND ENERGY http://science360.gov/obj/video/c5be5456-2e39-49a7-8118-218868df89eb/work-energy-power http://www.neok12.com/video/Simple-Machines/zX435c524c537c7d5d180606.htm

Simple Machines Machines help us do work by redistributing the work that we put into them. They do this by: Changing the direction of the input force. Change output force by changing the distance over which the force is applied (multiplying the force).

Simple Machines Simple Machines are the most basic machines. There are two families of simple machines: Lever Inclined Plane

Simple Machines The three simple machines in the lever family are: Pulley Wheel and Axle

Simple Machines Levers A lever is a board or bar that rests on a turning point. This turning point is called the fulcrum. An object that a lever moves is called the resistance or load. The closer the object is to the fulcrum, the easier it is to move.

Simple Machines There are three classes of levers. Depending on the class, levers may change the direction and amount of input force. F R E E R F F R E

Simple Machines Pulleys A pulley is a modified lever. It is a GROOVED wheel with a rope, chain, or cable around it. The point in the middle of the pulley is like the fulcrum of the lever, and the rest of the pulley acts like the rigid arm of a lever. Two Types: FIXED and MOVABLE

PULLEYS cont’d Because the distance of the fulcrum is the same on both sides of a pulley, a single, fixed pulley has a mechanical advantage of 1. Using moving pulleys or more than one pulley at a time can increase mechanical advantage. Multiple pulleys put together are called block and tackle or compound pulleys.

Simple Machines Wheel and Axle A wheel and axle is made of a lever or a pulley (wheel) connected to a shaft (the axle). Made up of two circular objects of different size that move together. Small input for large output. [Turn larger object to decrease input force needed; turn smaller object to increase distance.

Wheel & Axle Examples: steering wheel, screwdriver, crank.

Simple Machines The three simple machines in the inclined plane family are: Inclined Plane Wedge Screw

Simple Machines Inclined Plane A flat surface with one end higher than another (ex. Ramp) Inclined planes or ramps redirect the force applied on an object to lift it upwards. An inclined plane turns a small input force into a large output force by spreading the work over a large distance.

Simple Machines Inclined Plane

Simple Machines Wedge Functions like 2 inclined planes back to back. An object with at least one slanting side ending in a sharp edge. The longer, thinner edge decreases input force needed to overcome resistance. A wedge turns a single downward force into two forces directed out to the sides. Examples: axes and nails.

Simple Machines Wedge

Simple Machines Screw A screw is an inclined plane wrapped around the cylinder. Tightening a screw is like pushing an object up a ramp; it requires a small force acting over a long distance. Threads increase distance and output force as well as decrease input force .

Simple Machines Screw Examples: cork screw, jar lids, and spiral stair cases

Compound Machines Compound Machines Many devices you use every day are made of more than one simple machines. A machine that combines two or more simple machines is called a compound machine.

Mechanical Advantage Machines help us do work by redistributing the work that we put into them. They do this by: Changing the direction of the input force. Change output force by changing the distance over which the force is applied (multiplying the force).

Mechanical Advantage Mechanical Advantage is defined as the ratio between the output force and the input force. It is also defined as the ratio between input distance and output distance.

Mechanical Advantage MA= resistance force (fr) OR effort distance (de) effort force (fe) resistance distance (dr) *Remember: The units for force is Newtons (N) and the SI units for distance is meters (m). Mechanical advantage has NO units.

Mechanical Advantage Things to keep in mind when calculating mechanical advantage: Ramps: de dr

Mechanical Advantage Things to keep in mind when calculating mechanical advantage: Levers: de fe fr fr fe de dr dr

Mechanical Advantage Things to keep in mind when calculating mechanical advantage: Pulleys: The effort force is what you pull with and the resistance force is the load you’re lifting. Count the number of ropes pulling up to determine the MA. fe fe 1 fe 2 fr fr 2 fr

Mechanical Advantage fe = fr / MA fe = 1500 N / 10 fe = 150 N A mover uses a pulley system with a mechanical advantage of 10.0 to lift a 1500 N piano 3.5 m. What is the effort force required by the mover? fe = ? fr = 1500 N MA = 10 fe = fr / MA fe = 1500 N / 10 fe = 150 N fr fe x MA

Mechanical Advantage dr = de / MA dr = 3 cm / 0.85 dr = 3.5 cm An axe used to split wood is driven into a piece of wood for an effort distance of 3.0 cm. If the mechanical advantage of the axe is 0.85, how far apart (resistance distance) is the wood split? dr= ? de = 3 cm MA = 0.85 dr = de / MA dr = 3 cm / 0.85 dr = 3.5 cm de dr x MA