“Work and Energy” Chapter 13

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

Work W = F x d (J) (N) (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 mush 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? A car has run out of gas. Fortunately, there is a gas station nearby. You mush 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 = 715 N ? 27,200 J d = J / 715 N d = 38 m d = W / F W F * d

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

Power 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? 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 = ? 12 s 8,170,000 W 8,170,000 W x 12 s W = 8,170,000 W x 12 s d = 98,000,000 J W = P * t W P * t

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

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

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

Simple Machines There are three classes of levers: There are three classes of levers: F R E F R E E R F

Simple Machines Pulleys Pulleys A pulley is a modified lever. A pulley is a modified lever. 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. 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. 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. 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. 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. Multiple pulleys put together are called block and tackle or compound pulleys.

Simple Machines Wheel and Axle Wheel and Axle A wheel and axle is made of a lever or a pulley (wheel) connected to a shaft (the axle). A wheel and axle is made of a lever or a pulley (wheel) connected to a shaft (the axle). Small input for large output. Small input for large output. Examples: steering wheel, screwdriver, crank. Examples: steering wheel, screwdriver, crank.

Simple Machines The three simple machines in the incline plane family are: The three simple machines in the incline plane family are: Incline Plane Incline Plane Wedge Wedge Screw Screw

Simple Machines Incline Plane Incline Plane Incline planes or ramps redirect the force applied on an object to lift it upwards. Incline 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. An inclined plane turns a small input force into a large output force by spreading the work over a large distance.

Simple Machines Incline Plane Incline Plane

Simple Machines Wedge Wedge A wedge functions like two inclined planes back to back. A wedge functions like two inclined planes back to back. A wedge turns a single downward force into two forces directed out to the sides. A wedge turns a single downward force into two forces directed out to the sides. Examples: axes and nails. Examples: axes and nails.

Simple Machines Wedge Wedge

Simple Machines Screw Screw A screw is an inclined plane wrapped around the cylinder. 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. Tightening a screw is like pushing an object up a ramp; it requires a small force acting over a long distance. Examples: cork screw, jar lids, and spiral stair cases. Examples: cork screw, jar lids, and spiral stair cases.

Simple Machines Screw Screw

Compound Machines Compound Machines Compound Machines Many devices you use every day are made of more than one simple 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. 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. Machines help us do work by redistributing the work that we put into them. They do this by: They do this by: Changing the direction of the input force. Changing the direction of the input force. Change output force by changing the distance over which the force is applied (multiplying the 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 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 (f r ) OR effort distance (d e ) effort force (f e ) resistance distance (d r ) effort force (f e ) resistance distance (d r ) *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: Things to keep in mind when calculating mechanical advantage: Ramps: Ramps: dede drdr

Mechanical Advantage Things to keep in mind when calculating mechanical advantage: Things to keep in mind when calculating mechanical advantage: Levers: Levers: drdr dede drdr dede frfr fefe frfr fefe

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

Mechanical Advantage 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? 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 =fe =fe =fe =? fr =fr =fr =fr = 1500 N MA = 10 frfr f e x MA f e = f r / MA f e = 1500 N / 10 f e = 150 N

Mechanical Advantage 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? 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=dr=dr=dr=? de =de =de =de = 3 cm MA = 0.85 dede d r x MA d r = d e / MA d r = 3 cm / 0.85 d r = 3.5 cm