Work and Power

What does it mean to do work? In science, you do work when you exert a force on an object that causes the object to move a distance. Ex. You push a child on a swing Ex. You lift a bag of groceries

No work is done! In order to do work on an object, the object must move some distance as a result of the force. The force must be exerted in the same direction as the object’s motion. In the drawing at right, the person is pushing on a wall. Because the wall is not moving, no work is done.

How much work is done? The amount of work done on any object is found by multiplying the force times the distance. Example, You exert a force of 20N to push a desk 10m. How much work is done? Work = force x distance Work = 20N x 10m Work = 200 N·m or 200 J

The Joule The joule was named in honor of James Prescott Joule a physicist in the 1800’s. One joule (J) is the amount of work you do when you use a force of 1N to move an object 1 meter.

Power Power is the rate at which work is done. Power is calculated by dividing the amount of work done by the amount of time taken to do it. Power = work / time and is measured in watts. If a force of 8000N is needed to lift a beam 75 m in 30 s then the amount power done by the crane is found from the work done and the time. Power = 8000N x 75 m / 30 s = 20,000 watts

James Watt The watt is named in honor of James Watt who invented the steam engine. A watt is a small unit of power. 1000 watts equals one kilowatt (kW). A washing machine uses about one kilowatt an hour when it is running. James Watt also introduced the word “horsepower”. He compared his steam engine to the work of a horse hauling coal. One horsepower is the amt. of work done by a horse to lift a 33,000-pound weight a distance of one foot in one minute. One horsepower equals 746 watts.

How do you make work easier? Work is made easier by machines. A machine changes the amount of force you exert, the distance over which you exert the force, or the direction of the force. All modern machines are based on six simple machines.

Examples of Compound Machines

Inclined Plane The inclined plane makes work easier by spreading out the effort over a greater distance (the ramp), rather than trying to lift it.

Wedge This wedge acts to split the wood. Instead of requiring a large force, the wedge multiplies the small force to do the job.

Wheel and axle The larger the wheel, the less force required to move the load. Notice the force is moving in the same direction as the load.

Pulley The larger the wheel, the less force required to move the load (just like the wheel and axle). The load and force move in opposite directions.

Lever All levers have two parts: the bar and the fulcrum. The longer the lever, the less force needed to move the load. The load and force move in opposite directions. There are three classes of levers: first, second and third class.

Classes of Levers First class: the fulcrum can be moved closer or further away from the effort, and either multiply the force you apply or the distance. Think – seesaw. Move fulcrum closer to heavier person, then you multiply your force, and the seesaw balances! Second class: always multiply force. Think – wheelbarrow. Third class: Multiply distance but do not change the direction of the force. Think – a rake – sure makes getting all those leaves much easier!

Screw The screw consists of an inclined plane and a post. The longer the inclined plane, the less force required to move the load. Force and load move in the same direction.

Engineering feats! Machines have helped to create many of the world’s most beautiful and useful constructions.

Great Pyramid of Giza, 2550 B.C. Workers used wooden wedges to cut 2.3 million blocks of stone. The wedges were driven into cracks in the rock, which split the rock. Workers hauled the blocks up inclined planes to the top of the pyramid’s walls.

Theatre at Epidaurus, Greece 500 B.C. The Greeks used a crane powered by pulleys to lift the stone blocks to build the theatre. The crane was also used to lower actors to the stage during performances.

Yingxian Pagoda, China 1056 A.D. Slanted wooden beams called ang were used as levers to build up the roof of this pagoda. The of weight of the center of the roof presses down on one end of the beam. The other end swings up to support the outer edge of the roof.

Notre Dame Cathedral, Paris Many new inventions were needed to construct these enormous buildings. They needed to support the heavy roof, and stained glass windows. The wheelbarrow was invented to move materials around. Cranes, winches and steeplejacks were used to raise construction materials.

Empire State Building, NYC, NY It took one year and 45 days to complete the building. Large cranes were required to raise the steel girders to the higher floors. A railway car was built at to move materials at the site. This was more efficient than a wheelbarrow as it held eight times as much material.

The Chunnel, United Kingdom to France At a cost of 22 billion dollars, the tunnel connecting the UK and France is the most expensive construction project in the world. A tunnel-boring device was used to move the material out of the way while maintaining the wall structures.

Sydney Opera House It took three tower cranes to complete construction of the sails. Two mechanical stage lifts move scenery and props for performances.

Examples of Compound Machines