Chapter 8  Work, Power, and Machines

Work  Work- A force acting through a distance.  The distance that the object moves must be in the same direction as the force applied to the object. Lifting Force Movement Distance What force is having to be overcome by the lifting force? Weight! Weight

Work  Work- A force acting through a distance.  The distance that the object moves must be in the same direction as the force applied to the object. Pushing Force Distance Moved What force is having to be overcome by the pushing force? Friction!

Lifting Force Movement Distance  Work is not done every time a force is applied.  Work is done only when a force moves an object in the same direction as the applied force. Is the man doing work when he lifts the box? Yes. Is the man doing work when he holds the box? No! Is the man doing work if he carries the box? No! (But he IS applying a force)

Work Think about pushing on wall that does not move. A force is applied but the wall has not moved a distance. Is work done? No! But I did expend energy applying a force!

Calculating Work  Work = force (N) x distance (m)  W = F x D  The unit for work is a Joule (J).  If you lifted an object weighing 1N through a distance of 1m, you did 1 Joule of work: W = FD = 1N x 1m = 1 J A Newton is about ¼ of a pound. Is a Joule of work…very much work? No!

Formula for work Work = Force x Distance  The unit of force is newtons  The unit of distance is meters  The unit of work is newton-meters  One newton-meter is equal to one joule  So, the unit of work is a joule

Calculating Work  If you lifted an object weighing 200 N through a distance of 0.5m, how much work would you do? Work = force x distance = 200 N x 0.5m = 100 J.

Power  Power is the rate at which work is done, or the amount of work per unit of time. Two men can move a lot of sand using shovels… …but a front-end loader can do it in less time… …because the front-end loader has more POWER.

Power  Power is the rate at which work is done, or the amount of work per unit of time.  Power = work / time  or  Power = force x distance / time Because work = force x distance

Power  The unit for power is watt (W). One watt is equal to 1 joule per second (1 J/sec).  Large quantities of power are measured in kilowatts (kW).  One kilowatt equals _____ watts.  One million watts is a megawatt 1000

How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time Problems: Power Work Force

How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s a distance of 10 meters in 5 seconds?. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time Problems: Power Work Force Force=10kg x 2m/s/s Force=20 kg x m/s/s Force=20 N Work = 20N x 10m Work = 200 N x m Work = 200 Joules Power = 200J/5s Power = 40J/s Power = 40 watts

Machines  A machine is a device that makes work easier.  A machine is ANY device that helps you to do something.  They can be “simple” or “compound”

Machines : How do machines affect work?

 What are other examples of machines? CatapultWrenchSaw Combine SythePloughPulley

Machines : How do machines affect work? Hand Sewing and a Sewing Machine take the same amount of work to make a dress. But, the sewing machine is faster

Automatic Factory Looms

Machine Efficiency: Input and Output  There are always two types of work involved in using a machine.  Input work is the work that goes into the machine.  Output work is the work that comes out of the machine.

Machine Efficiency: Input and Output  The efficiency of a machine can be calculated: Efficiency = (work output / work input) x 100 Efficiency = (work output / work input) x 100 This is easy to remember…think about it… If you put 100 Joules of work into a pencil sharpener, but only got 80 Joules of work out, the pencil sharpener is 80% efficient: (80 Joules / 100 Joules) x 100 = 80% efficiency

 Machine efficiency can never be greater than or equal to 100% because some energy is always converted. Friction makes every machine less efficient.

Machine Efficiency  The friction in a machine “wastes” energy in the form of heat.  Machines with the smallest amount of friction are the most efficient.

Machines  Machines make work easier because they change the size or the direction of the force put into the machine.

Bolts and Work  It is a lot easier to remove a bolt with a wrench than your fingers.  The longer the wrench, the easier it is to exert the force to remove the bolt.

Determining How Helpful a Machine Is  Besides the efficiency of a machine we also can determine how helpful a machine is.

Determining How Helpful a Machine Is  What we mean by how helpful is how many times the machine multiples the effort force to overcome the resistance force Effort ForceResistance Force

Determining How Helpful a Machine Is  The number of times a machine multiplies the effort force is called the mechanical advantage.  This tells you how much force is gained by using the machine.  The more times the machine multiples the effort force, the easier it is to do the job.  Note: Use the links on the last slide to help you with calculating MA for each type of machine.

Six Simple Machines WHEEL & AXLE

Lever  Lever : A lever is a rigid bar that is free to pivot, or move about a fixed point. The fixed point is called the fulcrum. See Saw Hand Truck Wrench Crow Bar

Inclined Plane  An inclined plane decreases the size of the effort force needed to move an object. A ramp that reduces force necessary to climb

Wedge  Wedge: An inclined plane that moves.  In a wedge, instead of an object moving along the inclined plane, the inclined plane itself moves to raise the object. Ice Wedging

Wheel and Axle  A wheel and axle is a simple machine made up of two circular objects of different sizes. The wheel is the larger object. It turns around a smaller object called the axle.

Wheel and Axle  The mechanical advantage depends on the radius of the wheel and of the axle.

Screw  Screw : A screw is inclined plane wrapped around a central bar or cylinder to form a spiral.

Pulley  A pulley is a rope, belt, or chain wrapped around a grooved wheel.  A pulley can function in two ways. It can change the direction of a force or the amount of force. The Mechanical Advantage of a pulley is equal to the number of ropes supporting the pulley: MA = # of supporting ropes

Compound Machines  Two or more moving parts  They can increase efficiency, but still can never get to 100%.

Use this site to help you learn how to calculate the mechanical advantage for each machine. Click on each machine type to see an example of the calculation for mechanical advantage for that machine. Use this site to help you learn the types of machines and examples for each. LINKS TO HELP YOU STUDY