# 23 Unit 23 Work_and_Power

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### Work, Power & Simple Machines:

Work, Power & Simple Machines

### Work:

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 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!

### Slide 4:

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) (But he IS applying a force)

### Work:

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:

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 Think about how much force a Newton is…about ¼ of a pound. Is a Joule of work…very much work? No!

### Calculating Work:

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 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 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:

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. 1000

### Machines : How do machines affect work?:

Machines : How do machines affect work? A machine is a device that makes work easier. A machine is ANY device that helps you to do something.

### Machines : How do machines affect work?:

Machines : How do machines affect work? What are other examples of machines?

### Machines : How do machines affect work?:

Machines : How do machines affect work?

### Machines : How do machines affect work?:

Machines : How do machines affect work?

### Machines : How do machines affect work?:

Machines : How do machines affect work?

### Machine Efficiency: Input and Output:

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 (like turning a pencil sharpener). Output work is the work that comes out of the machine (like the grinding of the pencil). How hard was that??

### Machine Efficiency: Input and Output:

Machine Efficiency: Input and Output The efficiency of a machine can be calculated: 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

### Slide 18:

Machine efficiency can never be greater than or equal to 100% because the work output can never be greater than the work input. In other words, there is no machine that has a 100% efficiency! Why not? Think…it’s a force that opposes motion… Friction! Friction makes every machine < 100% efficient

### Machine Efficiency:

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.

### Machine Efficiency:

Machine Efficiency The closer the work output is to work input, the more efficient the machine. Toyota hybrid “concept car”

### Slide 21:

Many household appliances have energy guides that tell the consumer how efficient the appliance is. The more efficient the appliance the more money the consumer will save.

### Slide 22:

You can also look for the “energy star” label on more efficient machines.

### Machines:

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

### Machines:

Machines Most machines make work easier by multiplying either force or distance. Which does which? Force! Distance!

### Machines:

Machines What is multiplied, force or distance? Force Distance Force when “prying” the dirt free, distance when lifting it up out of the hole or into a truck or wheelbarrow

### Determining How Helpful a Machine Is:

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:

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 Force Resistance Force

### Determining How Helpful a Machine Is:

Determining How Helpful a Machine Is Effort Force or Resistance Force? ? ? ? Effort Force Effort Force Resistance Force

### Determining How Helpful a Machine Is:

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.

### Quick quiz –open notes:

Quick quiz –open notes 1. What is a machine? 2. Describe relationship between friction and the efficiency of a machine.

### 6 Kinds of Simple Machines:

6 Kinds of Simple Machines Inclined Plane Screw Wedge Lever Pulley Wheel & Axle

### Inclined Plane:

Inclined Plane Inclined plane: A ramp is an example of an inclined plane. It is simply a flat slanted surface. It has no moving parts.

### Inclined Plane:

Inclined Plane An inclined plane decreases the size of the effort force needed to move an object.

### Wedge:

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.

### Lever:

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.

### Wheel and Axle:

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. Video

### Wheel and Axle:

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

### Screw:

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

### Pulley:

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. 