Unit 25

Work, Power, and Machines

Work, Power & Simple

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?

Is the man doing work when he holds the box?

Is the man doing work if he carries the box?

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?

Is the man doing work if he carries the box?

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)

WorkThink 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!

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…

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

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?

Machines : How do machines affect work?

What are other examples of machines?Catapult Wrench Saw

Combine

Sythe Plough Pulley

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

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 lost.

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 Force

Resistance 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.

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

WrenchCrow 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.

Compound Machines Two or more moving parts They can increase efficiency, but still can

never get to 100%.