Work, Power, andSimple Machines

Book Chapter 14

Work

Power

Simple Machines

Work

Work is a force acting through a distance. In order for work to be done on an object, a force must move it. The object MUST move in the direction of the force. If there is no movement, there is no work done on the object.

Work is the product of a force applied to an object and the distance through which the force is applied.

Work = force x distance

Example

Lifting a barbell:

If a 100 N barbell is lifted 1 meter, there are 100 J of work done on the barbell by the person. If the barbell is lifted 2 meters high, twice the amount (or 200 J) of work is done. Once the barbell is lifted and held in air, NO work is being done – the object is not moving a distance.

Work falls into two categories:

1. Work done against another force

- An archer stretches the bowstring; she is doing work against the elastic

forces of the bow.

2. Work done to change the speed of an object

- It takes work to increase the speed of a car from 10 mph to 20 mph

Joule

The SI (metric) unit of work (and of energy) is the Joule (J).

1 Joule = (1 Newton)*(1 meter) = 1 Nm

(force) * (distance)

Calculating Work

A man picks a 350 N crate up and places it on a 2 meter high shelf. How much work is being done on the crate?

700JW

N)(2m) (350W

distanceforceWork

There are 700 Joules of work done on the crate.

Power

Power is the rate at which work is done.

The faster an object can do work, the more power it has.

Power and work are not the same thing!

Time

WorkPower

Power

Example: A car engine

One engine can do 100 J of work in 2 seconds. Its power is 50 J/s, or 50 Watts (W). An engine that is twice as powerful can do 100 J of work (same amount) in 1 second (Half the time). Its power is 100 Watts.

Units for Power

The SI unit of power is the Watt (W).

The unit we use (British/English) is the horsepower (hp).

1 hp = 0.75 kilowatts (kW) = 750 Watts

James Watt

When James Watt built his steam engine in the 1760’s, he wanted to use a unit of power that most people recognized. The horse was the most common source of power during those times, so he decided to compare the work his steam engine could do to the amount of work a horse could do. Watt determined that a good horse could move a 750 N object a distance of about 1 meter in 1 second, which is a power output of 750 Watts. He set 1 horsepower equal to this number of watts.

Calculating Power

A crane can lift a 250 N safe to a height of 5 meters in only 3 seconds. What is the power output of the crane?

WPs

JP

mNP

7.4163

1250sec3

)5)(250(time

distanceforceP

time

workPower

Simple Machines

A machine is a device that makes work easier by changing the size of the force needed, the direction of a force, or the distance over which the force acts.

The six simple machines are the basic components of every tool we use.

In each case, we use a simple machine to lessen the amount of work needed to get a desired result.

Work Input of a Machine

Because of friction, the work done by a machine is always less than the work done on a machine.

The work you exert on a machine is the input force.

The distance the input force acts through is the input distance.

The work done by the input force acting through the input distance is called the work input.

Work Output of a Machine

The force exerted by the machine is the output force.

The distance the output force is exerted through is the output distance.

The work output of a machine is the output force multiplied by the output distance.

Actual Mechanical Advantage

The Mechanical Advantage of a machine is the number of times that the machine increases an input force.

To find the Actual Mechanical Advantage (AMA):

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ForceInput

ForceOutputAMA

Ideal Mechanical Advantage

DistanceOutput

DistanceInputIMA

The ideal mechanical advantage of a machine is the mechanical advantage in the absence of friction.

Because friction is always present, the actual mechanical advantage is always less than the ideal mechanical advantage

Efficiency

The previous examples were ideal cases – all energy put in turned into work out. This is never the case; the amount of work out is ALWAYS less than the work put in.

%100xinputwork

outputworkefficiency

Efficiency

Example: 100 J of work put in and only 60 J of work done

Efficiency = (60 J out) ÷ (100 J in) = 0.6 or 60%

The wasted energy usually turns into heat. Brand new gas engine cars only reach 35% efficiency.

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Six Simple Machines

There are six types of simple machines:

1. Inclined Plane

2. Wheel and Axle

3. Wedge

4. Screw

5. Pulley

6. Lever

Inclined Plane

An inclined plane is a slanting surface connecting a lower level to a higher level

                                                                                                                   

Wheel and Axle

A wheel with a rod, called an axle, through its center lifts or moves loads.

                                           

                         

                              

Wedge

A wedge is an object with at least one slanting side ending in a sharp edge, which cuts material apart.

                                                     

                

                                                                     

Screw

A screw is an inclined plane wrapped around a pole which holds

things together or lifts materials. This simple machine is a modification of the wedge designed to yield a very large mechanical advantage in minimum space. The screw is

essentially a transfer device of motion and/or force.                                                       

Pulley

A pulley is a simple machine that uses grooved wheels and a rope to raise, lower or move a load.

A single pulley can change the direction of a force. Multiple pulleys can multiply the amount of input force.

The more strands of rope supporting the load, the higher the mechanical advantage of a pulley system.

Pulley

                                                                    

Lever

A lever is a stiff bar that rests on a support called a fulcrum which lifts or moves loads. 

                                                                                                          

Lever

The lever is used to lift up loads.

Work Input = Work Output

Since work = (force)(distance), then either force or distance can be changed.

Force Down

Force Up

Lever

The larger the lever, the less force needed to push down. The fulcrum, or pivot point can be changed to change the length of the lever.

The lever arm is measured from the pivot point the place where the force is applied to the lever.

Three Classes of Levers

Type 1: Fulcrum between the force and the load

Examples: Seesaw, crowbar

Three Classes of Levers

Type 2: Load is between the force and the fulcrum

Examples: Car jack, hand bottle cap opener

Three Classes of Levers

Type 3: Fulcrum is at one end and the load is at the other

Examples: Bicep, Construction crane

Compound Machines

A compound machine is a combination of two or more simple machines that operate together.

Examples: Automobiles, washing machine, clock