Chapter 8

Work, Power, Energy and Machines

Work: applying a force in the direction of motion

Work=Force x distance

Work is measured in joules (J). 1 joule is equal to a force of 1 N exerted over a distance of 1 m.

Work = Force|| x Distance

In this case, the distance is the magnitude of the displacement.

Only the component of force parallel to the displacement does work

x

F

xxdFW

xF

yF

PowerPower is equal to the amount of work

done per unit time.

interval time

donework Power

The unit for power is the Joule/second which is also called a Watt.

measured in watts (W)One watt of power is expended when one joule of work is done in one second.

Power

What is horsepower?

1 horsepower = 746 Watts

A horse exerting 1 horsepower can raise 330 pounds of coal 100 feet in a minute, or 33 pounds of coal 1,000 feet in one minute, or 1,000 pounds 33 feet in one minute.

More on horsepower...

1 horsepower (over the course of an hour) is equivalent to 2,545 BTU (British thermal units). If you took that 746 watts and ran it through an electric heater for an hour, it would produce 2,545 BTU (where a BTU is the amount of energy needed to raise the temperature of 1 pound of water 1 degree F).

One BTU is equal to 1,055 joules, or 252 gram-calories or 0.252 food Calories. Presumably, a horse producing 1 horsepower would burn 641 Calories in one hour if it were 100-percent efficient.

Energy…the BIG Energy…the BIG picturepicture

The Universe is made up of matter and energy.

Energy is the “mover” of matter. There are many forms of energy.Conversions from one form of energy

to another continually occur.Energy cannot be created or

destroyed.

Energy possessed by an object due to its motion or its stored energy of position.

It can either be potential energy or kinetic energy.

All forms of energy are measured in joules (J).

Mechanical Energy

(PE) stored chemical energy or energy of position.

Types: elastic, gravitational and chemical.

Potential Energy

Gravitational Potential Energy = mass x gravity x height

EPE = mgh

(KE) energy of motion. Kinetic energy depends on both mass and velocity

Kinetic Energy

Kinetic energy = ½ mass x velocity ²

Ek = 1/2mv2

Mechanical Energy Conversions …total energy is constant

Calculate speed at positions B,C and D.

More Energy Transfer

High speed winds are used to do work on the blades of a turbine at the so-called wind farm.

Mech. Energy from the air gives the air particles the ability to apply a force to the blades.

As the blades spin, their energy is subsequently converted into electrical energy (a non-mechanical form of energy) and supplied to homes and industries in order to run electrical appliances.

Energy Transfer...

This diagram shows that the boys potential energy is changing. Explain what is happening, in other words how is the energy changing?

Energy cannot be created or destroyed.

Energy can be transformed from one form to another, but the total amount of energy never changes.

Law of Conservation of Energy

Machine: a device

used to multiply forces or change the direction of forces.

Simple Machine: a machine that has only one motion. Two basic simple machines are

levers and pulleys.

A bar that freely pivots around a fixed point called the fulcrum. A lever multiplies a force

Lever

Classes of Levers

1st Class

3rd Class

2nd Class

Your Body as a Lever...

1st Class

2nd Class

3rd Class

Pulley: grooved wheel with a rope running through it. It changes the direction of the force.

Fixed Pulley

A pulley in which the wheel does not move.

change the direction of the effort force. It does not increase the size of the effort force. The effort force is equal to

the resistance force in a fixed pulley; therefore, the mechanical advantage (MA) of a fixed pulley is equal to 1.

Movable Pulley

A moveable pulley does not change the direction of the effort force but does increase the size of the force.

When the rope is pulled, the pulley and the load come up. The mechanical advantage

(MA) of a moveable pulley may be determined by counting the number of ropes that lift the load.

Pulley System

The pulleys are used to increase the mechanical advantage of the system.

A pulley system's mechanical advantage (MA) is equal to the number of supporting ropes.

Determine the MA of the following pulleys:

Other Simple Machines

Wheel & Axle

Screw

Wedge

Work input = work output assuming that there is no friction.

Mechanical Advantage: (MA) the ratio of output force to input force

An ideal machine would be 100% efficient.

Work input would equal work output.

This does not exist…due to friction, work output cannot equal work input.

In an ideal situation…

Efficiency: The ratio of useful work output to total work input.

E = (Work output / Work input ) 100

orE = actual MA / ideal MA