PHY3: WORK, ENERGY AND POWER

WORK, ENERGY AND POWER

Energy

This is defined as the capacity to do work.

Or It is the ability to do work.

The SI unit for energy is joule (J).

Forms of energy.

Chemical energy stored in coal, gas and oil can be released in form of heat when burned e.g. the food we eat has chemical energy in it. This energy is released by the chemical reactions so that the body can utilize it.

Electricity is also another form of energy.

The sun provides light or radiant energy.

Nuclear energy is released by nuclear reactions.

Mechanical energy.

This is divided into;

Potential energy

This is the energy a body has by virtue its position.

A body raised to some height has potential energy. A wound up string has potential energy and a stretched classic band has potential energy

Kinetic energy

This is the energy a body possesses by virtue of its motion.

WORK

Work is said to be done when the point where force is applied moves in the direction of the force.

Work = force x distance moved in the direction of force

The SI unit of work is the joule(J).

Definition of joule

The joule is the work done when the point of application of a force of 1N moves through 1m in the direction of the force.

Interchange of energy between potential and kinetic energy

As a pendulum oscillates its energy can be either kinetic energy or potential energy or both.

When the object has not yet been released it has potential energy and as it falls, it loses its potential energy and gains kinetic energy. Just as the object is about to touch the ground all its potential energy is converted to kinetic energy

The law of conservation of energy.

It states that energy cannot be created or destroyed but can be changed from one form to another.

E.g potential energy to kinetic energy, electrical to heat energy.(In iron and cookers) and light energy to chemical energy in plants.

POWER

This is the rate at which work is done or It is the rate of transfer of energy

The SI unit is  watt(W)

Definition of a watt

The watt is the rate of working of one joule per second i.e. 1 watt of power is used up when one joule of energy is used up per second

SOURCE OF ENERGY

• Wind energy

Wind mills transfer the energy of wind into mechanical energy which can be used to pump water or produce electricity.

• Electric energy

This is energy due to an electric current which flows in a circuit

• Chemical energy

This refers to the heat obtained from fuel such as wood, cool, oil, gas etc. it is used to generate other forms of energy, heat from chemical to light

• Solar energy

Concave mirrors collect sun rays and focus them on to special steam boilers which provide the mechanical force to run generators.

• Nuclear energy

This is energy released by nuclear reactions for example a bomb explosion

• Bio- thermal energy

This is obtained from cow dung that is organic plant and animal wastes.

• Geothermal energy

This is obtained from that hot springs and volcanoes. Energy from volcanoes can be used for creating electricity and that from hot springs can be used for boiling eggs and many other things.

MACHINES

This is any device that uses a force applied at one point to overcome another force applied at another point.

The force applied to the machine is called effort and the force it over comes is load i.e the effort is the force applied at one point to overcome the load.

The SI unit for effort /load is the newton(N).

Example of machines include;-

• Pulley
• Bicycles
• Wheel barrows
• Hammer
• Lever
• Screws
• Pair of scissors
• Indeed planes
• Gears
• Wheel and axle etc

Machines simply work by one or more of the following ways;-

1. Transforming energy for example a generator mechanical energy into electrical energy.
2. Transferring energy from one place to another for example in a motor car. Energy is transferred from a chunk shaft to drive shaft so as to move the wheels
3. Multiplying a force e.g. in pulleys
4. Multiplies speed in bicycles

The three basic terms used in relation to machines are the following

Mechanical advantage (MA).

This is ratio of load to effort.

It has no units

Velocity ratio [V.R]

Is the ratio of the distance moved by effort to the distance moved by load during the same time.

Efficiency of a machine (ρ).

This is the ratio of the work output to the work input expressed as percentage.

Examples of machines

Lever

This is a device (machine) that consists of a rigid body pivoted at a point called fulcrum.

It is any device which can turn about a fulcrum.

Classes of levers

There are three classes of levers. They are the 1^st, 2nsd and 3^rd.

• 1^st class lever

This is where the fulcrum is found between the load and the effort e.g. the crowbar

X and y are perpendicular distance of the load and effect from the pivot respectively.

Taking moments about a fulcrum;-

Load x load distance = effort x effort distance

Note: The M.A is equal to V.R.

Other examples of 1^st class levers are: claw hammer, see – saw, pair of scissors, spanner, and bottle opener etc.

• 2^nd class lever

This is where the load is in between the fulcrum and the effort e.g. wheel barrow and nutcracker.

• 3^rd class lever

This is where the effort is in between the pivot and the load e.g. tongs, tweezers.

Pulleys

A pulley is a wheel with a grooved rim over which passes a rope.

Pulleys are used in lifting heavy loads to higher positions. They are also used in cranes to off load heavy cargo from ships and in dock yards.

A single fixed pulley is used to raise small loads to the top of a building during construction. When we neglect the weight of the rope and a friction, the load is therefore equal to the one.

Since the distances moved by the effort and load are the same then V.R = 1. Therefore efficiency is 100%.

Maximum mechanical advantage implies that we obtain greater convenience and ease of being able to stand on the ground and pull the rope down wards, instead of pulling upwards from the top of the building

Note:

The velocity ratio is equal to the number of pulleys used or number of strings supporting the load.

Assumptions

1. – The groove of the wheel is assumed to be smooth.
2. – The wheel is light or weightless.
3. – The rope used is weightless and is inextensible.

SINGLE MOVABLE PULLEY

Neglecting the weight of the rope and friction, the tension in the rope is equal to the effort.

The total upward force on the pulley is therefore twice the effort (E) since the load is supported by 2 ropes. The total tension of the two section of the rope is equal to 2E thus load is 2E.

The M.A = 2 also V.R= 2 since the number of ropes that support the load is 2.

BLOCK AND TACKLE SYSTEM

A block is the name given to two or more pulleys that are filled on the same frame.

A block and tackle consists of two sections of pulleys that form the upper and lower blocks.

This arrangement of pulleys is used to achieve a higher velocity ratio and mechanical advantages.

The upper blocks are fixed while the lower blocks are movable.

If the number of pulleys is odd, the string is attached or tied to the lower movable block, however if the number of pulleys is even, the string is attacked to the upper fixed block.

AN EXPERIMENT ILLUSTRATES  M.A AND EFFICIENCY OF PULLEY VARY WITH THE LOAD

V.R = 4

M.A = 4

An initial small mass is added to the load pan and further are then added to the effort pan until the load just rises slowly with a steady velocity.

The experiment is repeated with different loads and the results are recorded in a table as shown below.

GRAPHS OF MECHANICAL ADVANTAGES AGAINST LOAD AND EFFICIENCY AGAINST LOAD ARE THEY PLOTTED

M.A increases with the load until value of 4

Efficiency increases with the load until 100%

Note

Weight of the string depends on the distance between the pulley and the blocks but the weight of the lower block remains constant.

The friction varies with load and becomes smaller as the load increases i.e. when the load is small work done due to the weight of the movable parts, a ropes/strings and friction is proportionally high.

But as the total load increases the work done to overcome friction and load becomes smaller and so mechanical advantage increases. The efficiency also increases with the load for the same reasons.

The efficiency of any machine e.g. pulley is never 100% because;-

1. Work is done in overcoming friction.
2. More work is wasted(done) in raising the movable parts of the machine.

Example.

A block and tackle has three pulleys in the upper fixed block and 2 in the lower movable block.

What load can be lifted by an effort of 200N, if the efficiency of the system is 60%?

This consists of a wheel to which an effort is applied and an axle to which the load is applied.

The wheel and axle both have the same axis of rotation.

For a complete revolution, distance moved by an effort is equal to the circumference of a wheel while the distance moved by the load

Advantages of the wheel and axle

A small force is applied at the wheel to overcome a large load placed at the axle.

Wheel and axle systems are widely used in gear boxes

Gears

These are commonly found in clocks, watches and cars

The gear system consists of two wheels or more that turn in opposite direction. They carry projection called teeth that interlock and causes the opposite motion.

The wheel that carries the effort is called the driving gear while the wheel that carries the load is called the driven gear. The V.R of this system depends on the number of teeth on these gears and is defined as follows.

Examples

Two gear wheels A and B with 80 and 20 teeth respectively, lock into each other. They are fastened on axles of equal diameters; such that a weight of 150N attached to a string wound round the first axle to raise a load of 450N attached to a string wound round the second axle as shown below.

Calculate;-

1. Velocity ratio
2. Efficiency of a system.

A – 80 teeth

Inclined plane

An inclined plane is used for raising a heavy load through some height more conveniently as compared to lifting the load, vertically upward.

If the length of the inclined plane  is L and the height through which the load is raised is h. Then;-

M.A is determined by using the principle of conservation of energy thus if the inclined plane is smooth, then work done on load is equal to work done on the effort.

Load x load distance = effort x effort distance

L x h                =          E X L

M.A = V.R is true if the plane is a perfect inclined plane.

Other machines

That includes; the screw/car jerk, the screw driver, the borer, the clump e.t.c

Have a section that contains threads which acts on the load and a rod on which the effort is applied.

When the effort traverses a distance equal to circumference swept by the rod. Then the load move through a distance equal to that between any two threads. This distance is called the pitch.

THIS VIDEO EXPLAINS MORE ABOUT WORK, POWER AND ENERGY.