Mechanics• In this presentation we will be dealing with WORK, ENERGY and

POWER.• This presentation will guide you and give you the foundation that

is needed by a person who want to succeed in the world of physical science especially in mechanics.

By T. J. Rantho

Science Pre-service teacher at the University of Johannesburg

WORKMy father brought a wheelbarrow of from his work and he was pushing it all by himself.

My father in the other hand always ask me, my bother and mother to help him, whenever he is working in the garden.

WORK

• According to James Prescott Joule, the only factors affecting the amount of work done are the size of the force, and how far the object is moved in the direction of the force.

• Hence, work is done by a force when the object it acts on moves.

WORK

• NO work is done by pushing against a stationary wall.

• Work is done when throwing a ball because the ball MOVES while being pushed during the throw.

Calculating Work Done

• Work done is given by

• (W) is work done in joules (J)

• (F) is the force applied in newtons (N)

• (S) is the displacement in (m)

• (θ) is the angle between F and s.

• The direction of the force (F) is assumed to be the same as the direction of the distance (d)

• A force perpendicular to the direction of motion of an object cannot do work on the object

𝑾=�́� ⋅ �́�=𝑭𝒔𝐜𝐨𝐬𝛉

Examples on work

Example:1. Push a box 8 m across the floor with a force of 100 N performs

800 J of work:

Calculate the work done in:

2. Pushing a 40kg box, 8m across a floor against a 15N friction force.

3. Lifting a 20kg box up on to a shelf 2m high.

4. Moving a 15kg box up a flight of 13 stairs. Each stair is 15cm high and has a tread length of 20cm.

JmNmNFdW 800800)8)(100(

Direction of Force

• When a force and the distance through which it acts are parallel, the work done is equal to the product of F and d

• If the forces are NOT parallel, work done is equal to the product of d and the projection of F in the direction of d.

ENERGYWe use energy in the form of electricity to cook, watch television and more

We also use energy stored in our body to lift heavy things, play and work and more.

Energy

• Energy is the amount of work a physical system is capable of performing.

• Energy can neither be created nor consumed or destroyed

• • When anything happens

in the physical world, energy is somehow involved.

Types of Energy

• Kinetic – Energy of Motion

• Potential – Energy of Position

• Chemical Energy • Food converted to energy

in our bodies

• Heat Energy• Heat from burning oil to

make steam to drive turbines

• Electric Energy• Electricity turns motors in

homes and factories

• Radiant Energy• Energy from the sun

Kinetic Energy

• Kinetic energy is the energy of a massive body due to its motion.

• It is given by:

• The kinetic energy of a body is a scalar quantity measured in joules (J)

• It has only magnitude, it has no direction.

Kinetic Energy

• Example 1:• Kinetic energy of a 1000kg car moving at 10 m/s is 50kJ

• ( 50kJ of work must be done to start the car from a stop, or stop it when it is moving)

Kinetic Energy

• Example 2:• Using a hammer with a 600g head moving at 4 m/s to

drive a 5mm nail into a piece of wood, what is the force exerted on the nail on impact?

Gravitational Potential Energy

• Potential energy is the energy stored in the set-up of a mechanical system

• When work is done on an object to move it in a gravitational field there will be a change in gravitational potential energy.

• Absolute Gravitational Potential Energy is a relative quantity and is defined as being zero at infinity (see Unit 6.1)

• In all real-world situations we are able to define our own “zero” and so work with Change in gravitational Potential Energy.

Gravitational Potential Energy

• Gravitational Potential Energy is defined as:

• Δ _ = Δℎ𝐸 𝑃 𝑚𝑔• It is a scalar quantity measured in joules (J)

• Technically absolute GPE is always a negative quantity.• If working in a situation where g is variable, then a

graph is generally used to find the area gh.

Potential Energy

• Gravitational Potential energy = the energy a body possesses because of its position relative to the ground

Potential Energy Example

• Potential energy of a 1000kg car pushed off a 45m cliff

• Compare with amount of KE done by a car moving at 30m/s (108 km/hr)

kJmsmkgmghPE 441)45)(/8.9)(1000( 2

Examples of Potential Energy

• Examples are almost everywhere• Book on the table• Skier on the top of a slope• Water at the top of a waterfall• Car at the top of a hill• A stretched spring• A nail near a magnet

POWERI heard that the power cut that took place this year (2014) was the first one since 2008. we need to save power by using alternative sources.

Yah, but not all forms of power can be safe to use, last year 2013 nine cricket school boys were stroke by lighting at school.

What is power?

• Power is the RATE of Doing Work…

• Rate is the amount of work done in a specified period of time

• The more powerful something is, the faster it can do work

• A truck is more powerful than a van, hence it does more work than a van.

How to calculate power

Example:• 500W motor can perform

500J of work in 1 s• … or 250J of work in 0.5 s• … or 5000J of work in 10 s

• Because Watts are very small units, Kilowatts are used most commonly

• Power is measured in Watts (W)

interval time

donework Power

kW 1 W 1000 kilowatt 1

Reference list• MsSallyHa. (2010).

http://www.slideshare.net/MsSallyHa/energy-5553449 ASSESSED ON 06/03/2014• Kennedy, J. (2011). http://

www.slideshare.net/thephysicsteacher/23-work-energy-and-power ACCESSED ON 06/03/2014• Astrayudha. M. (2010).http

://www.slideshare.net/Astrayudha/energy-work-and-power-presentation

ACCESSED ON 06/03/2014• Academy, L. (2011). http

://www.slideshare.net/logosacademyeduec/energy-9245948?qid=68b469c4-8d99-4126-bb92-52f05051074e&v=default&b=&from_search=7

ACCSSES ON 07/13/2014• Zamudio, N. (2013). http://

www.slideshare.net/nicolezamudio/energy-27372876 ACCESSED ON 07/03/2014