WORK AND ENERGY

WORK AND ENERGY

A. Work.

A force that work on an object can be said doing work if it causes the object to move.

- If object does not move, its work is zero.

- If force perpendicular with its displacement,its work

  is zero.

- If force is opposite with its diaplacement direction,

  its work is negative.

- If force is a direction with its displacement, its work

  Is positive  

1.    Work by a force     

                             Where W = work

                                         F   = Force

                                         S   = displacement

The unit of Work

If the unit of Force is Newton and the displacement is meter then the unit of Work is Joule.

If the unit of Force is Dyne and the displacement is cm, then the unit of Work is Erg.

So Joule = Nm and Erg = Dyne cm, the relationship between Joule and Erg is 1 Joule = 1= 10⁷ Erg

2.    Work by Some Force

a.    Work by same direction forces

 

b.      Work by opposite direction Forces

F₂                              F₁

                                      S

W = ( F₁ – F₂ ) . s = ∆ F . s

B. Energy

Energy is an ability to do work

1.      Forms of Energy

a.      Potential energy;gravitational potential energy of earth is energy had by an object because of the position to the earth.

        m

 

                                                  where :

h       E_p  = potential energy ( Joule )

         m   = object mass ( kg )

  g   = gravitational acceleration of earth ( m/s² )    h   =  height position object to the earth ( m ).

b.     

E k =  2. v2

Kinetic energy, energy that had by amotion object

                                      E_k  = kinetic energy (J)

                                      m  = object mass (kg)

                                       v  = speed ( m/s )

c.   Chemical energy; energy which is stored in a matter or fuel.

d.   Electric energy; energy that caused by electric current

e.   Light energy; energy from light e.g. sunlight.

f.    Sound energy ; energy is produced by vibrating.

g.   Nuclear energy; energy is produced by nuclear reaction of atom.

2.   Conservation Law of Energy.   

Energy can not be created or destroyed, but it can change from one form to another form.

3.   Transformation of Energy.

      Energy can change from one form to another form

      a. Electric energy to heat energy , e.g.electric iron

      b. Chemical energy to electric energy,e.g. battery.

      c. Electric energy to light energy , e.g. lamp.

      d. Electric energy to kinetic energy , e.g. fan

4.        Conservation Law of Mechanical Energy.                  “A body falss down on the earth surface, its mechanical energy will constant at every height if there is not external force or its fractional with the air is neglected.”

The law of mechanical energy conservation :

E_m = E_p + E_k  ;  E_m1  =  E_m2

          Law of mechanical energy conservation above can be described as follows :

a.       The smaller kinetic energy, then the larger its potential energy,

b.      The smaller potential energy, then the larger kinetic energy.

c.       The sum of kinetic energy and potential ener- gy is always constant.

C. Power.

     Power is velocity to do work or energy changes per unit of time.

       Mathematically, power is formulated as follows :

P  = Power ( Watt )

W = work ( Joule )

t    = time ( second )

The power unit is Joule equal to watt.

Another unit is Horse Power ( HP = PK )

       1 hp = 746 Watt = 1 PK

D.  Simple Machines

       Simple machines is tool to do work easier.

1.Lever

is a device used to move or lift load with smaller force than the load.

In its working  system, a lever consists of load, support point and power.While terminologies often used in the lever , are as follows :

a.    Load ( W ), namely weight force

b.    Support point (T), namely point between po- wer and load that not get displacement.

c.    Power (F), namely force required to lift load.

d.   Power arm (Lk)namely distance from support point to power.

e.    Load arm (Lb), namely distance from load to supprot point.

Based on position/location of power, load and sup- port point, the lever is divided into three kinds,i.d. 1. First Kind Lever

                                            ℓ_k

                    ℓ_b

                           

                            ^{Fulcrum                   F}

       W

       Example of application of first kind lever is crow- bar to lift stone, cutter forceps and so on.

       Lever’s principle :

       Power x Power arm = Load x Load arm

       Equations :  

                           

  

  Comparison between load and power or between power arm and load arm is called Mechanical ad- vantage.

    Where :    W    = load weight (N)

                     F     = power or force (N)

                     L_b    = load length

                     L_k      = power length

KM = mechanical advantage

2. Second Kind Lever.                                    F

 

             

                Fulcrum         W

       Example of the using of second kind lever is bottle plug opener , nail drawer and so on.

      

       3. Third Kinds Lever.         F

      

 

                   W                                                Fulcrum

       When your arm lifts a barbell,your arm is included into the 3rd kind lever with elbow as support point your arm as power and barbell in the palm as load.

2.    Pulley.

            Pulley is a simple machine in the shape of wheel which around it is given string.

       Pulley is used to lifts load easily. Work principle of pulley is the same with lever’s principle.

       Based on its connection, pulley consists of fixed pulley and moving pulley.

a.   Fixed Pulley.

       is pulley that never moves its place, but only revolves to its axis.

            In fixed pulley there is not change of force magnitude, but only happens change of force direction with purpose to simplify task or human work. So,function of the fixed pulley is to change force to simplify work.

The Point O = Support point

The Point A = Power point

The Point B = Load point

Length AO  = Power arm (Lk)

A       O       B    Length BO  = Load arm (Lb)                     

Lever’s Principle :

F x Lk = W x Lb  ;  Lk = Lb ,  so

F  = W  and M A  = 1

         F                          Pulley this kind is often used to bail water , banner lifter and so on.

b.  Moving Pulley.

is a pulley which is followed to move with the load 

The Point A   =  Power Point

                             The Point B    =  Support Point

The Point O    =  Load Point

Length of AB  =  Power Arm

Length of AO  = Load Arm

Lever’s Principle :

F x AB = W x BO

AB = 2 BO  , so   F x 2 BO  =  W x BO  ,

Mechanical advantage =w/F=2

3. Sloping Plane.

            Sloping plane is usually used to move or lift body from higher place.For example,to move box from the ground into a truck.

     Example of sloping plane is screw, wood drill, slo- ping plane curved, stairs, and so on.

     Consider the figure below !

                                                     F

                             s        h

                      w

    W_f = W_l ↔ F.s = w. h ↔ s / h = w / F  ↔  F =   therefor w = m.g  , so

    Its mechanical advantage is :

      M.A. =   =

    In which :   F    = power (N)

                        w   = load / weight of the body (N)

                        s    = length of the sloping plane (m)

                        h    = heght of the sloping plane (m)

                        W_f  = work from the power F

                        W_l  = work from the load w.

4. Gear. A bike equipped with a gear is a simple ma chine. A pressure force given at a pedal will rotate the axis;i.e.a big gear,which is connected to a small gear stuck in the rear wheel of a bike. The wheel will rotate along the rotation of the axis of gears.