Inttroducing work and energy

1. WORK & ENERGY
A very practical idea indeed

2.  What is Energy?
 Where is Energy?
 Kinetic Energy
 Gravitational Energy
 How would you define Work?
 How would you calculate Work?
 How do Energy and Work relate to each other?
 Energy cannot be destructed, it can only be transformed
 Conservation of Energy
 Why are machines more efficient? What is Power?
A Guide to Energy

3. Do they have Energy?

4.  Every physical object has a property named ENERGY
 We can learn about how much energy does a body
have by means of what we can observe when it
changes or undergoes any sort of transformation
 Because ENERGY IS NOTHING BUT THE PROPERTY OF
MAKING OR UNDERGOING CHANGES
What is Energy?

5.  ENERGY IS EVERYWHERE because it is a common
property of matter
 When we learn more about matter and energy we
finally realize that they are two faces of the same coin
 We find energy in moving objects, its transformation
is motion which is due to kinetic energy
 But we also find energy in a big stone hanging at the
top of a cliff whether it may fall down or remain still
Where is Energy?

6. Kinetic Energy
Ekin = ½ m·v2

7. Gravitational Energy
Epot = m·g·h

8. How would you define Work?
WORK IS DIFFERENT FROM EFFORT
WORK NEEDS CHANGE, EFFORT NEEDS ONLY A FORCE
THE MAN OR THE ENGINE DO WORK WHEN THEY RAISE A
LOAD
consum de combustible)- ha de ser proporcional a la força exercida.
Assenyaleu si es realitza o no treball en les situacions que es deta
continuació:
a) Pujar un sac a una altura determinada; b) picar una pedra; c) em
una paret; d) caminar sostenint una maleta.
Es treballa quan s’està sostenint un objecte? Aquesta qüestió exigeix una r
sada. Sembla evident que l’objecte (si no és deformable) no pateix transformacio
ant açò, tenim la impressió d’estar realitzant treball: el subjecte que suporta a l
rimenta transformacions (el seu cor va més de pressa, transpira...), però no represe
o sempre realitza treball: Per a que es realitze treball
nt.
de
per
i
ran
el
t.
Fig. 2. Diferència treball-esforç

9.  In simple situations where there is a constant force (F)
applied in a certain angle (a) with the direction where
the object moves along a distance (Dx):
W = F · Dx · cos a
How would you calculate Work?
We measure the amount of work
done as the amount of energy
transferred in JOULES (J)
1 JOULE equals the work made when
a force of 1 N is applied along 1 m distance
in an angle of 0 degrees

10.  We can change the amount of energy of a physical system
by means of work. There are THREE POSSIBLE SITUATIONS:
 Inside, outside and net forces applied on a system have
different effects, they can change different kinds of energy:
 NET FORCES CHANGE KINETIC ENERGY :
Wnet = DEkin
 INSIDE CONSERVATIVE FORCES CHANGE POTENCIAL
ENERGY AND MAKE IT DIMINISH :
Wins con = −DEpot
 OUTSIDE FORCES CHANGE TOTAL ENERGY :
Wout = DEtot
How do Energy and Work
relate to each other?

11.  One of the outmost principles in science is the
CONSERVATION OF ENERGY IN CERTAIN PROCESSES
 That is, when we are sure about having an insulated
system (Wout = 0), then DEtot = 0, Etot = constant, and that
means:
Et1 = Et2
Ek1 + Ep1 = Ek2 + Ep2
Energy cannot be destructed,
it can only be transformed

12. Conservation of Energy

13.  Power is a measure of efficiency
 We choose certain machines because of its better efficiency,
that is its higher power
 We can define POWER as the rate at which energy is transferred
between two systems or the amount of work done per time unit
 We measure power in watts (W) : one WATT is the power of a
machine transmitting 1 JOULE of energy per SECOND :
P = DE/Dt = W/Dt
Don’t get wrong with letters: capital W means work and watt
as well, but there is an important difference: W meaning
work is followed of an equal sign and then a number
followed by J (for joules). Instead, W meaning watts will
always be preceded by the number of watts (example: this
machine has a power of 100 W)
Why are machines more efficient?
What is Power?

14. THIS IS A FORGE USED FOR HEATING AND MODELLING IRON:
Since ancient times, machines have helped us
to do a better work by increasing humans’ power