Travail, Énergie et Puissance Flashcards
Define energy
the stored ability to do work
Name the various forms of energy. Describe/ give examples to each
- Kinetic Energy
energy associated w body in motion - Gravitational Potential Energy
energy associated w body due to its pos n in a gravitational field, eg raised objects & waterfalls - Elastic potential Energy
energy stored in compressed/stretched springs, bent springboards & stretched elastic band of catapult - Electrical Potential Energy
energy associated w forces electric charges exert on one another & their relative pos n to one another - Radiant Energy
energy that can b transmit by electromagnetic radiat n eg visible light, X-rays - Chemical energy
energy stored in fuels eg oil, wood, food - Nuclear energy
energy that can b released fr atomic bomb, nuclear reactors
Define work done. Give 2 formulae
Work done by force on body is product of force & displacement of body in direct n of force
Standard formula:
W=Fscosθ, where θ is angle btw direction of F & that of S
work done by gas formula:
W=p(Vf-Vi)
where Vf is final vol, Vi initial vol, p is Pa
What does work-energy theorem state?
net work done by forces on a body equals change in its Ek
State Principle of Conservation of Energy
states that energy cannot be created or destroyed, but it can only be converted from one form to another
What eqn can be formed thanks to principle of conservation of energy?
Ek,i + Ep,i + Ee,i + Esupplied = Ek,f + Ep,f + Ee,f + Edissipated
where i is initial, f is final
How to derive kinetic energy?
Step 1:
Consider object, mass m. It experience constant net force F over horizontal displacement s. Object’s velocity increases from u to v
Step 2:
By N2L, constant net force F produce uniform acceleration a.
Since a is constant,
v² =u² +2as
as=(v² - u²)/2
Step 3:
From def n of work done,
Work done by F
=Fs
=mas (since F=ma)
=m[(v² - u²)/2]
=0.5mv² - 0.5mu²
Step 4:
Take initial velocity u = 0,
work done by F
=0.5mv² - 0.5m(0)²
=0.5mv²
Step 5:
By principle conserv n energy, work done by F increases oni kinetic energy of block. Thus,
Kinetic energy = 0.5mv²
How to derive gravitational potential energy?
Step 1:
Consider object mass m near Earth’s surface, where acceleration of free fall is g. Object is raised vertically by height h at constant velocity by external force F
Step 2:
Since velocity uniform, by N1L,
net force = 0
F - mg = 0
F = mg
Step 3:
From def n of work done,
work done by F = Fh = mgh
Step 4:
By principle conserv n energy, work done by F oni increase gravitational potential energy Ep of object, as kinetic energy is constant. Thus,
change in gravitational potential energy,
ΔEp = mgh
What to take note when using Ep=mgh?
reference level
If calculated value for energy is negative, what does it mean? Give example
-ve work done (NOT negative direction)
eg
- diff reference pt for Ep
- gas compressed (gain energy fr environ)
- direct n of force on and displacement of object are opposite
What is the formula for elastic potential energy?
Ee=0.5kx²
where k is spring constant, x is displacement of spring from unstretched state
Define Power
work done per unit time
What is the formula of average power?
average power
= total energy/total time
OR
= total work done/total time
What is the formula for power when constant force acts on an object w velocity v in same direction as force?
P=Fv
How to derive P=Fv?
step 1:
consider object travels w velocity v & experience force F in same direction
step 2:
from def n of power,
power = rate of work done by force per unit time
step 3:
from def n of work done,
work done by force = force x displacement in direct n of force
step 4:
from definition of velocity
power of force = force x displacement in direct n of force PER unit time
step 5:
Therefore,
power of force P = force F x velocity v in direct n of force
–> P=Fv
