WORK, POWER AND ENERGY
• Work
(W) is energy transferred to or from an object by means of force acting on the
object.
W
= F ∙ d
* dot product
Note: This is the multiplication of two vectors in which the
product is a scalar quantity.
To obtain the magnitude of work , the following formula is
used:
W
= Fd cos θ
W = work
F = force
d =
displacement
θ = angle
• The
commonly used units of work is:
joules
(J) = 1J = 1 N-m = 1 kg m2/s2
calorie
(cal): 1 cal = 4.18J
electron-volt
(eV): 1eV = 1.6x 10-19 J
• Work
done by a single force
1.
A box was pushed with a 10N force causing it to
move by 5m to the same direction as the force. What is the work done?
2.
A 1.0kg ball was thrown upward; reaching up to a
4.50m above the point it was released. What is the work done by the gravity on
the ball?
3.
A 100.0 kg crate is being pushed across the
factory floor covering a distance of 24m. What is the work done by the normal
force exerted by the floor on the crate?
4.
A 100.0N force is applied at an angle of 40°
above the horizontal on a 5.0kg object, moving it with a displacement of 15.0m.
What is the work done
ENERGY
Energy - the ability
to do work
Mechanical, heat, light... Etch
MECHANICAL ENERGY
– Is
the energy possessed by a physical object due to its motion or state.
• FORM
– Kinetic
Energy – is the energy an object possessed due to its motion
– Potential
Energy – is the energy due to state of
the object; state could mean the position of the object or the extent it was
compressed or stretched.
• Kinetic
Energy
– Motion
of an object
– Depends
on its mass and speed
Kinds of Kinetic Energy
Vibrational – energy due to vibrational motion
Rotational – energy
due to rorational motion
Translational –
energy due to motion from one location to another.
KE = ½ mv2
where
KE- Kinetic Energy
m=
mass
v = speed
1.
What is the kinetic energy of a 72kg man running
at 3.0m/s
Potential Energy
is
the energy due to the state of the object; state could mean the position of the
object or the extent it was compressed.
2 Form
Gravitational Potential Energy PEg
Elastic
Potential Energy PE
• Gravitational
Potential Energy (PEg)
– Is
the potential energy associated with an object due to the position of the
object.
Peg
= mgh
PE
= Potential Energy
g - gravity
h - height
1.
What is the potential energy of a 5.4kg vase on
the balcony 3.5m above the ground
2.
At what height above the ground should a 4.0kg
bag of rice be placed to have a potential energy of 196J?
• Elastic
Potential Energy
– Is
the potential energy in a stretched or compressed elastic object
Hooke’s Law
Fspring = -kx
where k
= the spring constant
x = the
amount of compression
PEspring =
1/2kx2
• LAW
OF CONSERVATION OF ENERGY
• FORMS
OF ENERGY
– Electrical
Energy – energy due to the movement of electrons
– Radiant
Energy – is the energy that is carried by electromagnetic waves
– Thermal
Energy – or heat, is the energy that causes an increase in temperature
– Sound
Energy – is the energy transmitted through substance in longitudinal waves
– Chemical
Energy – is energy stored in the chemical bonds of atoms and molecules
– Nuclear
Energy – is the energy stored in the nucleus of an atom. Its an energy that holds
the nucleus together.
• SOURCES
OF ENERGY
– Non-renewable
energy source – are those energy sources that cannot be replenish
– Renewable
Energy – those that can be replenish in a short period of time
Ex. Hydropower, solar, wind, geothermal and biomass
POWER
• POWER
– The
rate at which work is done
– Defines
how fast work is done
P = W/t
where
P = power
W
= work
t
= time
• The
SI unit for power
– 1
Watt = 1J/s
Another unit used is
1Hp = 746watts
P = W/t
P
= Fd/t
P
= F x d/t
P
= F v
1
Watt = 1 N m/s
Little Nellie Newton lifts her 40kg body a distance of 0.25m
in 2 sec. Find the force she exerts, the work done, and the power she
delivered.
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