Transcript Work, Energy and Power

Work, Energy and Power
• Brainiac
• Solar Energy
Work
Work – Sample Qs 1
Work - Sample Qs 2
• A man carries a 50kg bag of cement up a stairs
at a steady speed. The vertical height through
which he rises is 20m. He exerts an upward
force on the bag equal to the weight of the
bag. Find the work done by the man on the
bag of cement.
• Ans: 9800J
Work – Sample Qs 3
• Find the work done in bringing a 100kg car
from rest to 30m/s in 10s.
• Ans: 450,000J
Energy
• Energy (E) is the ability to do work
• the amount of energy something has is the
amount of work it can do.
• Any time work is done energy is transferred.
• Units: Joules (J)
• Different Types: chemical, heat, electrical,
sound, nuclear, potential, kinetic
Kinetic Energy (K.E.)
K.E. – Sample Qs 1 & 2
• What is the kinetic energy of a race car of mass
1585 kg travelling at 252 km/h
• A particle of mass 20kg moving at a speed of
3m/s accelerates to a speed of 30m/s under the
action of a force of magnitude 12 N in the
direction of the motion. Calculate:
–
–
–
–
–
the initial K.E. of the particle
the final K.E. of the particle
the change in kinetic energy of the particle
the distance travelled
the work done by the force
When Work is done, Energy is
transferred.
Imagine pushing a car of mass 800kg a distance of
10m.
• Work is done and chemical energy in your muscles is
used up.
• The car gains kinetic energy as you move it
• Mathematically:
– If an object comes to rest, the kinetic energy lost
will be equal to the work done in bringing the
object to rest.
Sample Qs 3
• A bullet of mass 2g travelling with a speed of
150m/s penetrates 0.6m into a block of wood
before coming to rest. Find the force that the
wood exerts on the bullet.
Potential Energy (P.E.)
P.E. Sample Qs 1 & 2
• A high jumper of mass 65kg raises her centre
of gravity from 1.3m to 2.1m. What is her gain
in potential energy?
• A particle of mass 35kg is projected upwards
through a vertical height of 1200m. Find the
increase in its potential energy.
Principle of Conservation of Energy
• Energy can neither be created nor destroyed,
only converted from one form to another.
Relationship between K.E. and P.E. for
falling objects
• A stone is held at a height and has a P.E. =
5000J and K.E. = 0.
• When released the stone begins to fall and its
P.E. is converted to K.E.
• Just before it hits the ground, P.E. = 0 and K.E.
= 5000J.
• All the potential energy has been converted to
K.E.
P.E./K.E. Sample Qs 1 & 2
• A stone of mass 4kg is dropped from a height
of 200m. Find its kinetic energy and speed just
before it hits the ground
• A stone is projected vertically upwards with an
initial speed of 40m/s. Find the greatest height
reached
Consider an oscillating pendulum
PE at top=KE at bottom
At the
bottom the
bob has no
PE only KE
KE = ½ mv2
At the top of the
oscillation the
pendulum bob
stops. All it’s
energy is PE
PE = mgh
h
P.E./K.E. Sample Qs 3
A pendulum bob is released from a height. Find
the speed of the bob as it passes through a
vertical position 40cm below its starting
position.
Collisions
• When two bodies collide, kinetic energy is lost
in the collision.
• However momentum is always conserved:
• Momentum Before = Momentum After
• The loss of kinetic energy is usually converted
into sound energy or heat energy.
Newton’s Cradle
• Potential Energy is converted to Kinetic
Energy, which is transferred through the
spheres until at the end it is converted back to
potential energy.
• Why does it stop?
• Energy is also converted into sound and heat
Energy Conversions
• Think of examples of different energy
conversions.
• Clearly state the conversion taking place.
Example:
In a filament bulb, current is passed through a
thin tungsten wire, which becomes white hot,
which in turn gives out light
Electric Energy to Heat and Light Energy
Efficient Energy Use in the Home
List ideas to reduce energy in the home.
www.powerofone.ie
Power
Power
Power Sample Qs 1-3
• A man raises an object of mass 40kg through a
vertical height of 20m in 4s. Find his average
power.
• A weight lifter raises a mass of 30kg through a
height of 0.6m. He does this 50 time in half a
minute. Find the average power he develops.
• Calculate the average power developed by a 70kg
woman running up a set of stairs, given that each
of the 15 steps is 20cm high and it takes her 10s
to reach the top.
Efficiency
Efficiency Sample Qs 1 & 2
• The power rating on a food blender is 300W.
However it is only 75% efficient. The loss in
useful energy is due to heat. Calculate the rate
at which heat is generated by the blender.
• A 60kW motor in a crane lifts a mass of
4000kg through a height of 10m in 20s.
Calculate the % efficiency of the motor if it is
working at full power.