Transcript 3.3.1 work con of energy

Atlas Ball
Farmer’s walk
Who is doing more work?
When you do anything like running, jumping, writing
and even sleeping we know you need energy to do
them. However are you doing anything useful?
Work Done and Energy Transfer
Consider holding the flaming torch like the Statue of Liberty. It
would require you to consume food just to remain standing i.e.
an input of energy, however we could replace the person by
an inanimate object like a huge concrete statue which requires
no energy input.
Therefore is any work being done by the statue?
The answer is obvious as no energy is being put in and the
torch isn’t moving how can any work be done!!
In order to understand work we must first understand
energy.
Energy is the ability to do
work measured in Joules (J)
If you do work on an object such as lifting a bag, you lose
energy and the bag gains potential energy. Energy cannot be
created or destroyed as you know.
In Physics we sometimes talk about systems. A system is an
object or group of objects that we are considering the
Physics of.
If a system does work, its energy decreases.
When work is done on the system its energy increases.
Whenever a force causes an object to move, work is said to be
done. It is the product of the magnitude of the force and the
distance moved in the direction of the force.
Whenever energy is transformed from one
form to another work is said to be done!
1 Joule of work is done on an object when a force of 1 N cause
it to move a distance of 1 m in the direction of the force
This explains why the Statue of Liberty and you would
effectively be doing no work holding up the torch. There is no
resultant force and no movement!
In order to calculate the work done you use the following
equation.
Work Done = Force * Distance moved in direction of
force
Joules = N * m (DO NOT CONFUSE WITH MOMENTS)
W = Fd
When a system does work the amount of work it does is
equal to the amount of energy gained by the second system.
Hence
Work done = Energy Transferred
How much work must be done to lift a 5kg bag 1.2m
and place it on a table?
If the work being done within a system is not caused by a
uniform force it is not possible to calculate the work done
using the equation given. Instead we need to rely on
graphical methods.
Work done = The area under a Force – Displacement
graph
Force/N
Work Done
Displacement / m
A toy frog has a spring which causes it to jump into the
air. The force-compression graph for the spring is shown
below.
Force/N 22
0
Compression/m
0
0.04
Calculate the work done on the spring when it is compressed
by 4.0cm. Work done =...................(3)
The frog has a mass of 24 g and rises 0.60 m vertically into the
air. Calculate the gravitational potential energy gained by the
frog.
Energy =...................(2)
Compare your two answers for energy and explain how they
are consistent with the law of conservation of energy. (2)
(Total 7 marks)
Calculation of work done:
Work = area under graph/average force × distance (1)
= 1/2 × 0.040 m × 22 N (1)
= 0.44 J (1)
3 [Allow any correct unit, e.g. N
m. Penalise unit once only] [Fd ® + 0. 88 J gets 1/3]
Calculation of energy:
GPE = 0.024 kg × 9.81 (or 10) m s–2 × 0.60 m (1)
= 0.14 J (1)
2
Comparison: Some energy transferred to some other form (1)
Reason [a mechanism or an alternative destination for the
energy], e.g. (1)
Friction, Air resistance, Heat transfer to
named place [air, frog, surroundings etc], Internal energy,
Vibrational energy of spring
Sound OR quantitative comparison (0.3 J converted)
[No e.c.f. if gpe > work]
2
[7]
How much work must be done to drag a bag of
rubbish 10m across a field by pulling on a
string at 40° to the horizontal with a force of
250N?
Principle of
Conservation of Energy
States that ……
Energy cannot be created or destroyed,
merely changed from one form to
another
Whenever a “machine” does it’s job it changes one form
of energy to another.
However this change is never perfect. In the case of light
bulbs a lot of the energy is given off as heat.
There are many types of energy
changes we could investigate, one
of which is the change of
Gravitational Potential Energy into
Kinetic.
Many scenarios involve changes of
energy but sometimes they are not
so obvious. Where does the
chemical energy needed for this
lovely man to throw his darts go??
Ultimately all energy will change into heat due to friction
but in between it may go through several stages which you
should be able to identify.
A boy sliding down a grassy bank – his speed is constant
but where is the energy he is gaining from the
Gravitational Potential Energy going ?
If his speed is constant the Kinetic Energy is staying the
same so what is happening?
A car pulling a trailer with a constant force of 1000N
should be accelerating but if it is travelling at a constant
speed what is going on? If the force has been acting on the
trailer for a longer distance then more Work has been
done or more energy supplied to the trailer.
Heat energy is more correctly called Thermal Energy but
even more correctly called Internal energy which is
linked to the individual atom’s random kinetic energy.
Since there are electromagnetic forces between the
atoms, they create stores of potential energy, like
stretched springs, when they move around.
In the case of the boy sliding down hill, he is giving the
individual atoms in his bum and the ground more
internal energy all the time, supplied by the potential
energy gained from sliding down the hill.
Efficiency
This is a measure of how good a “machine” is at
transferring energy from the type supplied to the type
wanted.
The less is wasted the more efficient it is!
A car is apparently only 25% efficient as it wastes ¾ of the
energy in the petrol as heat and sound!
Therefore not all the energy supplied is used for the
intended purpose. The most efficient machine is
supposedly a person cycling uphill in the correct gear!
To calculate efficiency either in terms of energy
use the following equation.
or power you
% Efficiency = Useful output * 100%
total input
 WEP
Support
 Calculation sheet 5