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SECUNDARIA - 2º ESO - WORK&ENERGY - FÍSICA Y QUÍMICA - FORMACIÓN

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Subido el 21 de abril de 2020 por Cp santodomingo algete

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Well, so now we can do many calculus with forces. 00:00:00
So we can say that at least we have half the problem to understand physical world solved. 00:00:19
However, despite the concept of inertia and the relationship between forces and acceleration 00:00:29
are very important tools. 00:00:37
We cannot find out the speed of a bullet from a gun just by knowing how much powder it had. 00:00:39
Furthermore, we cannot find out the temperature of the bullet when it hits a wall. 00:00:48
So we need another tool, another concept. 00:00:56
Here it is, energy. 00:01:01
is used in ordinary life so we will have a rough idea about energy and I don't mean only 00:01:04
at human level. We will know that we need energy for heating our houses, cooking or washing our 00:01:14
clothes. So we will have an intuitive knowledge about energy as something we need to accomplish 00:01:24
a task make a work. That is, we define energy as the capacity of a system to 00:01:33
make a work. Which kind of work? Well, move something, change the shape of a 00:01:43
solid, compress gas, etc. And this reminds us forces. We know that force can change 00:01:51
the shape of a solid. Force produces acceleration. Thus, if energy is to 00:02:01
produce a work, and work is closely related to forces, then there must be a very 00:02:10
direct relationship between them. We can realize that the man on the 00:02:18
right, that is lifting a weight, would not make the same effort, would not consume the same energy 00:02:25
to lift one pound than to lift 100 pounds, for example. Then work proportional to force makes 00:02:34
sense. On the other hand, the man on the left picture is passing a box, and we can all agree 00:02:46
that the longer distance he's passing that box, the more energy he's spending. And so we conclude 00:02:56
that the force applied along a distance is just the work we are trying to find. 00:03:05
This definition of work as force multiplied by distance deserves a little analysis, 00:03:12
because both force and distance are vectors, but work is scalar, 00:03:20
so this product work is easier to deal with than forces, 00:03:28
but we have to take care that force and distance are aligned 00:03:35
in order to compute the product. Furthermore if distance is null then no 00:03:41
work is performed. For example the man on the right picture is lifting a weight so 00:03:51
when he gets that weight down again the effective work he has performed is zero. 00:03:59
Now, let's generalize the concept of work beyond the effort made by men, not only because we need to understand or calculate the energy required for a machine, 00:04:07
but also to understand the work performed by, for example, gravitational field or a magnet, etc. 00:04:23
And so, we will be able to connect the energy produced in a chemical reaction with the work performed by a car or similar machine. 00:04:33
For example, if the force is just the weight of something, we can easily calculate the work made by the gravitational field of Earth near its surface. 00:04:47
We simply multiply the force of weight by the height that something has fallen, by gravitational attraction. 00:05:03
And conversely, we can tackle this problem to calculate what energy do we need to lift 1 kilogram 5 meters, for example. 00:05:16
So, since nature is symmetric, that energy is the same work that gravitation would produce, downloading 1 kg 5 m. 00:05:30
This is the solution. 00:05:45
49 joules. 00:05:48
Joule, or julio in Spanish, is the unit for work. 00:05:50
Work for work or energy. 00:05:56
since you are noticing that these two concepts are two faces of the same coin. 00:06:00
The name of the unit for energy in the International System 00:06:07
comes from this scientist, James Joule, whose teacher was Dalton. 00:06:12
You remember him by the atomic model, right? 00:06:20
So James Joule worked on heat, energy, temperatures. In fact, he cooperated with Lord Kelvin, whose scale of temperatures you also know. 00:06:24
Well, Joule was the first man who linked work as force multiplied by distance with heat. 00:06:37
And hence before, everybody understood that heat can produce a mechanical work. 00:06:47
That was the beginning of the industrial revolution 150 years ago. 00:06:56
And that's why we have different units for energy. 00:07:03
The one for the international system, I repeat, is called Joule, or Julio in Spanish. 00:07:08
But we can also use calorie. 00:07:15
Calorie is the energy required to raise one gram of water one Celsius degree of temperature. 00:07:19
Its equivalence with joule is 4.18. 00:07:28
So you have to remember this forever. 00:07:35
One calorie is the same as 4.18 joules. 00:07:37
reason to have so many different units for the same quantity energy is because we have to deal 00:07:43
with it in different fields of physics. So despite you are also guessing that all these kinds of 00:07:51
different energies can be converted one into each other, to solve problems in atomic issues we use 00:07:59
electron volt. To deal with problems of domestic power we use kilowatt. To deal with thermal 00:08:09
problems of course we are using calories and for the rest we'll use always joules. Next year 00:08:19
we'll learn more about devices that can convert one energy into another. Thus far 00:08:27
this table summarizes different sources of energy, not different kinds of energy, 00:08:34
but different sources of energy that we can use for our welfare in industry, at home, 00:08:41
etc. By the way, it would be nice if you can organize this table as a mind map, so then 00:08:49
energy can be transformed into different kinds, but it's always the same stuff. We 00:08:59
understand that a car engine transforms chemical energy from the gasoline into 00:09:05
mechanical energy. Our body transforms food energy into mechanical energy and 00:09:13
and also heat and so energy transformation is a very useful tool because energy is not a vector 00:09:20
so we can add multiply etc but in this point we can ask ourselves which is the origin of all this 00:09:30
energy in our planet, for example? The obvious answer is sun. Life is sustained by the energy 00:09:40
that comes from the sun. But we also know that there are some elements, some stones if you want, 00:09:50
that have a property of radioactivity. So, they produce heat. And also, we know about volcanoes, 00:09:58
earthquakes, even the moon is the source of energy because tides are made because the 00:10:08
moon attracts the water in the ocean. 00:10:16
But please, do not mistake these origins of energy in general with the table we have seen 00:10:21
Before, there is a table with different sources that men have learned to take advantage of 00:10:30
in order to be provided with energy, useful energy in daily life. 00:10:38
Well, let's dig a little bit about this concept of energy transformation. 00:10:45
We can analyze the mechanical energy transformation. 00:10:51
In fact, a good example is the pendulum. In a pendulum, sometimes when it reaches the streams, 00:10:58
it stops. So the kinetic energy is zero in those stream points. And then it goes down 00:11:06
and increases its speed. And the question is, where has gone that kinetic energy the pendulum has 00:11:17
at the lowest point? Well, the answer is that kinetic energy at the lowest point has transformed 00:11:26
into potential energy at the highest point, where you remember the pendulum stops. 00:11:36
And so mechanical energy can be either potential or kinetic. Kinetic energy has to do with speed. 00:11:45
And potential energy has to do with what? 00:11:56
Well, potential energy is the capacity of a system to produce a work. 00:12:00
And that remembers us the very first slide of this video. 00:12:08
You remember this formula for the potential energy of gravitational field, right? 00:12:13
and we'll know that that potential energy will be transformed into kinetic energy 00:12:21
as soon as the object of mass m drops down. 00:12:29
So, this is the situation. 00:12:36
When the object is upstairs, its energy, its mechanical energy, is just potential energy. 00:12:38
When it drops down it increases its velocity and finally when it gets on 00:12:47
ground the total energy it has is kinetic energy. Its value is this one. 00:12:55
Half the product of mass multiplied by the speed squared. By the way, if the 00:13:03
object is elastic, like a ball for football for example, then you know it 00:13:14
will rebound from ground so it will reverse the process. It will change now 00:13:21
kinetic energy into potential energy because it will go up gaining potential 00:13:28
energy. So the situation is similar to the pendulum we have seen before. Let's 00:13:35
Let's perform this virtual experiment. 00:13:43
The skateboard girl or boy going downhill or uphill, I would like you to pay attention 00:13:46
to the record of kinetic and potential energy at the square of the right. 00:14:01
We can appreciate that the total energy, in other words, adding kinetic and potential 00:14:15
energy remains invariable, it's a constant. So this happens for any mechanical problem, 00:14:21
the pendulum, something that falls down or whatever. The total mechanical energy is constant. 00:14:31
So we can compare the two extreme situations when the object is on ground and when the object 00:14:40
is at the highest position. If the energy is the same at both situations then we can write 00:14:49
and so we can calculate the speed of something that's falling down from a given height 00:14:58
or conversely with the speed that we need to impulse something up to reach 00:15:06
a given altitude or simply calculate the altitude if we know the initial speed of 00:15:13
something we launch up. So now we can easily compute the altitude the object 00:15:22
will reach if its initial speed is 44.3 meters per second. Please check the 00:15:30
result. Besides, you can find out the speed at ground of something that is 00:15:40
dropped from 100 meters. And finally, just notice that the speed or height is 00:15:49
independent of the mass of the object. 00:15:58
Subido por:
Cp santodomingo algete
Licencia:
Reconocimiento - Compartir igual
Visualizaciones:
217
Fecha:
21 de abril de 2020 - 23:30
Visibilidad:
Público
Centro:
CP INF-PRI SANTO DOMINGO
Duración:
16′ 12″
Relación de aspecto:
1.78:1
Resolución:
960x540 píxeles
Tamaño:
94.45 MBytes

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