viernes, 19 de mayo de 2017

Physics and TV: Genius

A highly recommendable TV series for those who like physics is Genius. This show is on the screen on National Geographic Channel, available on some cable and satellite TV providers. The series tells us the life and work of one of the most brilliant scientists ever: Albert Einstein. It shows us not only the scientist and his crucial works but also his personal life and how the rise of nazism in Germany affected to the most famous Jew of the 20th century. Not only it is a very entertaining program but it is also very educational. It provides a quite comprehensible approach to his paramount contributions, such as his proofs of the existence of molecules, his explanations for the photoelectric effect or the most revolutionary physics ideas ever: the theory of relativity. An absolute "must" for those who like science or TV series.

viernes, 12 de mayo de 2017

Physics and movies: Up

I don't know anyone who dislikes Disney-Pixar's UP. This story of friendship between an elderly man and a boy is very inspiring. The movie deal with a physics principle: buoyancy. The movie tells us how a recently widower man tries to make his wife's dream real by traveling to South America. To do so, he uses his house as a mean of transport by making it float in the air. According to the plot, the house goes up thanks to the buoyancy force produced by thousands of helium balloons. Would that work? A National Geographic TV show tried to recreate the set-off of the house and got the result you can see in the video below.


Despite it is a lightweight house specially designed for this experiment, a lot of helium was used. Could something like that be done with an actual house like the ones that people live in? How much helium would be needed? According to an estimation by, a 150-squared-meter house may have a mass of roughly 100 tons. Are you able to estimate the volume of helium that could make a house fly?

Let's do the math.

First, we can get some info from the video. They use 300 balloons. Each of them has a diametre of 8 ft (2.44 m). The reporter says that it was almost freezing, so I will assume they are at STP conditions. We also need additional data, such as helium and air densities:
He: 0.179 kg/m3
Air: 1.295 kg/m3.

How much helium do they use?
Each balloon is a sphere with a radius of 1,22 m. Its volume can be easily calculated: 7.59 m3. All the balloons together add up a volume of 2277 m3. Multiplying by its density we figure out that the mass of helium inside the balloon is 407.5 kg which has a weight of 3994 N.

What buoyancy force can be obtained from that mass of Helium?
The buoyancy force is the weight of the fluid displaced by the balloon. In this case we have to calculate the weight of 2277 m3 of air.
E = 1.295 kg/m3 · 2277 m3 · 9.8 m/s2 = 28897N.

This buoyancy force has to be strong enough to rise not only the house but also the helium itself. The weight of cannot be greater than 28897 N - 3994 = 24903 N, so it has to be lighter than 2541 kg, roughly two and a half tons. To rise an actual house, like the one you may live in, with an estimated mass of 100 tons we would need forty times more Helium, which is 91080 m3. That volume is the equivalent of a sphere with a radius of 28 m. Take the volume of a party balloon (0,91 according to the website a party stuff provider company) into account we conclude that 100000 balloons would be needed. I honestly was expecting an even higher number of balloons. In fact, we would need much more balloons, at least to set the house off. Don't forget the force we have just calculated is the one required to lift the house but, to set off we have to pull it out from its foundations and higher buoyancy force would be required.

jueves, 4 de mayo de 2017

Physics and movies: Titanic

[Spoiler alert: the ship sinks]

In one of my last lessons, a controversy regarding the movie Titanic came up. Just after the sinking of the ship, the two main characters, Jack (Leo Dicaprio) and Rose (Kate Winslet), try to keep themselves alive by laying on a wooden door. After a few attempts, they reach the conclusion that it was impossible because of the fact that the door didn’t have enough buoyancy to hold the two of them. As a consequence, only one them could survive, so Jack decides to let Rose lay on the door while he remains in the water. Finally, he dies due to the low temperature of the seawater and sinks. My female pupils, who consider this scene is one of the most romantic ones ever shot, tell me off when I express my doubts about the buoyancy of the door. Possibly Jack could have survived if Rose had let him up.

I have surfed the net and found some information.
  • According to this frame of the movie and others we can estimate that the dimensions of the door are 2 m · 1 m · 15 cm.
  • Jack (Leo Dicaprio) has a mass of 77 kg
  • Rose (Kate Winslet) has a mass of 63 kg
  • Some kinds of wood were used to build this famous ship. Each of them has a different density:
    • Teak: 980 kg·m-3
    • Oak: 770 kg·m-3
    • Pine: 440 kg·m-3
  • The density of sea water at that low temperature (we can assume it was very close to 0 ºC or even lower) is 1.028 g/cm3.

  • Do the math and calculate the maximum buoyancy force exerted on the door.
  • Figure out the total weight of the system door+Jack+Rose for each of the three types of wood mentioned before.
  • Compare that two forces.
  • What do you think? Did Jack have a chance? Did he die because he romantically decided to sacrifice his life to save Rose’s or just because she was mean?

miércoles, 3 de mayo de 2017

Counting one mole

One mole is the number of molecules in eighty grams of water which is very little water, about three teaspoonfuls. Such a little volume of water contains 6.02·1023 molecules (the Avogadro’s number) which is a huge number of molecules. To picture how big this number is I wonder how long it would take to count all molecules in one mole one by one? One year? One century?… Let’s do some maths. To make the task a little bit easier I will suppose we are able to count ten units per second which is a very fast counting rate. Counting that fast, we would count…
10 molecules in 1 second
600 molecules in 1 minute
36,000 molecules in 1 hour
864,000 molecules in 1 day
315,360,000 molecules in 1 year.

As we have to count from 1 to the Avogadro’s number, we just have to divide it by the number of molecules counted in a year (6.02·1023/ 315,360,000), which leads us to an enormous number of years: 1.9·1015. This number is much more impressive if we don’t use scientific notation: 1,900,000,000,000,000 years, in other words, almost two quadrillion years (in Spanish: mil novecientos billones). Is it that long? Let’s compare it with other long times:
First human-ish beings appeared 4.5 million years ago
Earth exists since 4.5 billion (4.5·109) years ago
The Universe 14 billion (1.4·109) years ago.

The time we would need to count from one to the Avogadro’s number is about 1,364,000 times longer than the age of the Universe. Mind-blowing, isn’t it? To make the task shorter, let’s imagine that all people (7 billion = 7·109) helped us to count, in that case, the counting would be 7 billion times shorter, which is roughly 270,000 years. All the human beings in our planet working together would need two hundred and seventy thousand years to count the number of molecules in a mole.

As you can see the Avogadro’s number is really huge but it is just the number of water molecules in a sip of water.  

martes, 28 de marzo de 2017

La nueva "selectividad": EBAU

La polémica Evaluación final de Bachillerato, la mal llamada Reválida, solo tendrá, de momento, efectos académicos para el acceso de la Universidad. A efectos prácticos la Prueba de Acceso a la Universidad de estos últimos años se sustituye por la Evaluación del Bachillerato para el Acceso a la Universidad (EBAU) con una estructura muy similar.

¿De que hay que examinarse? Obligatoriamente de todas las troncales de 2º: Lengua Castellana y Literatura, 1ª Lengua Extranjera e Historia de España, además de Matemáticas (los del Bachillerato de Ciencias), Latín (los de Humanidades), Matemáticas aplicadas a las CCSS (los de Ciencias Sociales) o Fundamentos de Arte (los de Artes). La media de las notas de estos cuatro exámenes de la calificación de la EBAU. Hay que sacar un 5 ó mas.

¿Qué es la nota de acceso y cómo se calcula? Es la media ponderada de la nota final de bachillerato (60 %) y la calificación de la EBAU (40 %). Debe ser igual o mayor que 5 para poder acceder a la Universidad.
Nota de acceso = 0,6 · Nota de Bachillerato + 0,4 · Calificación de EBAU

¿Puede mejorarse la nota de acceso? Sí, realizando hasta un máximo de cuatro exámenes de materias troncales de modalidad. En el caso del Bachillerato de Ciencias (el que mejor conozco por razones obvias): Física, Química, Biología, Geología y Dibujo Técnico. No importa si el alumno las ha cursado como troncales, como específicas o que no las haya cursado. Cada una de estas materias tiene asignado un coeficiente (0,1 ó 0,2) dependiendo del grado al que alumno quiera acceder. Se tendrán en cuenta solamente aquellas dos notas que den un resultado mas favorable para el alumno. Estos exámenes permiten aumentar la nota de admisión hasta 4 puntos.
Nota de admisión = 0,6 · Nota de Bachillerato + 0,4 · Calificación de EBAU + c1·TO1 + c2·TO2

¿De que asignaturas me conviene examinarme para un determinado grado? Aquí os dejo el enlace de la tabla de ponderaciones de la UEX: ponderaciones UEX

¿Para qué grados pondera la Física en la fase de admisión?

Física II
Grados con
ponderación 0,2
Grados con
ponderación 0,1
C y T de los Alimentos
Ciencias Ambientales
Terapia Ocupacional
Todas las Ingenierías
Ciencias de AFD
Com. Audiovisual
Educación Infantil
Educación Primaria
Educación Social
Finanzas y Contabilidad
Relaciones Laborales
Trabajo Social
Información y Doc.

¿Para que grados pondera la Química?
Química II
Grados con
ponderación 0,2
Grados con
ponderación 0,1
C y T de los Alimentos
Ciencias Ambientales
Terapia Ocupacional
Educación Infantil
Educación Primaria
Educación Social
Ciencias de AFD
Algunas ingenierías:
I. civil
I. Agropecuaria
I. de Materiales
I. Eléctrica
I. Electrónica
I. Topografía
I. Forestal
I. Horticultura
I. Industrias Agrarias
I. Mecánica
I. Química industrial
Com. Audiovisual
Finanzas y Contabilidad
Información y Doc.
Relaciones Laborales
Trabajo Social
Algunas ingenierías:
I. Imagen y sonido
I. Diseño Industrial
I. Telemática
I. Informática

Planetary movement: Galileo Galilei

Prezi presentation by Rafael Tejada, Marta Thovar and María Santos.

They also have made a poster about Galileo's life and works:

They have found also this rap. You can learn a little bit of science with a rap. Thank you for finding it for me. I really love it.

miércoles, 22 de marzo de 2017

Sobre gráficas de experiencias de física...

En muchas ocasiones utilizamos gráficas para estudiar la relación entre dos magnitudes. Una buena gráfica tiene mas importancia de la que parece. Por ejemplo, hemos hecho recientemente una experiencia sobre la fuerza de rozamiento. En ella pretendíamos medir la fuerza de rozamiento que surge al intentar mover diferentes masas apoyadas en una superficie horizontal y relacionarla con la fuerza normal N ejercida por la superficie.

He aquí una de las gráficas presentadas por mis alumnos: 

OK: Los ejes están etiquetados, se indica las magnitudes que se están representando y en que unidades. Los ejes no están etiquetados, no sabemos que se está representando ni las unidades.
OK: Los ejes son proporcionales, la misma distancia en el gráfico representa la misma cantidad de la magnitud representada. La misma distancia en el gráfico representa diferentes cantidades de la magnitud representada: 140 – 105 = 35 mide lo mismo que 212 – 140 = 71.
OK: ambos ejes comienzan en cero.

OK: Se representan los puntos resultado de cada una de las medidas pero no se unen con una línea. Se representa una línea pero no los puntos de las medidas realizadas.
OK: Dado que los puntos parecen seguir una línea recta, se ha dibujado la recta de tendencia que mas se aproxima a todos ellos.

Mejorable: En el gráfico aparecen líneas de cuadrícula para cada división del eje vertical pero no cada división del eje horizontal. No aparecen líneas de cuadrícula.

miércoles, 15 de marzo de 2017

¿Por qué no venden fruta en los ascensores?

Este vídeo está tomado del blog de experimentos de Física y Química del profesor Manuel Díaz, al que felicito públicamente por su interesantísimo blog. Esta mañana hemos estado haciendo problemas de ascensores. En este vídeo pesan fruta en un ascensor, observa lo que ocurre con la lectura de la balanza electrónica.

miércoles, 22 de febrero de 2017

Simulaciones de movimientos rectilíneos

En la clase de hoy hemos trabajado con dos simulaciones de la página "Física con Ordenador" del profesor Ángel Franco de la Universidad del País Vasco.

Enlace a la experiencia simulada de MRU:

Enlace a la experiencia simulada de MRUA:


Siguiendo las instrucciones de la página realiza ambas experiencias y presenta un informe de la misma en el que se incluya:
- Breve introducción teórica.
- Tabla con las medidas realizadas.
- Cálculo de la velocidad (en el MRU) y de la aceleración (en el MRUA)
- Gráficas.
- Conclusiones.

La tarea deberá presentarse la semana que viene (hasta el 3 de Marzo) por parejas. Cada pareja elaborará un documento que en enviará a

1. Es mejor que en cada pareja haya un alumno de los que han estado hoy en clase y otro que no.
2. Es recomendable usar el navegador Firefox en vez del Chrome. Es un programa en lenguaje java si no funciona te pedirá que instales la última versión de Java. 

jueves, 16 de febrero de 2017

Descenso del nivel del agua en una probeta.

La bajada del nivel del agua cuando abrimos una probeta es puesto en muchos libros como ejemplo de movimiento rectilíneo uniforme pero ¿lo es realmente? Hoy nos hemos propuesto comprobarlo.

El procedimiento es simple:

- Medimos con una regla la altura en centímetros entre las marcas de 0 y de 10 mL en la bureta que resultó ser de 13,6 cm. De esta forma sabemos que nivel baja 1,36 cm por cada mL de agua vertida.

- Llenamos la bureta con agua hasta por encima del nivel de enrase.

- Abrimos la llave de la bureta dejando caer agua lentamente (gota a gota)

- Ponemos el cronómetro en marcha en el momento en que agua pase por el "0" de la bureta.

- Medidos la posición del nivel del agua cada 15 s.

Los resultados son los que se muestran en la gráfica. los puntos muestran cada una de las medidas realizadas. En un MRU deberíamos haber obtenido una recta. Se ha representado también la recta que mejor se ajusta a esos puntos. Podemos observar que los puntos obtenidos no parecen encajar muy bien con una recta.
Antes de precipitarnos en las conclusiones analizamos la velocidad para cada intervalo. Dividimos para cada intervalo la distancia avanzada entre dos medidas entre el tiempo trancurrida entre las mismas. Obtenemos los siguiente:

Como vemos se aprecia una clara tendencia descesdente aunque con ligeras fluctuaciones. Es mas acertado pensar en un MRUA que en un MRU. Se ha representado también la recta que mejor se aproxima a los datos representados. Dicha recta tiene por ecuación:
v = 0,15676 - 0,00015 t. (con v en cm/s y t en s)

Si comparamos con la ecuación de velocidad del MRU:
v = v0 + a t
podemos deducir que el valor de la aceleración es 0,00015 cm/s/s.

Conclusión: el movimiento del nivel de la bureta abierta se ajusta mejor a un MRUA que a un MRU.

viernes, 10 de febrero de 2017

Distance required to stop a car

If you see and obstacle on the road while you are driving two processes have to happen for you to totally stop the car:
- Reaction. It takes you a time to hit the brake. That is the reaction time or thinking time. During this while, the car continues moving at the same speed. It is a uniform movement.
- Braking. No matter how well the brakes work, the car will never stop immediately. A time and a distance is required to speed reduce to zero. This second process is a UAM or uniformly accelerated movement whose acceleration is opposite to its velocity.

In this video speeds are expressed in miles per hour (mph). Take into account the fact that 1 mile = 1,609.344 meters.

Considering the information provided in the video:
A) Calculate the reaction time.
B) Calculate the braking time and acceleration.
C) Draw the speed vs time graph for the different initial speeds: 20 mph, 40 mph and 60 mph.

lunes, 6 de febrero de 2017

Espumosa vengana química de Sheldon Cooper

A parte de por su sentido del humor, me gusta Big Bang Theory por que la ciencia que aparece en la serie es real. En este vídeo vemos a Sheldon utilizar la descomposición del peróxido de hidrógeno en agua y oxígeno catalizada por yoduro de potasio para vengarse de su "archienemigo" Barry Kripkie.


sábado, 28 de enero de 2017




    lunes, 16 de enero de 2017

    A modern version of the gold foil experiment

    A modern version of one of the most important experiment in the history of science.

    Watch the video and answer the questions

    1.- Who made this experiment more than one hundred years ago?
    2.- Which kind of particles are used in this experiment?
    3.- Why some of the particles pass straight ahead but others are deflected?
    4.- What does 1.5 microns refers to?
    5.- What does the bell sound represent?
    6.- Mention two differences between the original experiment and the modern one.
    7.- How many particles per second are detected by each detector?
    8.- What conclusion can we make from the results of this experiment?
    a) J. J. Thomson was right.
    b) Atoms are mostly empty space.
    c) Atoms are rigid spheres.
    d) None of above.
     9.- What happen to the particles passing closer the nucleus?

    domingo, 15 de enero de 2017

    Chemical equations calculations. Exercise 6.

    Alba Muñoz 4º B

    Concentration. Exercise 8.

    We need to prepare 150 ml of a 20 g/L solution of iodine. How much iodine and water do we need? Explain how to prepare this solution in the lab.

    Enrique Borrego 3º A

    Concentration. Exercise 10.

    When 33g of sugar are dissolved in 198 g of water we get 0,22 liters of solution. Calculate the mass concentration (g/L) and the percent of mass of the solution.

    María Ordóñez 3º A

    Concentration. Exercise 6.

    A solution has been prepared with 30 g of sugar solved in water till we get 200 ml of solution. Which is its mass concentration of sugar in g/L?

    Mª Dolores Godino 3º A

    Concentration. Exercise 7.

    We weight 5 g of sodium chloride and water is added till we have 250 ml of solution. What mass concentration (g/L) of sodium chloride we get?

    María Benitez 3º A

    sábado, 14 de enero de 2017

    Concentration. Exercise 1.

    Ana Quiñones 3º B

    Concentration. Exercise 4

    Carmen Mirón 3º B

    Concentration. Exercise 9.

    We need to prepare 300 ml of a ferric sulfate solution for fertilizing plants with a 12 g/L mass concentration. How much ferric sulfate mass do we need? Explain how to prepare this solution in the lab.
    Ana Ordóñez 3º A

    Concentration. Exercise 3.

    María del Arco 3º A

    Concentration. Exercise 5.

    Pedro Suárez 3º B

    Chemical equations calculations. Exercise 9.

    What mass of sulfuric acid do we need to react with 10 g of aluminum? The unbalanced equation for this process is:  Aluminum + sulfuric acid → Aluminum sulfate + hydrogen (H2). How many moles of hydrogen are produced? What is its volume at 800 mmHg and 20 ºC?

    Gonzalo Rodero 4º B

    Chemical equations calculations. Exercise 5.

     If 8g of CO2 is produced by the combustion reaction of C5H12, calculate the number of grams of C5H12 which was burned.
    Carmen Maján 4º B

    Chemical equations calculations. Exercise 20.

    Jin Suk Oh 4º A

    viernes, 13 de enero de 2017

    Chemical equations calculations. Exercise 13

    .4NH3(g) + 6NO(g)→5N2(g) + 6H2O(g). How many moles of each reactant were there if 13.7 moles of N2(g) is produced? What volume of nitrogen is produced at 1.3 atm and 27 ºC?

    Gabriel Ballesteros 4º A

    Chemical equations calculations. Exercise 10.

    Inés Cabello 4º B

    Chemical equations calculations. Exercise 2.

    In the combustion reaction C8 H18 + O2 → CO2 + H2 O,.
    a) What mass of water will be produced when 27.3 g of C8 H18?
    b) How many molecules of CO2 will be produced?
    c) How many atoms of H are in 2 mol of C8 H18?

    Julia Arroyo 4º A

    Chemical equations calculations. Exercise 8

    Photosynthesis is a chemical reaction in plants used to produce oxygen and glucose. If a plant uses 6.3 g CO2, how much water (in grams) is needed to complete the reaction? How much glucose will be formed?

    Marina Polo 4º B

    Chemical equations calculations. Exercise 12.

    .Given the reaction: Zn + CuCl2 → ZnCl2 + Cu. How many moles of copper will be produced by reacting 23.5 g of Zn? What mass of copper(II) Chloride is needed? 

    David Sánchez 4º B

    Chemical equations calculations. Exercise 4.

    Marta Thovar 4º B

    Chemical equations calculations. Exercise 18.

    Using the following equation: Pb(SO4)2 + LiNO3 → Pb(NO3)4 + Li2SO4. How many grams of lithium nitrate will be needed to make 250 grams of lithium sulphate, assuming that you have an adequate amount of lead (IV) sulphate to do the reaction?

    Andrés Meléndez 4ºA

    Concentration. Exercise 15

    Exercise 15. As we saw in the lab, a 330-milliliter Coca-Cola can contains 35 g of sugar. Knowing that the mass of 330 mL of Coca-Cola is 343.86 g, calculate the sugar concentration in Coca Cola in g/L and percentage by mass.

    Andrea Rey 3º B

    Concentration. Exercise 14

    A blood test finds that the cholesterol concentration in a person’s blood is 2.05 g/L. What mass of cholesterol does this person contains if he or she has five liters of blood? What mass of cholesterol did the lab actually find if the test mass made with 10 mL of blood?

    Carlota Cuellar 3º A

    Concentration. Exercise 12

    Chemical equations calculations. Exercise 7.

    If 33.6 liters of NO2 is decomposed, find the volumes of N2 and O2 produced at STP.

    Ángela Polo 4º B

    One comment. We don't need to use fractions to balance a chemical equation. This equation could be balanced also like this: 
    2 NO2 → N2 + 2 O2

    Concentration. Exercise 11

    Air is gaseous solution where the concentration of oxygen is 21 % by volume. How many liters of air are there in 25 liters of air? What volume of air contains 30 liters of oxygen?

    Chemical equations calculations. Exercise 16.

    Ángela Polo 4º B

    Chemical equations calculations. Exercise 14.

    Alberto Blanco 4º A

    Chemical equations calculations. Exercise 3.

    Rafael Tejada 4º B

    Chemical equations calculations. Exercise 1.

    Celia Álvarez 4º A

    Chemical equations calculations. Exercise 11.

    A common pollutant in atmosphere is the sulphur dioxide from the power stations. Sulphur dioxide can be treated by reaction with hydrogen sulphide which produce sulfur and water.
    a) Write and balance the chemical equation of this process.
    b) If a power station produces 500 kg of SO2 per day, how many liters of H2S will be needed to treat it? Suppose the process occur at STP conditions.

    Tomás Rodríguez 4º B

    Chemical equations calculations, Exercise 19.

    Ana Vila 4º B

    Chemical equations calculations. Exercise 15.

    Concentration. Exercise 13

    Alba Cruz 3º A

    Chemical equations calculations. Exercise 6.

    Ana Vázquez 4º B

    Concentration. Exercise 2.

    Exercise 2. A glucose solution is 30% mass. How much glucose and water has 100 g of solution?

    Pedro Pablo Álvarez 3º A

    Chemical equations calculations. Exercise 17.

    Using the equation: NaOH + H2SO4 → Na2SO4 + H2O. How many grams of sodium sulphate can be formed by reacting 200 g of sodium hydroxide with an excess of sulphuric acid?

     Álvaro Mejías 4º A