Chapter 2 Describing Motion

Chapter 2 Describing Motion



Chapter 2 describing motion


Concept knowledge

Classification of variables in physics

A world build with particles

To observe is to influence

Sensory observation

Measuring properties at a certain moment

Relationships between variables

The change of properties through time is governed by a universal law

Variables that describe interactions over time

Observing and studying motion

The quest for a conceptual language in physics education.

Content knowledge describing motion dynamics

The CBR motion detector from Texas Instruments

Content knowledge describing motion kinematics

The solution of content issues Chapter 2 Conceptual Integrated Science


Phet simulations as a tool to test your conceptual understanding of linear motion.

Concept knowledge

Clicker test force (see Toledo)

Concept practical (light a bulb with a battery and a wire, also see Toledo)

→ concepts are hard to grasp, differences exist between the intuitive and scientific interpretation of a concept, scientific concepts are difficult to apply in reality

= worldwide problem

= studied in educational research

→ knowledge of preconcepts and misconcepts

→ innovative methodologies: inquiry based, ict supported, hands on, cooperative learning, ….

Despite of many efforts, the concept shift from intuitive to scientific interpretation remains extremely difficult.


The basic ideas in science are easy to understand. The way we pack these ideas and present them to learners on the contrary are extremely cryptic. We present them as isolated concepts (↔ classification). We work within models (↔ reality). We use an abstract language of formulas and definitions, math (↔common language, visualization, sensory information). This course aims to introduce a science didactics that introduces scientific ideas and concepts starting from real contexts, using common language, sensory observation, from a general to a detailed description, similar to the way we naturally learn.

Classification of variables in physics

A world build with particles

In the same way that a house is built with bricks, a LEGO house is built with LEGO blocks, a puzzle with pieces, a sentence with words, we and the world that we live in is built with building blocks called ‘particles’. Apparently, a set of 12 particles is sufficient to build the entire world . they are called

To observe is to influence

Humans experience the world, observe their surroundings and try to make sense of the observations. These observations give a snapshot of the surroundings and information concerning the properties of these surroundings at a specific moment. Observations over time provide us with information about changes of the properties of our surroundings. Especially this information about changes is vital in the survival of living beings.

By observing the world, humans influence it. This influence is called the forces the person exerts upon his surroundings . Influence works two ways. The person and his surroundings influence each other. The influence of the surroundings on a person are called the forces exerted by the surroundings upon the person. In symbols:


Also objects/ substances influence each other. Objects/substances exert forces upon their surroundings and the surroundings exert forces upon objects/substances. In symbols:


(Example falling ball, rubbed balloons, magnet and nail)

In common language we refer to forces as ‘pushing and pulling’. To visualize the influence, we draw an arrow.

Try to visualize the force you exert upon the given object in the following situations:

You push a car/ You lift a bucket/ you hold a book/ you sit on a chair / you swim/ you jump upward

In nature, 4 fundamental influences appear to exist. At that level, we call them interactions: the interaction between objects and substances with mass, the interaction between objects/ substances with charge, the interaction between particles within the nucleus of an atom: protons and neutrons and interactions between the building blocks of these particles: quarks.

Interactions always cause a change of properties. Sometimes these changes are so small or so slow that they cannot be observed macroscopically. When you look at an object for example, this object was influenced by light and inevitably was disturbed. This disturbance is so small that you cannot observe a change in the properties of the object while you are looking at it. Sometimes the disturbances can be observed. When you touch a snowball for instance, it starts to melt.

Sensory observation

In order to observe, humans naturally observe the surroundings using their senses.

We poses 5 senses: we see, feel, smell, taste and hear the world. Fundamentally these interactions take place between charged particles (electromagnetic interactions) and between masses (gravitational interaction).

With the information gathered from his observations, a person gains information about the properties of objects and substances.

See: in order to see an object/ substance, light that interacted with this object / substance must reach your eye. Light is a kind of electromagnetic disturbance that progresses . This disturbance is generated by the relocation of an electron in an atom. This disturbance is detected when it interacts with your optical nerves. The electromagnetic disturbance that it triggers, is interpreted by our brain and translated into optical information concerning the object/ substance. To communicate this information we use words as : color, how tall, volume, position, height, mobility, viscosity, … .

  • color : overall information about the wavelengths of detected light

red bleu

  • Position, how high, distance to, shape, volume: the direction of the incoming and the angle between the incoming light for both eyes.
  • Mobility: how quickly is the position changing?
  • Viscosity: how quickly is the shape of the object / substance changing?

Smelling: in order to smell an object/ substance small molecules originating from the object, must reach your nose and interact with the sense of odor in your nose. The electromagnetic disturbance that it triggers, is interpreted by our brain and translated into odor information.

Tasting: in order to taste an object/ substance molecules originating from the object/ substance, must reach your tongue and interact with the senses of taste in your tongue. The electromagnetic disturbance that it triggers, is interpreted by our brain and translated into information which we call taste.

Feeling: in order to feel an object/ substance, you mast touch it with your skin and interact with the sense of touch . The electromagnetic disturbance that it triggers, is interpreted by our brain and translated into information which we call:

  • hardness/ softness
  • shape
  • compressibility
  • weight
  • how warm/ how cold it feels

Hearing: in order to hear an object, a pressure wave generated by the object must be transported through the air, reach your ear and interact with your sense of hearing. The electromagnetic disturbance that it triggers, is interpreted by our brain and translated into information which we call sound, noise, music, … .

Sensory information has a limited accuracy and precision and is determined by a place and a moment. Sensory information is also subjective as it is the result of personal interpretation.

Based on the information we obtain by sensory observation, our understanding of the world around us becomes more detailed. We succeed in categorizing objects and substances into metals, liquids, soft, …. .

Measuring properties at a certain moment

In order to obtain a more detailed picture of the world, sensory information is supplemented with measurements. Measuring instruments are similar to senses, measuring instruments interact with their surroundings. Similar to sensory interactions, these interactions are fundamentally electromagnetic and gravitational.

Information obtained from measurements is more detailed, more precise, more objective, but also subjected to interpretation in our brain. We read data, interpret sound produced by measuring devices … .

A property that is measured by a measuring instrument is called a variable. Examples of variables are: time, temperature, mass, volume, position, charge, … . the result of a measurement is expressed in a number and a unit.

This extra information gives us a better understanding of the properties of the world surrounding us. The snapshot of the world evolves from cloudy to clear. The following scheme shows a limited view of the information obtained from measurements.

Sensory observation of properties → snapshot
seeing / feeling / smelling / tasting / hearing
color / shape / Position
Height / mobility / How much space it takes / viscosity / Hardness/ softness / compressibility / formability / weight / Electric charge / How warm/ how cold / odor / taste / sound
Measurement of properties→snapshot
Clock / Spectroscoop
Light sensor
prims / ruler / ruler / Microscoop / Measuring cup / Balans / Elektroscoop / Thermo-meter / Voltmeter
Ampère-meter / Indika-tor
pH meter / Manometer / …
Time / wavelength / Length / Position / shape / Volume / Hardness / Mass / charge / Tempe-rature / Current, voltage / acidity / pressure / …

Relationships between variables

Between some of the variables relationships exist. Some variables are related to each other.

For instance: there is a relationship between the mass of a liquid and its volume: the greater the mass, the greater the volume. Moreover: when the mass doubles, the volume also doubles.

Relationships like this reveal hidden information about the properties of nature.

The previous example possibly gives us information about the arrangement of the particles in a liquid. Imagine we know that a liquid is built with small particles. The relationship between mass and volume might suggest that distance between particles is fixed. Doubling the mass, means doubling the amount of particles. When the distance between particles is fixed, this would result in doubling the volume.

This is a possible conclusion. More measurements would be necessary in order to be sure about the conclusion.

The measurements give information about one specific aspect of the particle model. What about the arrangement of particles in solids and gases? What about the mobility of particles?

Another example:

The mass of a moving object and its velocity both determine how difficult it is to stop it. Mass and velocity appear to be related. This relationship is expressed by combining them in a new variable which we call impulse.

The previous relationships give rise to the definition of new variables and are summarized in a formula.

for instance stands for the relationship between mass and volume and is defined as density ρ.

More examples:

For an object with uniform motion on a straight track, its displacement is proportional to the elapsed time. Their ratio is called average velocity

Electric resistance expresses the relationship between the electric voltage across a conductor and the electric current through the conductor

The information gathered from measurements clarifies the microscopic meaning of variables. What for instance is the meaning of the variable temperature? Measuring it with an analog thermometer, relates temperature to the length of a liquid column. But what is the fundamental meaning of temperature? When you put a drop of ink in some water, the ink diffuses in the water. Now change the temperature of the water and repeat the experiment. You discover the following relationship: the higher the temperature of the water, the higher the diffusion rate. This relationship alights the nature of the concept temperature. Apparently the macroscopic concept of temperature is related to the microscopic concept of particle speed: the higher the average speed of the particles, the higher the temperature of the water.

The change of properties through time is governed by a universal law

Properties change through interaction, but apparently these changes are not random.

For instance: imagine you drop an object in vacuum. A relationships exists between the initial height of the object and its velocity just before it reaches the ground.: .

Or apart from a small correction a relation exists between the height from which a person is falling and the maximum length of the bungee that holds him: .

These relationships between properties before and after changes are more complex, not at first sight clear. The formulas that express these relationships are more complicated. They point to a deeper law in nature. This law governs all changes in nature. As far as we know this law has not been violated. It is called the law of the conversation of energy.

The law poses that you can calculate a certain quantity for every object/ substance, which we call ‘the energy of the object/ substance’. Through the manifold changes that nature undergoes, the total energy in the universe remains unchanged.

The energy of an object is determined by the properties of the object at a certain moment: the kind of matter it is built of (chemical energy), the velocity of the object (kinetic energy), the velocity of the particles that form the object (thermal energy) , the position of the object in relation to its surroundings (potential energy): its position in relation to earth (gravitational potential energy), its position in relation to charges (potential electromagnetic energy), its position in relation to a string (elastic potential energy) … . In case you made measurements of velocity, position, mass, temperature, … and you know the correct relationships, you can calculate the energy of the object at that moment by adding up all different contributions.

If anything changes with the object, the context changes, variables change and its energy will change.

This is a very abstract statement. It is a mathematical principle. To ever object you can connect a quantity that changes according to a fixed law. You can calculate the total energy for a specific state of the universe. Although the universe goes through many changes this total energy remains unchanged.

Variables that describe interactions over time

Interactions cannot be observed, but the effects of the interactions can. Objects and substances change shape, velocity and position, temperature and color , etc. . Some variables are defined to describe the effect of these interactions over time:

Work: a variable that describes the change in position as a result of interactions

Power: a variable that describes the velocity at which the interaction is happening

Heat: a variable that describes the change in temperature as a result of interactions


Observing and studying motion


Observe and describe the following motions.

Describe the following motions by measuring.

Look for relationships between measuring results.

Present the results in a written paper using the scientific method.

Presents the results in an oral presentation.

Practical1: motion of an air bulb in an oil filled tube.

Practical2: motion of a falling object

Practical3:motion of a mass on a pendulum

Practical4: motion of a mass on a spring

Didactical remark:

Note that we start the study of the chapter by posing a number of research questions and performing experiments. This methodology is called inductive. Learning starts from an offered real -life context and experiences within this context. The methodology aims to engage the student to connect to the subject, to give him experiences in different facets of the subject, to confront him with the personal preconcepts he holds on the subject.

The quest for a conceptual language in physics education.

(BPS 2012, also see the ppt on Toledo: natural sciences 1 content/ materials chapter 2)

The physics didactics course focusses on the ways we communicate in physics. How do we communicate, how effective do we communicate, how can we intensify class communication. Effective communication is the prime condition that must be met in order to facilitate the transfer of scientific knowledge.

Looking back I believe my personal focus on science communication was triggered when I myself was a physics student. I remember the phrase, ‘what does this mean, what do you mean’ as the most frequently used sentence amongst my fellow students during late night discussions. We had the mathematical description, but ‘what did it mean’?

Later, when I was a very young teacher, during summer holidays, I prepared pupils that had failed their physics examination. At the beginning of the first lesson I asked them ‘Tell me, what the problem is, what is it that you don’t understand.’ They replayed: ‘I don’t understand anything about it’. They had the abstract explanation, but failed to understand.

My students at the teacher training are all passionate about the subject. They chose to become physics teachers. Their educational background is divers. Many of them start the course without a good preparation in science or mathematics. But they share my passion for the subject and often become great physics teachers. Largest part of class time is dedicated to discussion, translation of abstract language into everyday language, the conceptual approach. Most time is invested in answering the question ‘what does it mean and very much how shall I explain’.

Planet TWILO

To illustrate the problem I would like to use the story told by Leon Lederman (The God particle). He tells the story of the inhabitants of the planet Twilo. These intelligent extraterrestrial creatures, look like humans, speak and act as we do, except for one thing: they are unable to observe objects with a strong contrast between black and white. Zebra’s for instance are invisible to them. When a Twilo delegation visits planet earth, they are invited to the a football game. The Twillians are politely observing the game, enjoying the atmosphere, but at first are completely unaware of the point of the game. It takes them quite some time and close observation before they link the cheering of the crowds to a small deformation of the goal net.