This week I am doing work and energy with my Y3 students. We started with the concept of work and went on to the activity bouncing ball activity shown below.
Bouncing Ball Activity
Explain:
Describe the conversion of energies during the motion from point of release to just before hitting the surface
Explain why the returned height after each bounce is less
Students' explanation on why the ball has a lower height after each bounce
It is a rather common for learners to associate any lost in mechanical energy as "being converted to heat". It is probably because of the fact that they have experienced the warmness due to phenomena such as rubbing their palms, hitting a nail, etc.
Qualitative understanding of the energy exchange during the bounce
What really happens requires the understanding of the following concepts:
Temperature is a macroscopic observable/measurable of the average kinetic energy of random molecular motion
The contact between the ball and the floor increases the random molecular motion at the expense of the ordered mechanical energy. This causes the temperature of the ball and floor to rise or we say that the internal energy of the system increases.
Assuming that the ball and floor is at thermal equilibrium to the surrounding, the increase in their temperatures result in a net transfer of heat to the surrounding, i.e. the warmness we feel if we are to touch them (the temperature difference is likely to be less than what we can perceive in reality).
In addition, we can also apply the same thinking to conclude that the movement of the ball through the air also increases the internal energy of both the ball and the surrounding air.
Feynman's explanation below (from about 1min45sec to 3min15sec) really sums it for learners and most importantly encourages them to enjoy thinking about science.
Fostering a growth mindset in learners (children, adults) may be one of the key to improve learner's motivation to learn, ownership of learning, perseverance, etc. As teachers, we may not have been tackling the root cause of these learning characteristics.
Learners will benefit greatly from a changed mindset about learning.
The benefits of a Growth Mindset - Carol Dweck
The Power of Belief - Mindset and Success: Eduardo Briceno at TEDxManhattanBeach
I couldn't help it but to apply what I know in Physics to explain the white/gray haze as a result of the smoke particles from the Sumatran fires.
To begin, let's try to understand how we see the colours of the buildings and the trees. We "see" the colour of the light that is scattered off the surface of the building, leaves, etc, as shown.
Scattering of light from surface (e.g. green wavelength em waves from the leaves) causes us to "see" the objects' colour
Light is also scattered by the air molecules (~ nanometer in diameter) along the path from the sun to the object by a process known as Rayleigh Scattering (refer to my previous post) towards the observer. However intensity from this scattering is low compare to the intensity of light reaching the objects and scattering to the observer. Generally our eyes cannot sense this low intensity in the presence of the light from the objects.
Effect of Rayleigh Scattering is small and hence don't produce the white haze
With the dust particles, diameter in the region of microns (0.000001 m), suspended in the air, Mie scattering dominates. The scattering intensity by this process is larger than Rayleigh Scattering. The larger intensity of white light scattered from these particles (consisting of all visible wavelengths) is now more obvious to us.
Mie Scattering off the dust particles produces the white haze seen the above photo
The additional scattering also explain why the sky and the sun appears red around 6 pm, something we usually observe around 7pm when the sun is close to the horizon.
Red sky and sun around 6pm
For more information on the different type of scattering, you may want to refer to this.
Line Spectral with Tracker
Created this instructional video on "using Tracker for spectroscopy". Youtube link is here.
Lessons that took place before this activity
Students could have
observed the emission spectrum of elements such as sodium, neon, mercury, kryption, etc. although they do not have hands-on experience of setting up and adjusting the apparatus.
gone through the theory and tutorial on the section of spectral lines.
Resources
Below is the worksheet that students will attempt.
Experiments on spectral lines are time consuming due to the difficulty in setting up and calibrating the spectrometer.
The other big challenge is the availability of a suitably dark room to observe the emission spectrum of discharge tubes.
The availability of sufficient apparatus for experiments.
Advantages of using Tracker
Cheap.
Easy to deploy.
Easy to operate to obtain quality results for comparison and analysis because students can now focus on the actual physics rather than on the setting up of the experiment.
This is something that should be related to your experience when shopping with your family at supermarkets, etc. Observe the behaviour of the wheels and provide your responses to the questions after the video.
Newton’s 3rd Law (N3L)
presents the greatest challenge to students unlike the first two laws of motion
because what it claims is contrary to many daily experiences and hence counter-intuitive.
Newton's Laws of motion allows us to break down the problem systematically by first
looking at the forces exerted on bodies, compute the resultant force and predict
its motion.
Challenges to learners include:
·N3L only work for specific
situations, e.g., system at rest
In using N3L, we are looking only at the forces between two bodies at the
point/surface of direct contact (except for the fundamental forces which
are action-at-distance[1]). If the two bodies are in contact, the force on each body due to the other body
must be equal and opposite. There is no need to look beyond the contact point
when using N3L.
·Object being pushed experience a
larger force exerted by the imparter (e.g., a person) while the force exerted
on the imparter by the object should “logically” be less.
This is a compromised version of N3L.
Students recognized and know that they need to apply N3L but preconceptions compel
students to come up with alternate N3L to accommodate daily experiences. The
single most difficult misconception to rectify in students since it is contrary
to daily experiences.
·Using N3L to describe the forces
acting on the same body
Forces like gravitational force (or
weight) and normal force exerted by the table are taken to be the pair of
forces in accordance to N3L because they are “equal and opposite”.
·Dealing with massless string
In basic mechanics, the sole purpose of
the string is to connect various bodies so that they behave as a single system
yet they are not in contact. N3L is not applicable to the forces between two
bodies even though they satisfy “equal and opposite forces on two different
bodies” condition because they are not in contact. N3L describe the pair forces
between each end of the string with the body. For the string, the forces on
both ends must cause it to be taut and have equal magnitude regardless of the
state of motion of the system. This is a consequence of Newton’s 2nd
Law
Resultant force on string = mass x acceleration
=> Tleft – Tright
= 0 x a
[1] Actually most of the forces we are
discussing in Mechanics are action-at-a-distance electromagnetic interactions.
We don’t observe this because we perceive only the macroscopic form of the
interaction which is the contact between objects.
Complete the following questions for the 3 cases (see pictures)
(a) Identify the forces acting on A and B separately even if they are interacting
(b) Identify which pairs of forces that are a consequence of Newton' 3rd Law.
(c) Calculate the magnitude and indicate the direction of all the forces on A and B separately
(d) Calculate any forces on the rope if applicable
Below is a rather normal textbook example for Newton's 2nd Law:
A person pushes a box, mass xx kg on a frictionless surface with force yy N, determine the resultant force and acceleration of the box.
Very few teachers will spend their time to discuss about the person pushing the box since identifying the forces on the box is sufficient to illustrate the use of F = ma. However if one is to take a closer look at the forces on the person pushing the box, there will be plenty of inconsistencies. This is the kind of textbook problems that plague physics teaching which are the source of confusion among students because it cannot exist in real life. There is no way the person standing on a frictionless floor to push the box for more than an instant! There is a contradiction between actual experience and physics principles. To deal with the discrepancy, students resorted to assimilate the solutions of this kind of problems through mental accommodation rather than through a consistent mental model. Many researches have pointed out that such laymen's preconceptions of how the world functions continue to persist in learners even after years of high quality science education.
What Newton's 3rd Law claims and how we perceive how forces work in the real world can be poles apart. To master it, learners have to stop relying on personal perceptions and use the laws which are unbias and self-consistent. Learners should start to build a consistent mental model from ground up rather than accomodate Newton's Laws with pre-existing perceptions. For the record, it took the ancients (from Aristotle to Newton's) almost 2000 years to realise that over-reliance on the senses lead to inconsistent scientific model. To master Newton's Laws of motion, we may want to take a leaf from a small part in a famous martial art novel 《倚天屠龙记》.
《倚天屠龙记》里张三丰教张无忌太极拳里的片段
Finally I appeal to all teachers and anxious parents to allow students the time to struggle with Newton's Laws. The more they struggle, the more they can distil the essence of the laws. Trying to shorten the 2000 years' of journey taken by scientists is not practical. If a teacher can teach the laws [in the textbooks] and the students can understand them with a few examples and assignments, why do we need teachers? We can easily replace teachers with DVDs of lectures and demonstrations (Hewitt or Walter Lewin just to name a few)!
The following conversation may shed some light on the struggle of learners have in order to reconcile real world experience with Physics model.
Student - If I am pushing a box on a frictionless surface, then the box should push back at me with the same amount of force. But if the box is moving in the direction i am pushing it, doesnt that mean that i am exerting a larger force on it?
Teacher - Why do you think you need a larger force to move the box in the direction of motion? Maybe you want to break down the question into the forces acting on the box and the man. Maybe sketch and drop me a pic?
Student - Because there needs to be a resultant force for the box to move? But if that's so then newtons third law doesn't apply... So it only applies for stationary objects?
student's sketch
Teacher - Newton's third law applies 100% of the time! Looking at what you have drawn, the box will accelerate to the right but the person will accelerate to the left! Do you think there is something missing on the person? Can you walk on a frictionless ground?
Student - Then the person should have a force by muscles letting him walk?
Teacher - Nope. No one can walk on a frictionless floor! To walk, you need to be able to exert a force on the floor backward relative to you which gives rise to a force on you by the floor (see diagram). In fact you need to be able to exert a force by your legs larger than what you apply to the box in order for the situation to be possible in real life! Your question is the kind of textbook problems that plague physics teaching which led to confusion among students (You have brought up an excellent problem). If you look at the box alone, no problem, most students will be able to work out the acceleration of the box easily. However contradiction arises because the person can't accelerate forward with the box unless the floor under him miraculously turn into a rough floor for him to exert a force on it backward!
Teacher's sketch
Student - But then if we use your first question of two boxes as an example, when box A pushes on box B, by N3L box B should push back with the same amount of force. But box B will move, which means its pushing back with less force. then how does N3L apply then? Or rather, why is there even resultant force in the world?
Question that is referred to by student
Student - Oh. Is it cause the amount of force with which B pushes back on A doesn't affect the resultant force of B itself cause it's on A, not B?
This video (shot at 120 frame-per-second) can be used to show that the spring balance does not measure the weight/gravitational force of the object hanging below rather it is measuring the force exerted by object.
Concepts:
The spring extends because the object exerts a force on the spring balance and the spring balance exerts an equal force on the object (Newton's 3rd Law). The force on the object exerted by the spring balance is equal to the gravitational force on the object when the system is in equilibrium (either at rest or at constant velocity).
When the system is accelerated (or free fall in this video), the force exerted on the spring balance by the object is different (zero at free fall) while the gravitational force on the object remains constant
Stumble upon this through the Modelling Physics community in USA.
Using Direct Measurement Videos
"Direct Measurement videos show situations that students can use to learn and apply physics concepts. Grids, rulers, and frame-counters are added as overlays on the video. These allow students to make precise measurements of quantities such as position and time, without needing additional video analysis software." - http://serc.carleton.edu/sp/library/direct_measurement_video/index.html
Download the 2 videos on your iPad from dropbox (Y3 LTL Class->Assignments->02 Kinematics->02 Assignments Video x.yyy)
Observe the motion of the ball in each video.
Post your ideas about the motion of the balls for the following questions HERE.
Research the observed phenomena
Conduct an in-depth study of the 2 motions of the balls using your knowledge of kinematic motion, graphs and equations.
Present your research findings using the lab report format (modified from modelling instruction course material 2007). A sample of a lab report (handwritten version) can be found here.
Submission
You are advice to spend one and a half hour for this assignment.
iMessage me for clarification by 9/2/13.
Submit your report as a pdf (type written or neat handwriting using iOS app with any images collected) to me via email by 13/02/13 (Wed).
Technical Advices
Frame-by-Frame analysis using CoachMyVideo
In videos that we film for physics analysis, there is always a reference length (a metre rule, calibrated stick, etc) near to the object in motion. In the 2 videos, the distance between successive markings on the stick is 10 cm.
You can use your own ruler to measure the position of the object and make use of proportion to determine the position of the object from a reference point.
You may want to use adhesive tape, that is not too sticky, to secure your ruler to the iPad screen for ease of taking measurements.
iPad apps for collating lab report
Penultimate - handwritten notes with ability to attach images and sync with Penultimate
Evernote - typewritten report with image attachment. However difficult to write formulae
Neu.notes - similar to penultimate but is a paid app
Pages - microsoft equivalent for iOS devices. Very expensive!
1. Download the following video from Dropbox using your iPad (procedure to do that can be found here)
Dropbox app -> "Y3 LTL Class" -> "Classroom activities" -> "02 Kinematics" -> "20130125 Motion of an object II"
Download "table tennis ball.mp4"
2. Complete the following questions:
Determine the ball's positions, distance from the starting point of the motion, clock reading and time interval from the start of the video. Record it in a table with the following column headings. (The centre of the ball begins its motion at the position of 2.7 cm when the clock reading is 0 s, each white line represent 10 cm interval on the metre rule)
Describe the motion of the ball qualitatively (i.e., with words)
Compare with the assignment you have just completed (refer to here), state the difference(s) between the motion of the table tennis ball and the toy cars.
Create a possible motion map of the table tennis ball.
Use Graphical Analysis app to determine the linearised relation between the distance from starting point (y-axis) and time interval (x-axis).
3. Find a way to put all your work above in a single document (I would suggest Evernote or some handwriting app that you can insert pictures). This is in anticipation of future larger projects and experiment.
Hint to complete the assignment using CoachMyVideo app Do you need to capture the positions of the ball at equal position intervals?
Access "Video1.mp4", click the star icon on the top right hand corner to start the download.
Once downloaded, click on the "download icon" at the extreme top right hand to "Save to Photo Library".
2. Open the video using CoachMyVideo app to view downloaded video, capture the object's positions and clock readings (the reading on the top left hand).
3. Use Graphical Analysis app to determine the relation between the 2 quantities.
4. Repeat step 1 to 3 for "Video02.mp4" for the 2 moving objects.
5. Take photos of your data collection and screenshots of the graph produced and email to me.
Questions
After completing the data collection and analysis, please complete these questions.
Download the notes on "20130114 Y3 Measurement notes 2.pdf" from the shared dropbox folder (refer to your email).
Vernier Callipers
Read the section on "Vernier Callipers"
Download this iPhone app (http://itun.es/sg/FVJBH.i). Refer to pictures below regarding the setting and how to use the app. Use the app to gain an understanding on how to use the Vernier Callipers.
Complete Example 10 and 11.
Micrometer Screw Gauge
Read the section "Micrometer Screw Gauge".
Watch the following video on how to read micrometer screw gauge
An object (eg. a rubber ball) is dropped from the 4th floor.
Predict the motion of the ball.
Task
As a group, design a experiment to investigate the motion of a ball dropped from the 4th floor. Appoint a group leader and a scribe/photographer for the activity.In your account youshould pay particular attention to
the variables you will consider for falling objects,
the apparatus you use,
the procedure to be followed,
any safetyprecautions you would take,
Represent the data collected in a manner that helps you to justify your predication of the motion of a falling object.
Identify the possible error(s) in your experiment.
(Hint: You will probably need to
refer to the height measurement task that you did on Monday 7/1/13
use Movie app/CoachMVideo app to capture the fall
use CoachMVideo app for frame-by-frame and for position/timing)
Submission
Email completed softcopy writeup before 1159 pm on 13/1/13. We need the data for the lesson on Mon 14/1/13.
Physics Matters - 1.2 Physical Quantities and SI Units (Pg 3 - 5)
Video assignment. Click here to access the assignment.
"Physics is the study of how nature work from the smallest, like the atoms, to the largest, like the galaxies. The range of measured quantities in the universe is there fore enormous. This video will demonstrate the scale of things that physicists, and you, are dealing with everyday."
