Young Scienties

a way of Science - Solutions

I have created this site to advertise some science education resources that I have produced for use in the classroom.
Problem solving from student questions.
Best of all they will save you lesson preparation time.

Best Wishes,
Pristiadi Utomo

                      

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WORK ENERGY AND POWER

Imagine a porter carrying a load on his head. Is he doing any work? Yes, one would say! He would be paid for carrying that load from one place to another. But in terms of Physics he is not doing any work! Again, imagine a man pushing a wall? Do you think he is doing any work? Well, his muscles are contracting and expanding. He may even be sweating. But in Physics, we would say he is not doing any work!

 If the porter is carrying a very heavy load for a long distance, we would say he has lot of energy. By this, we mean that he has lot of stamina. If a car can run at a very high speed, we say it is very powerful. So, we relate power to speed. However, it means something different in Physics. Let us understand what is meant by work, power and energy in Physics.


Work

 Consider the simplest possible case of work done. A force ‘F’ is acting on an object. The object has a displacement ‘S’ in the direction of the force. Then the work done is the product of force and displacement.

 What will happen in the case when the applied force is not in the direction of displacement but rather at an angle to it. In such a case we will consider the component of force in the direction of displacement. This component will be effective in doing work as shown.

F

S

θ

 

Component of force in the direction of displacement is F Cos θ.

W = (F Cos θ) S = ∑F. S

Work done is a scalar quantity.

Cases where work is not done

 Let us consider some cases where work is not done:

• Work is zero if applied force is zero (W=0 if F=0): If a block is moving on a smooth horizontal surface (frictionless), no work will be done. Note that the block may have large displacement but no work gets done.
• Work is zero if Cos θ is zero or θ = Π/2. this explains why no work is done by the porter in carrying the load. As the porter carries the load by lifting it upwards and the moving forward it is obvious the angle between the force applied by the porter and the displacement is 90^o.
• Work done is zero when displacement is zero. This happens when a man pushes a wall. There is no displacement of the wall. Thus, there is no work done. Similarly when a car is moving on a road, there will be a frictional force applied by the road on the. There will be work done by the frictional force on the car. What is the work done on the road by the car? From Newton’s third law we can say that to every action, there is an equal and opposite reaction. Thus the force applied by the road on the car will be equal and opposite to the force applied by the car on the road. Since, there is no displacement of the road, there  will be no work done on the road. 

Units of Work

Since work done W = F.S, its units are force times length. The SI unit of work is Newton-meter (Nm). Another name for it is Joule (J).

 1 Nm = 1 J

 The unit of  work in cgs system is dyne cm or erg

 Note that 1 J = 10⁷ ergs.

 The dimensional formula of work is [ML²T^-2]

Table 1:

System	Unit of work	Name of the combined unit
SI	Newton meter (Nm)	Joule
cgs	Dyne centimeter (dyne-cm)	Erg

 

Positive and Negative Work

We have seen the situations when the work done is zero. Work done can also be positive or negative. When 0^o <= θ < 90^o, work done is positive as  Cos θ is positive. Work done by a force is positive if the applied force has a component in the direction of the displacement. When a body is falling down, the force of gravitation is acting in the downward direction. The displacement is also in the downward direction. Thus the work done by the gravitational force on the body is positive. Consider the same body being lifted in the upward direction. In this case, the force of gravity is acting in the downward direction. But, the displacement of the body is in the upward direction. Since the angle between the force and displacement is 180^o, the work done by the gravitational force on the body is negative.

Note, that in this case the work done by the applied force which is lifting the body up is positive since the angle between the applied force and displacement is positive.

Thus work done by the  applied force which is lifting up the body is positive since the angle between the applied force and displacement is positive. Thus work done is negative when 90^o < θ <= 180^o as Cos θ is negative. We can also say that work done by a force is negative if the applied force has a component in a direction opposite to the displacement.

Similarly, frictional force is always opposing the relative motion of the body. When a body is dragged along a rough surface, the frictional force will be acting in the direction opposite to the displacement. The angle between the frictional force and the displacement of the body will be 180^o. Thus, the work done by the frictional force will be negative.

 
Examples on Work Done

 Example1:

A box is dragged across a floor by a 100N force directed 60^o above the horizontal. How much work does the force do in pulling the object 8m?

60o

F = 100 N

N

W

 F = 100N

θ = 60^o

S = 8m

W = (F Cos θ) S

=(100 Cos 60^o) 8

= 100x1/2x 8 = 400 J

 

Examples on Work Done

Example 2:

A horizontal force F pulls a 10 kg carton across the floor at constant speed. If the coefficient of sliding friction between the carton and the floor is 0.30, how much work is done by F in moving the carton by 5m?

 The carton moves with constant speed. Thus, the carton is in horizontal equilibrium.

 F = f = μN = μmg.

 Thus F = 0.3 x 10 x 9.8

= 29.4 N

 Therefore work done W = FS

=(29.4 Cos 0^o)%

=147 J

 
Variable Force

 

Force in everyday life is usually not a constant force. So far, when we have discussed work done, we have assumed that the force is constant. What happens if the force is variable? The figure below is a graph of varying force in one dimension. 

Let us first understand how de we find work done from a graph. If we are talking about a constant force, the area under the graph (which will be a rectangle) gives us the work done.

 
Example on Variable Force

Example 3

 A force F = 2x + 5 acts on a particle. Find the work done by the force during the displacement of the particle from x =0m to x = 2m. Given that the force is in Newtons.

 Work done W  = ∫F(x)dx

 Thus W = = ∫F(x)dx Cos 0^o

= ∫F(x)dx

= ∫(2x + 5)dx

= 2x²/2 + 5x |

= 2² + 5 x 2

= 14 J

 

                 

Important to Use Internet on Learning

Discuss the process of learning will be up to the teaching of media play an important role in the learning process. Learning activities has even shifted to the lecture method and the reduced move towards the use of media. In general, the media has a role to make the learning process more important, high efficient, current and interesting. In particular, useful for the media to simplify the learning of complex material, a great zoom, enlarge the small, slow down and speed up the process, the approach distance, keep at a close, record important moments-moments, perpetuate and disseminate, showing the validity of a process and so . Given the importance of media in the learning process, teachers are expected to have the skills to make media.
One of the important media is the internet. Internet is a window of the world without limits. Through the internet can be a wide range of information, ranging from the political, economic, social, entertainment, even science and technology. In addition to the internet as a window of information, it can also serve as media of learning.
Learning Science is dynamic, Learning developments in other countries using the media and methods vary and the model is known and easily obtained. For example tool experimental Physics for movement in this country we are still using the trolley, sloping field, ribbon, and the ticker timer (in fact, still many schools that have not yet).
 
For such tools, can it used to moving on plat field or even rise? Only the derivative field
But in other countries already use the equipment with a mechanical precision and high accuracy.
    
A panel used to change movement variations selected button on the car. This tool producing record impressions to movement parameters. Tracking can  made variable, even, including derivative and climb around the track.
After know that, the physics teacher can start to change. Also change is the basic law of nature. Nature is always changing, something that never changed in nature is change itself.

 

We teachers are no longer sitting pretty waiting for the changes given by the education policy (government, education office, headmaster) but independently develop competency dynamic is not one of them ambivalent to the Internet.
Physics is one of the basic science that will support the development and growth science and technology on future. Physics website on the internet have a very large amount. Search engine (search engine) Google, for example, the number 1800000 in the top of the page (homepage) is related to the keyword physics. The large number of this page will not be useful if teachers and physics students do not utilize.
physics learning with the internet media strongly support the constructive approach, which focuses the teacher as a facilitator. The main objective of physics with the media the Internet is:
1. Create atmosphere of learning may be interesting and even interactive.
2. allows teachers to create interactive whiteboard with access to teaching materials physics and running through the projector.
3. Make examples of real physics (physics of daily life).
4. Facilitate distance learning (eg time practice have in vocational school
My study of potential teachers and students more time to spare even outside official hours learning in schools, it is possible to chat with, group mailing list, comments visit, or the other, the more the visual quality of a short message or through the telephone.

 

 

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This is internet's science learning web site, for students, parents and teachers. I offer a friendly environment to support public & private school science education and home learning. Young Scienties is chock full of information, news, links, pictures, with the best content from leading colleges & universities.
Young Scienties is the place to learn about the universe, or space, find information on volcanos, and global warming, explore plants, animals or microbes, and study physics , mathematics or chemistry.

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New ways of teaching physics

Dr. Hans Laue
Department of Physics and Astronomy

In this article, you can find information about some of the things that have been happening in recent years in the design and delivery of physics courses at our university. Although the reported changes have taken place in the context of a particular subject, I think that some information may have applications in other areas. The following topics will be addressed:

• Curriculum changes in first-year physics
• Computers in physics teaching
• Cooperative learning in first-year physics courses
• Discovery-style laboratories in first-year physics
• Curriculum review of senior physics courses and course descriptions
• Future plans.

Curriculum changes in first-year physics

Last spring, members of the physics department designed a major change in the first-year curriculum for physical science majors. The first-year physics curriculum has been static across North America; the last major changes were made in the 1960s. The typical first-year physics course begins with an introduction to classical physics (e.g., Newton's laws, and work and energy) and has little room for the physics of the twentieth century. In the United States, studies are now being conducted into how modern physics can be included in the first-year curriculum.¹ Our new pair of introductory physics courses for physical science majors, Physics 215/217, appeared in the 1995-1996 University Calendar for the first time.

Physics 215 is quite different from the beginning course in the previous traditional sequence, Physics 231. The new Physics 215 is a response to course evaluations, which stated that first-year physics seemed to contain little that was different from high school physics (the fact that the level of the presentation is different does not matter.) Physics 215 will have as an underlying theme the dichotomy of the wave/particle nature of matter. It will attempt to present technological applications of modern physics as well as open questions at the research frontier. The course will start with an introduction to optics. Technological applications will include X-ray diffraction, which was helpful in the discovery of the structure of DNA, and optical instruments. The course will go on to the twin pillars of modern physics, the theory of relativity and quantum physics. The theory of relativity has given us the idea of mass-energy equivalence, as expressed in the equation E = mc², which has led to applications in nuclear energy technology. Quantum physics has introduced the notion of probability as a fundamental property of matter and has made profound technological change possible, e.g., the use of ionizing radiation in the treatment of tumours, spectroscopy as an analytical tool, lasers, and solid state physics devices like the transistor - essential in all computer technology.

After the glamour and glitz of Physics 215, Physics 217 will then take the students back in history to Newtonian mechanics. By this time, students have taken a semester of calculus, which makes it possible to present mechanics in more elegant fashion (not surprising since Newton invented calculus.) A modern feature of the course will be the use of computers to calculate motions like those of a skydiver or of a space craft in the presence of the earth and the moon. We started using computers in this way in our first-year curriculum a couple of years ago. We think computers can help students focus on the physics of the problem, because the computer will give nonsensical results if an incorrect force expression is entered into the machine. Computers provide immediate feedback. At the same time, students learn about an important tool for doing calculations. The exercise is structured so that programming skills are not required. Our experience has shown that after students have been introduced to computers in a hands-on fashion, they can learn to do the numerical exercises that we pose and can begin to appreciate an important way in which scientists use computers.

Computers in physics teaching

Using computers as calculating machines has been described above. This is a natural transfer from research experience into the teaching arena. Another instance of this kind is using computers in the laboratory for automatic experimental data processing. As soon as relatively inexpensive desktop computers became available, around 1980, we started using computers in undergraduate laboratories for automatic data processing.

Now, computers are being used widely for computer-assisted learning purposes. The department was a pioneer on this campus in this area. Soon after the arrival of the Macintosh computer in 1984, a fairly powerful machine with excellent graphics capabilities, software was created in the department using animated graphics to teach the detection of radioactive radiation. In 1987, the CALiPH (Computer Assisted Learning in PHysics) project began. To my knowledge, this software is now the most comprehensive physics software of its kind on this continent.

CALiPH is a computer-delivered tutorial system that deals with conceptual difficulties in introductory physics. The impetus to create this software came from research showing that conceptual matters are usually poorly understood after exposure to the typical lecture course. At the same time, research demonstrated that this material could be learned in an effective manner through one-on-one instruction.² Since the human resources to provide this kind of instruction were unavailable, we decided to use the computer as a substitute. Although computers have obvious limitations when compared to human tutors, they also have certain advantages over human tutors. Students appreciate being able to take their time when working with computers and prefer the anonymity and independence provided by this form of tutoring.

A few prototype programs were designed in the summer of 1987, and their effectiveness was tested with a group of more than 50 student volunteers from our first-year classes in the fall of 1987.³ The results were positive, and it was decided to create a computer- delivered tutorial program for the entire first-year curriculum that would emphasize conceptual aspects of physics and exploit the computer medium's strengths, i.e., animations and user interaction. We decided to design the tutorial for the Macintosh because of its superior graphics capability and friendly user interface. We also decided, after some experimenting with authoring systems, to write the programs in BASIC, a language that takes advantage of the Macintosh's built-in graphics and user interface capabilities, provides a minimum of restrictions, and could be easily learned by summer students who were hired to help with the programming.

CALiPH now contains 36 separate instruction programs, called modules, and a separate testing program. Each module takes about two hours of a student's time to complete. CALiPH supports the first-year sequence for physics majors (the new course, Physics 215, will require additional modules), the first semester of the general physics sequence, as well as parts of several second-year courses. It is being made available to over 1000 students annually. The department has a lab with 35 low-end Macintosh computers. The lab has been relatively inexpensive to set up. A number of machines were given to us as surplus; others were bought with department and faculty funds. CALiPH is also available in the student microcomputer lab in the library tower. At present, CALiPH is available on the Macintosh family of computers only, but plans for adapting it to the Windows environment exist. Our experiences with integrating CALiPH into the curriculum have been described in the literature.⁴

Many people have helped to create CALiPH, either as programmers or by making suggestions. Student feedback has brought about significant change. Financial support was provided in a variety of ways. The University Endowment Fund helped support the pilot project. Summer support for student programmers was made available via provincial and federal STEP and SEED grants. Canada Immigration and Employment, through its job training program, provided funds for several programmers. Support from the Royal Bank Teaching Development Fund was provided through a grant and via an allocation from the Faculty of Science. The Department of Physics and Astronomy helped in topping up the salaries of some summer student programmers.

CALiPH is used at several other Canadian universities, although we have made no systematic attempt so far at distributing this material. CALiPH was also presented to students at a college in Madras, India. It was very well received, especially among female students who discovered that through CALiPH they had access to tutorial help that was otherwise unavailable to them, because of the structure of Indian society.

Cooperative learning in first-year physics courses

Two different approaches to cooperative learning are presently being used in first-year physics courses. One of them involves bi-weekly workshops; the other one involves in-class group work.

i) Workshops

For several years, we have run bi-weekly three-hour workshops in our first-year courses that alternate with bi-weekly laboratories. The workshops have replaced traditional individual assignments. Worksheets for the workshops are handed out one week ahead of time, and students can work on these before coming to the workshop. Help with the worksheets is available, for example, via the drop-in CALiPH computer tutorial. Remaining questions can then be handled during the workshop, where students work in groups and can seek help from the lab instructor. The latter is, in most cases, a graduate student or a sessional instructor. Students are expected to write up their solutions individually and are required to hand them in at the end of the workshop for marking. Typically, 15% of the course grade is awarded for the workshops. The purpose of the workshops is to help students learn the material. The structure of the workshops is deliberately flexible so that students can do what works best for them.

ii) In-class group work

In a new experiment this year, five to ten minutes of every class were set aside for answering one or two questions projected on an overhead. The students were asked to work in groups of three or four for this purpose. The composition of the groups remained the same for a month; the following month entirely new groups were formed. The questions were related to the same lecture, and students had to hand in their answers for marking. The marked answers were returned at the beginning of the next class, and students received bonus points for their work in this exercise. This worked well on the whole. Students indicated on their course evaluations that they preferred to be given questions related to the previous rather than the present class in order to have some time to understand the material before being given a question. This suggestion will be followed next time.

Discovery-style laboratories in first-year physics

During the last two years, the laboratories in the first-year physics courses for physical science majors have been significantly revised. We felt that the first-year laboratory should give students a sense of the spirit of experimental inquiry and that it was less important at this time to teach technical skills related to data analysis. Thus, free-form laboratories were introduced in which students were given various pieces of equipment along with some suggestions for questions that they might try to answer with the given equipment. Precise instructions on how to set up possible experiments were not provided. As a result, one can now observe physics students riding skate boards in the hallways or riding the elevators in the Social Sciences building to observe forces during accelerated motion. One can also see students splashing around with water trying to discover the properties of fluid flow in pipes. The lab reports for experiments of this kind are expected to be like those of a working scientist, with an initial statement of the intended purpose of the experiment and a description of the proposed experiment, a log of the experiment as performed and the observations made, a conclusion, and a raising of open questions. Good marks are given for the consistency of the experimental inquiry and observations, not for the accuracy with which any existing theory might be verified by the observations.

Not all laboratories follow this free-form format. Others do give instructions on how to set up an experiment and ask for specific measurements. However, in these structured labs too we have abandoned lab reports that follow a rigid format. Instead, we are asking students to turn in lab reports that they themselves have organized along the lines described above. This way, students learn to write and think for themselves.

 

Curriculum review of senior physics courses and course descriptions

Curriculum development is an ongoing process in our department. In 1987, we had a major overhaul of the senior physics curriculum where we improved the sequencing of topics in the curriculum, introduced new courses, and trimmed some low-demand courses. That curriculum has worked well.

Last summer, we undertook another review of the physics curriculum from second to fourth year. The purpose of the review was to make the courses really mesh and to spell out explicitly what background would be expected in each course. This review was done jointly by faculty and students. The outcome is the "Green Book", a set of detailed descriptions, including background preparation for each course, that have been agreed to by students and by faculty members involved in teaching particular courses. This has been a fruitful exercise because it became apparent that various topics had to be moved from one course to another to achieve a better fit. The Calendar descriptions of some courses had to be changed as a result. The new 95/96 University Calendar reflects these changes.

We divided the material from each course into core material and optional suggested material. We agreed that the core material of a course should always be taught, either because the core was deemed to be an essential part of our students' education or because it was part of the expected background for a subsequent course. The optional material was intended to allow individual instructors to structure a course according to their preferences. The "Green Book" is going to be revised on an annual basis according to our experiences with its content. Apart from being useful for students and faculty in the course delivery, the book has proved useful in communicating with people from outside of the department and outside of the university. One of our students presented a paper at this year's Undergraduate Physics Conference in Ottawa on the work that went into the green book.

Future plans

In the current climate of rapid change at universities, much may happen in the way we teach physics that we do not foresee. However, whatever may happen, I am confident that, because of its past track record, the Department of Physics and Astronomy will be up to the challenge.

In our first-year physics teaching, we may move towards more conceptual aspects of physics.⁵ What is the point of working out mathematical equations if one does not know what the equations are all about? We may also start experimenting with presenting tutorials on the Internet to reach a wider audience, both in this province and beyond. We welcome the input of others on what they would like to see.

Acknowledgment

Many individuals in the department, students, support staff, and faculty, as well as outsiders, were involved in various aspects of what I have described. They all deserve credit for what has been accomplished.

For more information, contact Hans Laue: 220-6909, messages 220-5410.

1. J.S. Rigden, D.F. Holcomb, and R. DiStefano, Physics Today 46(4), 32 (1993).
2. D.E. Trowbridge and L.C. McDermott, Am. J. Phys. 48, 1020 (1980); 49, 242 (1981); F. Reif, Physics Today 39(11), 48 (1986), Cognitive Science 11, 395 (1987), J. Res. Sci. Teach. 24, 309 (1987).
3. R.B. Hicks and H.Laue, Am. J. Phys. 57, 807 (1989).
4. R.B. Hicks and H.Laue, Physics in Canada, 48(5), 293 (1992).
5. W. Brouwer, Physics in Canada, 51(1), 55 (1995).