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Work done and Kinetic/Potential Energy

Hi!

My Physics Class 11 question says that a climber who is 65kg climbs 600m. What is the work done by the climber?

I thought I would have to add both the change in KE and PE. But the answer says that you should simply use mgh (PE).

This is confusing because there should be some work done through movement. Why is KE omitted from calculating the work done that in this case?

Leo asked

Answer:

man climbing a treeMany problems in Physics are solved by making certain approximations and assumptions to avoid complications. If we consider all factors contributing towards the expenditure of energy there may still different factors, but of less importance for the problem under consideration.

In the question we assume that there is no change in Kinetic Energy involved. Work is not to be done to maintain the KE but to change the KE. Though there may be some changes in speed during the process, we disregard the changes and concentrate only on the change in potential energy, which is actually the work done against gravity.

Even if there is a change in Kinetic Energy, then also the work done against gravity will only be the change in potential energy (mgh). He may be doing work against friction and also the change his speed during the climbing process, but the work done against gravity would remain the same (mgh)

If the question had mentioned this (work done against gravitation) then confusion could have been avoided.

 

Hope that the idea is clear now. If you need further clarifications please respond via the comment form.

BOARD QUESTIONS FROM WAVE OPTICS (CBSE/AISSCE)

Download the collection of questions asked in previous CBSE board exams from the chapter Wave Optics.

Practising the previously asked questions is a good idea to prepare for the exams.

Download (PDF, 607KB)

Mechanical advantage of pulley configuration

It has been a long time since I have been in a physics class.  My question should, I believe, be relatively simple.  We are developing a prototype using a spool, cable, and pulleys to lift a weight.  I would like to know the mechanical advantage presented in the design I am attaching as well as any explanations you might have in how the determination was arrived at.  I am thinking it presents a 6:1 or 7:1 lifting advantage, but I may be way off.  Please feel free to disabuse any assumptions I may have on this.

2016-04-13

 

Doug Rasmussen asked

Applications on Newtons Second Law

“A particle of mass 2m is on a plane inclined at an angle 36.86 to the horizontal. The particle is attached to one end of a light inextensible string. The string runs parallel to a line of greatest slope of the plane, passes over a smooth pulley at the top of the plane and then hangs vertically carrying a particle of mass 3m at its other end. The system is released from rest with a single taut. Find the acceleration of each particle and tension in the string when the particles are moving freely, given that the plane is smooth.”

 

Asked Gina

Score More in Physics without fear – A Physics teacher’s advice

When I interacted with students of class 11 and 12, I came to know that Physics is one of the subjects they FEAR most during exam time. Though it seems difficult to score high marks, it is not actually so. Here are some suggestions you can easily follow to score more in Physics without much strain.

“Nobody plans to fail, but plans to fail”

Yes, A well planned study habit is essential to score more.

If you analyse the pattern of exam in class XI and XII (CBSE) the most of the marks are distributed among 3 marks questions and 5 marks questions which emphasizes the knowledge of concepts. SO, if you find it difficult to pass, then the following plan of action will easily work out.

  • Identify all portions from which 5 marks questions can be asked and practice them well.
  • Identify all portions from which three marks questions can be asked and practice them well.
  • Learn all Laws, definitions, principles
  • Practice all diagrams.
  • Practice all solved problems given in text

These links may be helpful

http://physics.2lv.in/must-practice-questions-for-cbse-physics-class-12-solved/

http://www.plustwophysics.com/set-of-must-practice-questions-for-cbse-physics-class-xii-aissce-physics-2011/

Just try this routine and see whether you are scoring more or not.

For those already got pass marks can also try the above to better plus

  • Solve all exercises and additional exercises from NCERT Textbook.
  • Get a collection of all one marks questions and Two marks questions preferably from previous ten years’ question papers and find answers to them.

Those who want to excel further and look forward to entrance examinations, IIT, MBBS etc the following links to books will help.

 

Numericals from Newton’s Laws of motion based on F=ma (For calss XI students of Kendriya Vidyalaya Pattom)

Download the questions and solve them and submit on or before 9 Oct 2014

  1. A force acts for 10 s on a body of mass 10 kg after which the force ceases and the body describes 50 m in the next 5 s. Find the magnitude of the force. [Ans: 10 N]
  2. A  truck starts from rest and rolls down a hill with constant acceleration. It travels a distance od 400 m in 20s. Calculate the acceleration and the force acting on it if its mass is 7 metric tonnes. [Ans: 2 m/s2, 14000N]
  3. A motor car running aat the rate of 7 m/s can be stopped by applying brakes in 10 m. Show that total resistance to the motion, when the brakes are on is one fourth of the weight of the car.
  4. In an Xray machine, an electron is subjected top a force of 10-23 N. In how much time the electron will cover a distance of 0.1 m,? Take the mass of the electron = 10-30 kg

Speed of Light and Protons

If the definition of the speed of light is for massless particles, then how is a proton, having mass, accelerated to the speed of light?  This sounds like a conumdrum.

Posted Thierence

Answer:

No massive particle has not so far crossed the speed of light. Even the extremely light particles – the neutrinos – couldnot cross the speed of light. Protons have not crossed the speed of light. When it nears the speed of light, the relativistic mechanics will become prominent and the it is to be treated is different from the classical mechanics.

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