When a particle is flying from the ground slowly upward against gravity. Since the kinetic energy is zero throughout the process what is bringing the change in potential energy?
1Is the work done by wind to take the body up bringing the change in potential energy??
71Could the block rise automatically? Or is their any agent which pulls it up?
1Air is the agent i guess!
Energy conservation works in only conservative forces..... Force by air is non conservative.....I think thats why we are getting variable PE+KE.
But by applying Work energy Theorem, we get change in KE is equal to Work done by external force which so is zero then we may not say that the change in PE is brought by external agent?
1well its not at all a silly doubt....
potential energy is dependent on state of the body...like...for example....in a ball falling from certain height or in a simple pendulum...we take a particular position of the object (ball or bob respectively) and then define wat energy it is possessing (i mean how much)
i think in this problem..the potential energy increases or decreases due to the the position of the body..
for example...if u hav to climb 10 floors in a building...it doesnt matter if u go by stairs or lift or even if u high jump to the tenth floor...the work done to reach and potential energy at 10th floor will be the same...mgh
bt still u feel more tired if u go by stairs than by lift...ryt??
in this case ur supposing that air is applying variable force in one direction...so total forces acting on particle is gravity and air...
however...air is a fluid...the way u consider buoyancy for liquids...do the same with this...and luk at it by that view point....
similar to liquids...this particle is experiencing lot of forces...i know i have spoken out rubbish n not presented it well...bt i hope u get my point
1n the wind thing ur talking abt...consider it that particle is in a beaker of liquid n wen u put the beaker in motion how the scenario will be
or wen u put sumthin in flowing river....
frankly speaking even i am not able to solve it completely....bt if possible....try imagining the scenario...hope u get some possible idea which i havent got....take care
1while applying work energy theorem ,one should consider the forces acting on the body.
1) external force acting on the body(wind in this case)
2)weight of the body
viscosity and buoyancy can be neglected.
work done by gravity is negative and the work done by the external force is positive
therefore,if the object is raised by a height h.
work done by gravity is = -mg*h
work done by external force is = f*h
therefore work done = .5 mv^2
since the object does not acquire any velocity f*h-mg*h=0
f*h=mg*h
i.e work done by external force = change in potential energy
1Well, how much I know only Work by External forces only equals Change in Kinetic Energy!
1the things you know is incomplete the work done by any external force is equal to the change in the total energy of the particle let it be kinetic potential or any other kind of energy as there is a most basic law of physics called the law of energy conservation which states the energy is always conserved and till your level there is no mass energy transfer so dont bother about transformation of energy