IIT JEE , AIEEE Forum - Physics , Mechanics - a conceptual doubt regarding conserv. of energy in sun planet system

mrityunjay

consider a sun and a planet system.when they are at infinite distance from each other we say that the energy of system is zero.as they come closer the energy of system<mechanical> becomes more and more negative. but since the total energy should be conserved the total energy everywhere should b zero.then where does this energy come from.or is the energy zero everywhere in its path?

master_purav

The negetive energy as you said increases is the Garvitational Potential Energy. There will be an equivalent increase in Kinetic Energy so that the total energy remains constant. As the bodies approach each other the GPE increases (in negetive) and there is equivalent increase in velocity.....

mrityunjay

but my ques is that why is energy not zero everywhere bcoz at infinity its zero and energy is conserved

nishant_88

The bodies will approach each other due to gravitational attraction. The work done by the gravitational force will be converted into energy.

shubham_sachdeva

arrey wats the matter. purav has explained it i suppose.....
at infinity E = 0
now as it approaches sun gpe becomes -ve & k.e increaeses such that K.E. = - (G.P.E.)

now u will say that when at infinity E = 0 then why planet moves...?? may be due to its inertia

mrityunjay

common yaar u even didnt understand the question. at infinity E=0,by conservation of energy , everywhere e should b zero or the planet should radiate energy sothat when it goes closer to sun its t.e. decreases but still e is conserved

akshaybansal

Mrityunjay,

your question says " as they come closer "

how do they come closer

if they are coming closer that means some force must be acting

that force is the source of this energy.

master_purav

Hello

You mean to say that the Law of Conservation of Energy is violated.

"Energy of an isolated system remains constant"

Here sun and planet form isolated system.

So their total Potential Energies = Their Kinetic Energies

Kinetic Enegy also includes the spin of sun, rotation of earth, etc.

Since net G P E is atrractive in nature, its sign is negetive. (You will come to know as you study other forces)

So total enegy of the system = 0

Hope u r clear........

krishna.gopal

Mrityunjay in case two particles at infinite seperation come closer to each other under its mutual attraction then total energy of the system will remain zero at all the time (so you are right in this aspect.) For earth sun system the total energy is negative. But you can't ask why it is not equal to zero because sun and earth never started from infinite seperation.

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