# Physics Predictions/Thoughts (1 Viewer)

#### Arrowshaft

##### Active Member
kepler's laws though? do we just disregard the first two and only talk about the third? i don't see the relevance of the question to the first two kepler's laws.

(i'd also appreciate it if you weren't so arrogant, thanks.)
I would start of by deriving the energies; kinetic and total to be exact (which aren’t too difficult). Then you can derive Kepler’s third law by analysing $\bg_white \dfrac{2\pi r}{T}$ and equating it with orbital velocity (might’ve to derive that as well)

#### Hscbuzman

##### Active Member
the first law "all planets move about the Sun in elliptical orbits" the energy stuff really falls apart huh?
Yes bcos eplitcal means kinetic and GPE change but total is same. That means u can find out v from it. If it was circular u can't do that

#### Arrowshaft

##### Active Member
Just seems like a mostly derivation based q to me

#### worldno17

##### Active Member
Well if u direagard the first then the energy stuff falls apart. The second is useful aswell for extra deets. You really need to get back to the textbooks...
ok dude. i was literally just asking an ordinary question. the first law has nothing to do with energy unless you wanna talk about hohmann transfer orbits (which was part of the old syllabus and isn't required for this one). i think you're speaking to yourself here.

(but thanks for the questions though)

the first law "all planets move about the Sun in elliptical orbits" the energy stuff really falls apart huh?
thanks for echoing my thoughts!
how much do we rlly need to know about elliptical orbits though?

#### akkjen

##### Active Member
Yes bcos eplitcal means kinetic and GPE change but total is same. That means u can find out v from it. If it was circular u can't do that
i am confused by this can you show how its done please.

#### Hscbuzman

##### Active Member
Just seems like a mostly derivation based q to me
Yes and no. For 9 marks I would say deriving would be half of it. The rest and the harder part (hence why it is a 9 marker) is about linking them together and having an overall understanding

#### Hscbuzman

##### Active Member
i am confused by this can you show how its done please.
How the mighty have fallen. One moment ago I am wrong and now u r begging for help.

#### Arrowshaft

##### Active Member
i am confused by this can you show how its done please.
Its because as a satellite or planet moves closer to the object being orbited, it gains kinetic energy as it 'falls', which by the law of conservation of energy means that it loses gravitational potential energy, since the total energy must be constant

#### Arrowshaft

##### Active Member
Yes and no. For 9 marks I would say deriving would be half of it. The rest and the harder part (hence why it is a 9 marker) is about linking them together and having an overall understanding
Yeah i know, hence why i said "most". But yeah, i would make evaluating statements for each and compare them

#### akkjen

##### Active Member
Its because as a satellite or planet moves closer to the object being orbited, it gains kinetic energy as it 'falls', which by the law of conservation of energy means that it loses gravitational potential energy, since the total energy must be constant
ye isn't that the second law.... objects sweep equal areas or something like tht

#### Hscbuzman

##### Active Member
ye isn't that the second law.... objects sweep equal areas or something like tht
No. Its not a law but is comes due to the 1st law thus its important

#### akkjen

##### Active Member
How the mighty have fallen. One moment ago I am wrong and now u r begging for help.
i still don't believe that statement, you can't analyze total energy using the first law

#### Arrowshaft

##### Active Member
ye isn't that the second law.... objects sweep equal areas or something like tht
Not really, that law is based off Kepler's observations on the solar equinoxes of the Earth iirc, he reasoned that since at different paths along the elliptical orbit the rotating object possess varying angular and linear velocities, its areal velocity must be the same across equal time periods.

#### Hscbuzman

##### Active Member
i still don't believe that statement, you can't analyze total energy using the first law
It provides the basis for it. In a three marker, it would be worth 1 mark for linking it.

#### Arrowshaft

##### Active Member
Really just an empirical observation

#### Minecraft

##### New Member
When doing special relativity questions, do we consider both time dialation AND length contraction in calculations? For example a question asking how long it takes for a spaceship travels to a distant planet a given distance away.

#### Arrowshaft

##### Active Member
But I guess you’re right in the sense you can explain the 'speeding up' as gaining and losing K and U respectively, so you’re right

#### Arrowshaft

##### Active Member
When doing special relativity questions, do we consider both time dialation AND length contraction in calculations? For example a question asking how long it takes for a spaceship travels to a distant planet a given distance away.
Depends what they're asking. If they ask for time or distance, just choose one

#### Arrowshaft

##### Active Member
You guys should join the physics question thread! it'll be good revision, we can contribute to each other's understandings

#### worldno17

##### Active Member
When doing special relativity questions, do we consider both time dialation AND length contraction in calculations? For example a question asking how long it takes for a spaceship travels to a distant planet a given distance away.
depends on the variables given, i think it'd be good if you could provide an example q.