Comet Question

[SkyWriting]

If I drop a 16 lb. bowling ball, and then an 8 lb. bowling ball, which will land first?

Your fingers will cramp over time and you'll drop the 16# first.
But the second one will smash your swollen toes, and hurt more.
And you'll forget all about the first one.

So I say you'll only recall the 8 pounder.

 

[SkyWriting]

They lose mass as they blow off steam, but that won't alter their orbit, unless the steam they blow off acts like a mini jet engine, and increases their speed slightly; in which case they will go into a slightly lower orbit.

Comets tend to fall into the sun because the side facing the sun discharges vapor or steam and slows their speed.

 

[Chesterton]

Correct. Gravity is an external force, one of the few fundamental forces that exists.

This goes to my earlier point - gravity is determined by mass, so if mass changes, the force of gravity changes, therefore the speed/velocity must change, therefore the orbit must change. Where am I wrong?

 

[BL2KTN]

This goes to my earlier point - gravity is determined by mass, so if mass changes, the force of gravity changes, therefore the speed/velocity must change, therefore the orbit must change. Where am I wrong?

You are correct. The change of mass is relatively tiny, however, and thus the orbit's adjustment is likewise.

 

[essentialsaltes]

This goes to my earlier point - gravity is determined by mass, so if mass changes, the force of gravity changes, therefore the speed/velocity must change, therefore the orbit must change. Where am I wrong?

The force of gravity is proportional to the comet's mass.

However, the equation of motion (Newton's second law) is that F = ma

i.e. the force on the comet equals the mass of the comet times the comet's acceleration.

The acceleration tells us how the comet moves. If we rearrange the equation to

a = F/m

the dependence on the mass of the comet cancels out. The force is proportional to the mass, but then you divide by the mass and get the same acceleration.

This is exactly the same as with a heavy rock and light rock dropped at the earth's surface. They experience the same acceleration, although the forces and the masses are different.