Physics question -pls help with this one:)

I watched this vid https://www.youtube.com/watch?v=pwhW..._tDTS&index=30
and got confused as to why at 7:21, M does not change but at 9:30 M does change?

Reply 1

alevelphysicist

14

Original post by h26

I watched this vid https://www.youtube.com/watch?v=pwhW..._tDTS&index=30
and got confused as to why at 7:21, M does not change but at 9:30 M does change?

The first problem deals with just the Earth (which has a constant mass) and another object. The second problem deals with a comparison between the Earth and another hypothetical planet (with a fraction of the mass of the Earth).

In these problems, M just represents the mass of the Earth and doesn't change, but when doing calculations about the hypothetical planet, the fractional mass of the planet in comparison to Earth is used.

Reply 2

h26

OP

13

Original post by alevelphysicist

The first problem deals with just the Earth (which has a constant mass) and another object. The second problem deals with a comparison between the Earth and another hypothetical planet (with a fraction of the mass of the Earth).

In these problems, M just represents the mass of the Earth and doesn't change, but when doing calculations about the hypothetical planet, the fractional mass of the planet in comparison to Earth is used.