Tidal Friction

https://isaacphysics.org/questions/tidal_friction?board=da382f18-f7dc-49ab-8817-a44abb4564c7&stage=a_level

I was confused with this q ages ago but never got around to asking about it, and I have a few qs about it.

1. what is the green arrow in the diagram showing? Is that the moon's orbit? If so, why does this cause a tidal friction on the earth? surely if the earth was spinning CW and the moon orbiting CW, then sure at first there would be a tidal friction pointing ACW, but then when the earth catches up to the moon again, wouldn't the moon be pulling the earth CW, so the tidal friction would then be pointing CW as well, so how does this slow the earth's rotation?

2. I don't really get the logic of how you get to the answer.
Say angular momentum of system L = i_m * w_m + i_e*w_e, where w_e and w_m is the angular velocity of the earth and moon, and i is the moment of inertia
then if w_e decreases, and i_e is constant then (i_m * w_m) must increase so (mr^2 * w_m) must increase.
So either w_m goes up and r goes down, or vice versa but which how do you eliminate one of them? I tried using centripetal force but ended up with the same final argument of the angular momentum calc.

(edited 1 month ago)

Reply 1

mqb2766

Original post by mosaurlodon

https://isaacphysics.org/questions/tidal_friction?board=da382f18-f7dc-49ab-8817-a44abb4564c7&stage=a_level
I was confused with this q ages ago but never got around to asking about it, and I have a few qs about it.
1. what is the green arrow in the diagram showing? Is that the moon's orbit? If so, why does this cause a tidal friction on the earth? surely if the earth was spinning CW and the moon orbiting CW, then sure at first there would be a tidal friction pointing ACW, but then when the earth catches up to the moon again, wouldn't the moon be pulling the earth CW, so the tidal friction would then be pointing CW as well, so how does this slow the earth's rotation?
2. I don't really get the logic of how you get to the answer.
Say angular momentum of system L = i_m * w_m + i_e*w_e, where w_e and w_m is the angular velocity of the earth and moon, and i is the moment of inertia
then if w_e decreases, and i_e is constant then (i_m * w_m) must increase so (mr^2 * w_m) must increase.
So either w_m goes up and r goes down, or vice versa but which how do you eliminate one of them? I tried using centripetal force but ended up with the same final argument of the angular momentum calc.

Havent spent much time, but I presume that the earth is stationary, so the green arrow represents the relative angular velocity of the moon. The moon and earth are usually represented anticlockwise
https://science.nasa.gov/moon/tides/
but relative to a stationary earth (you watching the moon), the moon moves clockwise.

In the diagram, the tidal bulge is ahead of the moon a bit due to the earths rotation. Therefore the earth must be experiencing a -F (anticlockwise) force/torque on the tides. As its a force pair, this must exert a force F (clockwise) on the moon.

I think that should cover your first point,
https://explainingscience.org/2014/05/27/the-days-are-getting-longer/
has a bit more detail about the days etc, so see if those two links help and post if youre still unsure.

The isaac info is a bit terse (again).

(edited 1 month ago)