Tides tl;dr: Tidal forces are the difference in force felt on one side of a body versus the opposite side. In the context of astronomy, tidal forces arise from the fact that gravity depends on the distance to the (other) massive object and that objects (like planets and moons) have non-zero size.

Details: Take for example a planet experiencing tides as a result of its moon. The acceleration felt on the near side of the planet (near to the moon) is a_ns = GM/(r-R)² , where r is the distance from the planet's center of mass to the moon's center of mass, and R is the radius of the planet. On the far side: a_fs = GM/(r R)² . The tidal acceleration is: a_ns - a_fs =

= GM [ 1/(r-R)² - 1/(r R)² ]

= GM [ (r R)² - (r-R)² ] / [ (r R)² (r-R)² ]

= GM [ 4rR ] / [r⁴ .....]

= 4GMR / [r³ .....]

Thus, for tides on some object being perturbed by a massive object, the strength of tides is proportional to the mass of the perturber, the radius of the object being perturbed, and inversely proportional to the cube of the distance between the two objects.

Tidal locking tl;dr: Tidal forces raise a tidal bulge that points towards and away from the perturber. If the tidally distorted body rotates at a different rate then the perturber orbits around the body then the bulge will get rotated away from the line directly from the body in question to the perturber. Here's a diagram for the case where the body orbits faster than the perturber orbits. The perturber will torque on the tidal bulge and try to pull it back in to line. This will change the rotation rate of the body (and the orbital rate of the perturber) until the rotation rate of the body and the orbital rate of the perturber are equal. In other words, until the same side of the body is always facing the perturber. Many moons are (or are expected to be) tidally locked to their planet. Also, many extrasolar planets that orbit close to their star are expected to be tidally locked.

Tides in general:

• How would oceanic tides be affected if the moon were orbiting at an ISS altitude?

• Since the moon has an influence over the Earth's oceanic tides, would it have any effect over my ability to jump?

• Why are the times of ocean tides so different between relatively nearby areas?

• Are things lighter below the moon?

• When the tide goes out, where does the water go?

• Can someone explain why there is an antipodal high tide?

• How is the moon's gravity strong enough to affect so many millions of litres of water to create tides, yet we feel no effects?

• tides on a moon

• Why are there two spring tides per lunar month?

• Why do both sun-earth-moon syzygies cause stronger tides?

• Are tides stronger or weaker at the poles?

• How exactly does the Moon effect the tides of Earth?

• A few questions about tides

• How do I develop physical intuition for the tidal force?

Tides from multiple bodies:

• How would multiple moons affect a planet's tides?

• Pretend we have a second moon, basically identical to our current one, orbiting perfectly on the opposite side of the planet as our own. Would we still have tides?

• If Earth had a second moon, how would it affect the tides?

• What would happen to the tide on a planet with more than one moon?

• Multiple questions about having 2 moons

Tidal locking:

• Why is the same side of the moon always facing Earth? Is this common among satellites?

• Tidal Locking Earth to The Sun

• What causes tidal locking?

• Why we always look at the same side of the moon?

• Why do we only see one side of the moon?

Orbital evolution and tides:

• Why is the the Earth's Moon gradually drifting further away from us, rather than gradually spiraling in as what I would seem to consider more intuitive?

• Could we make the Earth rotate faster by bringing the Moon closer?

Relevant Wikipedia articles:

• Tides

• Tidal force

• Tidal acceleration

• Tidal locking

• Tidal heating