What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios

Summary

In this video, the host debunks common misconceptions about ocean tides, revealing that the true cause is more subtle than commonly believed. The Moon's gravity does not stretch the oceans but creates a pressure differential that causes water to bulge in certain areas, akin to a hydraulic pump. Lakes and smaller bodies of water do not experience noticeable tides due to their limited size and poor hydraulic response. The video also touches on related concepts such as the Sun’s influence on tides, the effect of Earth's rotation, and the complexities of tidal forces in various environments.

Takeaways

• 😀 The traditional explanation of tides being caused by the Moon's gravitational differential is incorrect. The real mechanism is more subtle and involves tidal forces.
• 🌊 Tidal forces are not about stretching or lifting the oceans, but instead result from the cumulative sideways pushes on water across the Earth's surface, creating pressure that causes bulges.
• 🌍 A non-inertial frame of reference, like Earth's, makes tidal forces appear as if there's anti-gravity, but this is a fake force in Newtonian physics.
• 🌔 The tidal force's differential gravity from the Moon is small—only about 1/10,000,000th of Earth's gravity—and isn't strong enough to lift objects or water.
• 💦 Tides are similar to how a pimple bulges when squeezed from the sides, showing how the Moon's gravity compresses the oceans, rather than stretching them.
• 🌒 Tides are generally not noticeable in smaller bodies of water like lakes, bathtubs, or swimming pools, due to the lack of sufficient area to generate noticeable pressure differences.
• 🌊 Very large lakes like Lake Michigan may experience minimal tides, but they are often smaller than the effects of wind or boats, making them hard to notice.
• 🌞 The Sun's effects on tides are similar to the Moon's but weaker, about a third as strong. When Earth, the Moon, and Sun align, the effect is stronger, leading to spring tides.
• 🌐 Tides are influenced by Earth's rotation, the position of continents, and local geographical features, which can lead to dramatic variations in tidal range from place to place.
• 🎬 Tidal effects on fictional worlds, such as the planet in the movie 'Interstellar,' may also be explained by similar principles, with squeezing forces being more important than stretching ones in extreme conditions.

Q & A

Why is the commonly explained model of ocean tides, based on the Moon's gravitational differential, considered incorrect?

The model that explains ocean tides as a result of the Moon's gravitational differential stretching the oceans is incorrect. While there is a gravitational differential, it doesn't cause the oceans to stretch or bulge. Instead, the tides are a result of the cumulative sideways pushes on water caused by tidal forces, which lead to the ocean's bulge along the Earth/Moon line, similar to a blister or pimple.

How do tidal forces work on objects relative to Earth's frame of reference?

In Earth's frame, tidal forces appear as a non-inertial acceleration. Objects experience a difference in acceleration due to the Moon’s gravity: blocks closer to the Moon experience a stronger pull, while blocks farther away are pulled weaker. This causes an outward separation force, called the tidal force, which acts similarly to anti-gravity.

What is the main cause of the ocean's bulges during high tide?

The ocean's bulges are caused by pressure building up from the cumulative sideways pushes exerted by tidal forces, not by stretching. The Moon's gravity pulls water toward the Earth/Moon line, causing it to pile up in that area, like a blister being squeezed from the side.

Why don't smaller bodies of water, like lakes or bathtubs, experience noticeable tides?

Smaller bodies of water like lakes and bathtubs don't experience noticeable tides because they lack the vast surface area required for tidal forces to accumulate enough pressure to create noticeable changes in water level. The effect is much smaller, often on the order of microscopic shifts.

Can large lakes, like Lake Michigan, experience tides?

Yes, very large lakes like Lake Michigan can experience small tidal changes, but these are typically only a few centimeters in height. However, these changes are too small to be noticeable due to other factors like winds and boats that create much larger disturbances.

Why are tidal forces in the ocean more noticeable than in small bodies of water?

Tidal forces are more noticeable in the ocean due to its vast surface area, which allows tiny sideways pushes from tidal forces to accumulate into significant pressure. In contrast, smaller bodies of water don’t have enough surface area for these small forces to produce noticeable changes in water level.

What role does Earth's own gravity play in the phenomenon of tides?

Earth's gravity creates a much larger force than the tidal forces caused by the Moon, so it prevents objects, like water, from being lifted during high tide. Earth's gravity also helps to maintain the overall stability of the ocean, meaning the tidal shifts are more about pressure redistribution than any actual lifting of water.

How does the Sun affect ocean tides compared to the Moon?

The Sun also causes tides, but its effect is only about one-third as strong as the Moon's, because while the Sun is much more massive, it is also much farther away. When the Earth, Moon, and Sun align, their tidal effects combine to create larger spring tides, and when they form a 90-degree angle, the tides are smaller, known as neap tides.

What are spring and neap tides, and how are they related to the alignment of the Earth, Moon, and Sun?

Spring tides occur when the Earth, Moon, and Sun are aligned, either during a new or full moon, leading to higher than average tidal fluctuations. Neap tides occur when the Moon is at a 90-degree angle to the Earth-Sun line, resulting in weaker tides due to partial cancellation of the gravitational forces.

Why do some places, like the Bay of Fundy, experience extreme tidal swings?

Extreme tidal swings, such as those in the Bay of Fundy, are caused by the geography of the region, which amplifies tidal effects. The shape and size of the bay, along with its position relative to the Moon's orbit, cause the water to pile up, leading to tidal variations of over 10 meters.