This is second part in the series of posts about Spaghettification. In this part we go over the idea of how gravity squeezes objects. We will also briefly discuss how to make tidal forces stronger.
Squeezing
Now let’s rotate the rod so that it is oriented with both ends equally distant from the Earth, while the rod’s mid point is still located at a distance R from the Earth’s center. Unlike the case when the rod was vertical, here the Earth pulls at both these ends with an equal force directed towards the Earth’s center (white arrows in Figure 2). The force at the left end (point A) can be resolved into two components – one perpendicular to the rod and another along the rod towards its center (see dashed blue arrows). What this resolution of forces means is that a part gravity wants to pull the rod downwards and the other wants to push it towards the right. The force at the right end (point B) can similarly be resolved into two components – one pulling it downward and the other to the left. However, note that the force components along the rod are directed towards the the center of the rod (dashed arrows blue arrows pointing to the rod’s center). Thus the rod feels squeezed (or compressed). Here again I choose the extreme ends to illustrate the general idea of squeezing – but the tidal force will cause compression across the entire length of the rod.
So gravity stretches and squeezes things at the same time! In fact this is one way to make noodles or spaghetti – a combination of stretching and squeezing. The reader can play with this idea even more by replacing the rod with objects of several shapes and sizes, such as a rectangle, a circle, or even a human being!
Feeling Tidal Forces
If we calculate the tidal acceleration felt by the rod near the Earth’s surface we find that it is millions of times smaller than the acceleration due to gravity. Immersed in Earth’s gravitational field, where we spend our lives, the resulting tidal forces aren’t enough to cause real life stretching / squeezing effects (Earth is shitty stretcher which is great news for us 😊). Another way to think about this is that the adhesive forces that hold the objects around us together can easily overcome these small tidal forces / accelerations. So how can we experience stronger tidal forces?
Tidal forces increase in strength in the following ways :
- the mass of the object producing the gravitational field increases
- the extended body is brought closer to the extended object
This does not mean that if we go deeper into the Earth we will be experience stronger tidal forces – because as you go deeper into the Earth the effective mass that contributes to gravity decreases as well.
In other words, tidal forces will get stronger where the gravitating object has a lot of matter crammed into a small volume. Such an object creates a strong field due to its large mass and in addition, a stronger gradient is experienced as one can reach closer to the object’s center due to it’s compactness. And nothing in the universe crams a lot of mass in a small volume than a black hole!
Summarizing: Gravity can stretch and squeeze objects. Fortunately, Earth’s gravity is not strong enough to induce tidal forces that can cause human beings like us to be stretched and squeezed.
In the next and the final part of this series we will examine the tidal influence of black holes on objects in their vicinity.
