Video Transcript
The diagram shows the first part of how the solar system formed. Which of the following is what causes the cloud of gas and dust to contract? (A) Pressure from the surroundings. (B) Electrostatic attraction between the particles in the gas and dust. (C) Random motion of the particles in the gas and dust. (D) Gravity.
Our solar system formed from a cloud of dust and gas, which contracted and went on to
form the Sun and the planets. In this question, we must identify what caused the original cloud of dust and gas in
space to contract. Let’s look at each answer choice in turn.
Answer choice (A) supposes that pressure from the surroundings of the cloud caused
the cloud to contract. When particles collide with other things, they exert pressure on them. However, we may recall that space is mostly empty. When the occasional dust or gas particle moves freely around space, it’s very
unlikely that it’ll bounce into anything and exert pressure on it. Outside the massive cloud that eventually formed our solar system, there was probably
not enough matter to exert enough pressure on the cloud to cause it to move or
change. So, answer choice (A) is incorrect.
Answer option (B) is a little more straightforward to think about. Electrostatic attraction occurs between charged particles, like ions, electrons, or
protons. Opposite charges attract, while like charges repel. We may remember having experienced electrostatic attraction ourselves when rubbing
our hair with a rubber balloon or when getting an electric shock while walking
across a carpet in winter. However, there’s no evidence to suggest that the cloud of dust and gas that formed
the solar system contained enough charged particles for electrostatic attraction to
have had a significant effect. Therefore, answer choice (B) is incorrect.
Answer choice (C) asks us to consider that the dust and gas cloud formed our solar
system because of the random movement of particles. However, if the dust and gas particles were moving randomly, it’s highly unlikely
that enough of them would have moved toward each other at the same time and caused
the cloud to contract. We can therefore conclude that option (C) is incorrect.
That leaves us with option (D), gravity. Gravity is an attractive force between all objects that have mass. Since all gas and dust particles have mass, these particles in the cloud would have
experienced the attractive force of gravity, pulling them together. This means the most likely option is that gravity caused the cloud to contract. Therefore, our answer is choice (D), gravity.
