IB Physics Sub-topic D1 Notes

Newton's law of gravitation

At this point in the syllabus, gravity has been mentioned as few times. In Topic A.2, it was mentioned that the gravitational force can be regarded as a centripetal force in some scenarios. In Topic D.1, you learn more about the gravitational force.

The gravitational force is governed by Newton's law of gravitation, which states that every mass in the universe attracts every other mass in the universe. This occurs because every mass releases gravitons that exert the gravitational force on other masses.

The more mass there is, the more gravitons are released and thus the higher the exerted gravitational force. However, the further away the other mass, the weaker the exerted gravitational force. If you remember, this is very similar to how the electrostatic force behaves.

The formula for gravitational force is given by:

$F = G \frac{m_{1}m_{2}}{r^{2}}$

In this, G is the gravitational constant, equal to 6.67 x 10^-11 Nm^-2 kg^-2.

Although the gravitational force is present between all objects, you likely know yourself that this is not felt unless the masses are huge. For such masses, let’s explore how we describe gravity via their gravitational fields.

Gravitational fields

Like an electric field is the area wherein there is an electrostatic force per unit test point charge, a gravitational field is defined as the area wherein there is a gravitational force per unit test point mass. 

The strength of this gravitational field (g) at any point is the force exerted per unit point mass, measured in N kg^-1 or ms^-2. On Earth, the gravitational field strength is 9.8 ms^-2.