Centripetal Force

3 (g) describe qualitatively motion in a circular path due to a constant perpendicular force, including
electrostatic forces on an electron in an atom and gravitational forces on a satellite.

3 (h) discuss how ideas of circular motion are related to the motion of planets in the solar system.

Whenever an object is in a circular motion, there is a force acting towards the center of the circle in which it is moving. This force is called centripetal force. Look at the picture below.

The object is moving in a circle. As a result, there is a force acting from the ball towards the centre of the 
circle, which we call the centripetal force.

Centripetal force plays an important role in our universe. In an atom, we know that electrons move in circles around the nucleus. They do so because of centripetal force. The nucleus has positive charge on it, while the electrons have negative charge on them. As a result, the electrons are attracted towards the nucleus, resulting in a centripetal force being evolved, which in turn causes the electrons to move in a circular motion around the nucleus.

Natural satellites like moons also revolve around the planets due to the centripetal force. The moon is attracted towards the planet due to the gravitational pull of the planet, and as a result starts moving in a circular path around the planet.

Similarly, planets revolve around the sun, due to this same principle. The sun's gravitational field applies a pulling force on the planets of the solar system, and as a result, they circle around it.

Now let us consider another scenario. Look at the following picture.
What would happen if the centripetal force suddenly stops acting on an object which was in circular motion? The object would break out of its circular motion, and continue at right angles to the centripetal force that was acting on it. This is indicated by the green arrow in the diagram above.