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Planets move around the sun in a nearly circular orbit. This type of movement is known as circular motion. For an object to follow a circular path, it must be constantly changing direction. This means there must be a force acting upon it at all times, pulling it towards the center of the circle. This force is called centripetal force.

Circular motion is everywhere in our world. Think of a car turning around a bend, or a rollercoaster looping in a circle. In each case, there's a force that keeps the object moving in its circular path.

When we talk about circular motion, we must understand the role of centripetal force. Centripetal force is what keeps an object moving in a circle. Without it, the object would move in a straight line due to inertia. For planets, this force comes from gravity pulling them towards the sun.

Consider a ball tied to a string and swung in a circle. The tension in the string is the centripetal force that keeps the ball moving in a circular path. If the string breaks, the ball will fly off in a straight line, not continue in a circle. Similarly, planets stay in their orbits because of the centripetal force provided by gravity.

Sir Isaac Newton formulated the Law of Universal Gravitation. According to this law, every mass in the universe attracts every other mass with a force that is directly proportional to the product of their masses, and inversely proportional to the square of the distance between their centers.

This gravitational force is what keeps the planets in their nearly circular orbits around the sun. The formula for gravitational force is given by
\( F = G \frac {m_1 m_2} {r^2} \),
where \( F \) is the force of attraction between two masses \( m_1 \) and \( m_2\), \( r \) is the distance between the centers of the two masses, and \( G \) is the gravitational constant, approximately equal to \( 6.674 \times 10^{-11} \) N(m/kg)^2.

Gravity is the force of attraction between two bodies. It is the reason we stay grounded on Earth and why planets orbit the sun. Without gravity, there would be no centripetal force to keep planets in motion around the sun.

Gravity is a fundamental force that affects everything with mass. On a smaller scale, it's what keeps you standing on the ground. On a larger scale, it's what keeps the entire solar system together. Although it is the weakest of the four fundamental forces of physics, its effects are profound on a cosmic scale.

Remember, the strength of gravity depends on two things: the mass of the objects and the distance between them.