Question

1. how does gravity cause the planets to move in a circle around the sun?

Explanation:

Gravity from the sun acts as a centripetal force. A centripetal force is a force that pulls an object towards the center of a circular path. For planets, the sun's gravitational pull continuously pulls them towards the sun. At the same time, the planets have a tangential velocity (a velocity perpendicular to the direction of the gravitational force). The combination of the inward - pulling gravitational force (centripetal force) and the tangential velocity causes the planets to move in a circular (or nearly circular, in the case of elliptical orbits which can be approximated as circular for basic understanding) path around the sun. The gravitational force provides the necessary centripetal acceleration $a = \frac{GM}{r^{2}}$ (where $G$ is the gravitational constant, $M$ is the mass of the sun, and $r$ is the distance between the planet and the sun) that keeps the planet in its orbital path, changing the direction of the planet's velocity vector continuously so that it follows a circular trajectory instead of moving off in a straight line (as per Newton's first law of motion, an object in motion will stay in motion in a straight line at a constant velocity unless acted upon by a net external force).

Answer:

Gravity from the sun acts as a centripetal force, pulling planets toward the sun. Combined with the planet's tangential velocity, this force continuously changes the planet's velocity direction, causing it to move in a circular (or near - circular) orbit around the sun. The gravitational force provides the centripetal acceleration ($a=\frac{GM}{r^{2}}$) needed to maintain the orbital path, preventing the planet from moving in a straight line (per Newton's first law).