Newton's Third Law

A force is an interaction between objects, and forces exist in equal-and-opposite pairs. These statements are summarized by:

Newton's third law: when one object exerts a force on a second object, the second object exerts an equal-and-opposite force on the first object.

"equal-and-opposite" is short for "equal in magnitude but opposite in direction".

Although the forces between two objects are equal-and-opposite, the effect of the forces may or may not be similar - it depends on the relative masses of the objects.

If we drop a 100 g (0.1 kg) ball, it experiences a downward acceleration of 9.8 m/s2, and a force of about 1 N, because it is attracted towards the Earth. The ball exerts an equal-and-opposite force on the Earth, so why doesn't the Earth accelerate upwards towards the ball?

It does, but the mass of the Earth is so large (6.0 x 1024 kg) that the acceleration is much too small (about 1.67 x 10-25 m/s2) for us to notice.

When objects have similar mass, the equal-and-opposite pairs of forces are much easier to see.