[Hossein Mozaffary]

Hossein Mozaffary asked the Naked Scientists:
   
Hi! It is said that the force of gravity is always attractive. This is a contradiction of Newton´s third law. Could you elaborate, please? Thank you.

What do you think?

[JP]

Hmm... this is an old one, but it's got an easy answer.

Newton's third law, usually phrased "for every action, there is an equal and opposite reaction" holds for Newtonian gravity. Imagine two large masses sitting in space.  Mass A pulls on mass B, but at the same time mass B pulls on mass A.  If mass A is pulled to the left, mass B is pulled to the right.  One of these pulls is the action.  The other is the equal and opposite reaction.

It's clearer if you imagine a tug of war with a rope.  If person A pulls the rope, person B gets pulled towards them.  But at the same time, person A feels a tug in the opposite direction, back towards person B.  So the two people feel equal and opposite tugs.

[CliffordK]

If you have a mass that weighs 1kg.
And the earth that weighs about 5.9736×10²⁴kg

And you drop your mass from a height of 1m down to earth.
In theory, you should also move the earth by about (1/5.9736×10²⁴) meters in the direction of the object you dropped.

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The moon orbits the Earth due to the centripetal force pull on the moon. 
Without the Earth (or any other celestial objects), the moon's path would be governed by Newton's first law (An object in motion tends to remain in motion, and an object at rest tends to remain at rest).
Earth's gravity changes the moon's direction into a roughly circular motion around the Earth.

However, the Moon also pulls on the Earth. 
This causes the earth to wobble from the moon's pull. 

While the center of mass of the earth/moon system lies within the earth, the wobble induced by the moon causes the high tides on the side of the earth away from the moon.

Anyway, so the Earth pulls the moon to create the orbit.  But, the moon pulls the earth to create the wobble, and the two high tides.