A tractor of mass 1000 kg is connected by a tow-bar to a trailer of mass 1000 kg. The total resistance to motion has a constant value of 4000 N. One quarter of this resistance acts on the trailer. When the tractor and trailer are moving along horizontal ground at a constant speed of 6 m s⁻¹, what is the force exerted on the tractor by the tow-bar?

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