The potential energy of a pair of hydrogen atoms separated by a large distance x is given by u(x)=−c6/x6, where c6 is a positive constant. is this force attractive or repulsive?

We need to see what happens to the energy depending on their distance. If the atoms get closer, we have that x gets smaller and their energy smaller; 1/x^6 is getting bigger but accounting for the minus sign, u gets smaller. If their distance increases, x gets bigger and similarly u increases. The forces act in a way to decrease potential energy, in every case of power. In this case, since potential energy is reduced when distance is decreased, we have that the force is attractive.

Idon’t know wish i could

Less force and energy was applied