8 A box of mass 80 kg is to be pulled along a horizontal floor by means of a light 100 N rope. The rope is pulled with a force of 100 N and the rope is inclined at 20 to 80 kg rope -- 20 the horizontal, as shown in the diagram. floor (5) Explain briefly why the box cannot be in equilibrium if the floor is smooth. act the floor is not smooth and the box is in equilibrium. (ii) Draw a diagram showing all the external forces acting on the box. (ifi) Calculate the frictional force between the box and the floor and also the normal reaction of the floor on the box, giving your answers correct to three significant figures. The maximum value of the frictional force between the box and the floor is 120 N and the box is now pulled along the floor with the rope always inclined at 20 to the horizontal. (iv) Calculate the force with which the rope must be pulled for the box to move at a constant speed. Give your answer correct to three significant figures. (v) Calculate the acceleration of the box if the rope is pulled with a force of 140 N