1 A small block is pulled along a rough horizontal floor at a constant speed of \(1.5 \mathrm {~ms} ^ { - 1 }\) by a constant force of magnitude 30 N acting at an angle of \(\theta ^ { \circ }\) upwards from the horizontal. Given that the work done by the force in 20 s is 720 J , calculate the value of \(\theta\).