Calculate the magnitude of the normal force on a 17.7 kg block in the following circumstances. (Enter your answers in N.) (a) The block is resting on a level surface. (b) The block is resting on a surface tilted up at a 30.8° angle with respect to the horizontal. (c) The block is resting on the floor of an elevator that is accelerating downward at 2.53 m/s^2. (d) The block is on a level surface and a force of 115 N is exerted on it at an angle of 30.8° above the horizontal.