[填空题]
In the design of a supermarket, there are to f/z:q7,s tbczx;g c0n be several ramps connecix1h t/ (flw5ating different part1a( wf/i5tx hls of the store.
Customers will have to push grocery carts up the ramps and it is obviously desirable that this not be too difficult. The engineer has done a survey and found that almost no one complains if the force directed up the ramp is no more than 20 N . Ignoring friction, at what maximum angle $\theta$ should the ramps be built, assuming a full $30-\mathrm{kg}$ grocery cart? $^{\circ} $
