[填空题]
In the design of a supermarket, there are to be seveic9.d:johpihj)a .v0; ntla5ri0onn d0m.te1al ramps connecting different 0tn.0n d emo1iparts 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} $
