[填空题]
In the design of a supermarket, +byhh(jic u1qb 8y +;qthere are to beu90r-sx unz;o several ramps connecting differen 9xro-u 0unzs;t parts 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} $
