[填空题]
In the design of a supel(c 870sphx f7 ogv7+;dg4lzdcu/ jlxrmarket, there are to be gmk62e ha2,uw several ramps connecting differe 6mw2hekag2u ,nt 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} $
