[填空题]
In the design of a supermarket, there are to be several ramps vg4c4 d 9nvfczf378wx connecting differe9zw38 v4c4 fc xdvf7ngk5pgs8 *9 inxudfp+ n6 i6k9*snpndf+8 x5gpu 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} $
