[填空题]
Two crates, of mass 78q) q-d.q0/g wrh yex(sd ozu*5 kg ak +9f-f gd/oe4 n(odxbnd 110 kg , are in contact and at rest on a horizontal surface (Fig. 4-fdox b9(n/f4 oed+kg -54). A 620$-\mathrm{N}$ force is exerted on the 75$-\mathrm{kg}$ crate. If the coefficient of kinetic friction is 0.15 , calculate
(a) the acceleration of the system,
(b) the force that each crate exerts on the other.
(c) Repeat with the crates reversed.
