1 2500kg railroad car moving with a speed of 3 m/s is linked with a 3500 kg railroad car moving toward the first car with a speed by 1 m/s. Both are moving along a straight track. Find the speed of the two cars just after they link up.

Consider the growth of a 20-nm-diameter silicon nanowire onto a silicon wafer. the temperature of the wafer surface is maintained at 2400 k. assume the thermal conductivity of the silicon nanowire is 20 wm-1k-1 and all its surfaces including the tip are subjected to convection heat transfer with the coefficient h = 1×105 wm-2k-1 and t∞ = 8000 k. when the nanowire grows to l = 300 nm, what is the temperature of the nanowire tip (t (x =