Usual y, we do not walk or even stand on a lightweight boat or raft because of the danger of falling into the water. If you have ever stepped off a small boat onto a dock, however, you have probably noticed that the boat moves away from the clock as you step toward the dock or out of the boat. A heavy dog running on a long lightweight raft presents a similar situation. At first, the raft and the dog are at rest a r A with respect to the water (see figure A below) so that v,- = 0. The dog then runs on top of the raft at vd = vdi with respect to the water (see gure B below). The dog has four times the mass of the raft. Find an expression for the velocity of the raft after the dog began running. (Use the following as necessary: vd.) A yr: 4vd