Am = 500 kg sack of coal falls vertically onto a m = 2000 kg railroad flatcar which was initially moving horizontally at 3 m/s. no external forces, including friction, act on the flatcar. after the sack rests on the flatcar, the speed of the flatcar is:

You can view this as an inelastic collision between a 2000 kg object moving at 3 m/s, and a stationary 500 kg object.
 The total momentum of the objects before the collision equals their momentum after collision.
 Before:
 flatcar = 2000*3 = 6000 kg*m/s. 
 coal = 0*500 = 0.
 After:
 flatcar & coal = 2500*V
 then
 2500*V = 6000,
 V = 2.4 m/s. 
 Additional:
 the definition of an inelastic collision is that the objects "stick together" after colliding.

This can be pointed out as the inelastic collision between a 2000 kg object moving at 3 m/s, and a stationary 500 kg object.
The total momentum of the objects before the collision would be equal to the momentum after collision.
So before collision: flatcar = 2000*3 = 6000 kg*m/s. coal = 0*500 = 0.
After collision: flatcar & coal = 2500*V = 6000, V = 2.4 m/s.

Answer: buoyancy forceexplanation: in simple terms, the principle states that the buoyancy force on an object is equal to the weight of the fluid displaced by the object, or the density of the fluid multiplied by the submerged volume times the gravitational acceleration, g. this is also known as upthrust.