As you may know some objects float others do not, this is because some objects contain air or oxygen. for example an orange with a peel floats because the peal contains oxygen and an orange without a peel sinks because it does not contain air at oxygen
By Newtons Third law this is an action reaction pair of force
So the momentum is the same
30*20=50*x where x is velocity
x= 12m/s
direction is towards left since they act in opposite directions
Tha atmosphere has no effect on gravity.
The population is growing rapidly