# Q & A: conservation of momentum

Q:
hello!! i have a question and i will be happy if you answer it!!:D we have a container that is full of a liquid this container is on the ground and has wheel for move then we make a hole at the right bottom of the container!! now the question is that the container move left or right?? please answer my question thanks
- peyman (age 16)
iran
A:

That depends on whether the liquid falls straight down or of toward the side. if the hole is put on the side, the liquid will flow a bit toward the right. Then conservation of momentum means that the container must move to the left. If the hole is on the bottom the liquid may flow straight down. So the net sideways momentum of the rest of the fluid and the container will be zero. However, the fluid inside will be flowing a bit to the right to reach the outlet, until it drains, so the container will move a bit to the left, then stop. Again, that's because with no sideways forces from the outside, the total momentum of the container+water can't change. While the water flows to the right, the container must move to the left.

The result makes sense, because it leaves the container a little to the left of where it started and the fluid a little to the right of where it started, on average. So the center of mass didn't move sideways, which makes sense because there was never sideways momentum.

Mike W.

(published on 11/29/2013)