The lower hole has a taller column of water above it. The weight of that water makes the pressure higher there, so water comes out of the lower hole faster.
Picture an open bottle with two small holes in its side. Both holes are below the waterline. What happens when you uncover them at the same time?
Pressure is higher deeper down
Gravity pulls the water down. Deeper water supports the weight of a taller column of water. That pressure also pushes sideways against the wall of the bottle. To compare the two holes, look at how far each one is straight down from the water's surface.
The water level changes the jet
Air pushes on the outside of the hole. Below the surface, the water in the open bottle is at a higher pressure than the air outside. A bigger difference makes the water leave faster. As the bottle drains, the water level falls. There's less water above the same hole, so the jet gets weaker.
Exit speed and how far the jet goes
Once the water leaves the hole, gravity pulls the jet down. A low hole is closer to the surface that catches the water. Its jet starts out faster but has less time to travel sideways. A jet from higher up can therefore reach farther.
Watch one hole at two water levels
- Ask an adult to make a small hole near the bottom of an empty plastic bottle. Cover the hole with tape and stand the bottle upright in the sink. Leave the cap off.
- Fill the bottle with tap water. Peel off the tape and watch the jet from the side. Look at how far it goes from the hole before it bends noticeably downward.
- Wait until the waterline is just above the hole. How has the jet changed? Refill the bottle and see whether the jet gets stronger again.
Only an adult should make the hole. Use a plastic bottle, work in the sink and wipe up any water that splashes onto the floor.
The takeaway
The higher the water stands above a hole, the bigger the pressure difference with the air outside and the faster the water comes out.
What happens at the same hole as the water level falls?
Sources & further reading
Idaho State University · Pressure Dependent on Depth ↗Purdue University · Liquid Stream Trajectories ↗Our stories explain the main idea in our own words. Pictures and models simplify reality and are not drawn to scale.
