Pressure, Winds, Storms, and Cyclones · Lesson 2 of 8
How Liquids Exert Pressure
“Investigate how water pushes on the bottom and walls of a container and why height matters.”
• Infer from paired pipes that pressure at the bottom depends on water-column height rather than pipe width alone. • Explain why increasing water depth increases pressure. • Use a bottle investigation to show that liquid pushes sideways and in all directions. • Apply these observations to tanks, taps, pipe leaks, and dams.
Water has weight and pushes
When you fill a container, water pushes on its bottom. Unlike a solid block, water also presses against the walls that keep it in. We can discover what determines this pressure by comparing the deformation of identical rubber balloons stretched across the bottoms of two upright pipes.
In the investigation, the transparent pipes have equal length but different diameters. Fill them with water to the same height. Both bottom balloons bulge by about the same amount, even though the wider pipe contains more water and has more total weight. The important observation is equal pressure at equal depth for the same liquid, not equal total weight of water.
Height changes the pressure
Now add water to just one pipe. Its bottom balloon bulges more. The extra water raises the water column above that balloon, and the pressure at the bottom increases. Repeat in small steps: as the height increases, the bulge grows. This gives the pattern observation → explanation: greater height of the same liquid above a point means greater pressure there.
The two experiments must be read together. Pipe width changes the total amount of water without changing the bottom pressure when the height is equal. Raising the water level changes the bottom pressure. The chapter does not require a numerical formula for liquid pressure; the qualitative comparison is enough here.
Problem
A narrow and a broad pipe have identical balloons and hold water to the same height. Predict the bulges.
- 1.Compare the vertical height of water above each balloon: it is equal.
- 2.The liquid is the same, so at the same depth the pressures are equal.
- 3.The balloons should bulge about equally, although the broad pipe contains more water.
Problem
Water is added to only the narrow pipe so that its level becomes higher. Which balloon now bulges more?
- 1.The narrow pipe now has a taller water column above its balloon.
- 2.A greater height produces greater pressure at the bottom.
- 3.Its balloon bulges more; the pipe need not be the wider one.
Water presses sideways as well
Make four small holes around the side of a used plastic bottle at the same height, seal them, fill the bottle, then uncover them together. Water jets leave the holes in several directions. The jets show that the liquid pushes against the side walls, not only the bottom. The holes at the same depth are useful because their pressures can be compared fairly. An adult should make the holes; avoid heated needles in a student demonstration.
A pipe leak may spurt sideways for the same reason. Liquid pressure acts on the walls of the pipe, and the water moves out when an opening appears. In a stationary liquid, pressure at a given depth acts in all directions. The direction of a jet depends on the opening it escapes through.
Why tanks are high and dams are broad
An overhead tank is placed above the taps. The greater vertical difference between its water level and a tap helps give a stronger flow at the tap. If two taps in the same building are open at different floors below one tank, the lower tap generally has a greater height of water above it and can deliver a stronger stream, although the plumbing and flow losses also matter in real buildings.
The water against the lower part of a dam is deeper, so its pressure is greater there than near the top. A dam has a broad base for support and to withstand the strong horizontal push near the bottom. The water also presses downward on the floor. Do not confuse the dam’s broad base with a claim that it makes the deep water exert less pressure; the structure is made stronger where the load is greater.
Problem
A tank supplies one tap on the first floor and another on the second floor. Which is more likely to have the stronger stream, other conditions being alike?
- 1.The first-floor tap is farther below the water level in the tank.
- 2.It has a greater water-column height above it.
- 3.That gives greater available pressure at the lower tap, so its stream is usually stronger.
At equal depth in the same liquid, a wider container may hold more water yet the pressure at a point on its bottom is the same as in a narrow one. Total force on an entire bottom can still differ because bottom area differs.
Check your thinking
Imagine moving one observation at a time: change the width, change the height, or move an opening to a side wall. Decide which change affects the pressure at the point being discussed.
Quiz
Two pipes of different widths contain water to the same height. What happens to identical balloons at their bottoms?
What happens to pressure at the bottom when the height of water above it increases?
What does water escaping from holes in the bottle’s side show?
Why is the base of a dam broad?
Which tap normally has a greater water-column height above it when supplied from the same roof tank?
Practice Problems
- Explain the equal bulge of two balloons beneath pipes of unequal diameter but equal water height.
- Predict what changes if more water is poured into just one of the pipes.
- Describe the bottle-hole investigation, its observation, and what it means.
- Why can a leak in the side of a water pipe spurt outward?
- Compare water pressure near the top and bottom of a deep dam and connect this to its shape.
- Two floors are supplied from one rooftop tank. Explain which tap may have greater pressure and give one real-world factor that can affect the actual stream.
Key Takeaways
• For the same liquid, greater water-column height gives greater pressure at a point below it. • Equal heights in pipes of different widths give approximately equal pressure at their bottoms. • Liquid pushes against the bottom and sides of a container, in all directions. • Elevated tanks provide pressure at taps, and dams resist especially large sideways pressure near the bottom.