Why does water shoot out of a bottle, and why does it go farthest from the lowest hole?
Your guess
Pick one before you read on. It is fine to be wrong.
By the end you can
Show that water pressure grows with depth using a bottle with holes
Explain how air pressure lets you drink through a straw
Explain floating as an upward push balancing weight
Make a hole in the side of a bottle full of water. A jet of water shoots out. Water is pushing on the sides of the bottle, and it pushes out through the hole.
Water pushes in every direction: down, up and sideways. This push is water pressure. The depth is how far you are below the surface.
The deeper you go, the more water is above you. So the pressure grows with depth.
The water pushes out of the wall. The deeper the point, the longer the arrow, because the pressure is bigger.
Draw it first
Draw a bottle with jets
1 Draw a bottle with water 20 cm deep, on a tall block. Make a hole 4 cm below the surface. A short jet leaves it.
2 Add a hole 10 cm below the surface. Its jet leaves faster and lands farther out.
3 Add a hole 16 cm below the surface. Its jet lands farthest of all.
The deepest hole has the biggest pressure behind it, so its water leaves fastest. The bottle must stand high up. Then the water has a long way to fall, and a faster jet lands farther out.
If the bottle stood on the ground, the lowest jet would have almost no height to fall. It would land close, even though its pressure is the biggest. So stand the bottle on something high.
See it three ways
As a picture
In words
A deeper hole has a bigger water pressure. The water leaves faster and lands farther out, as long as the bottle stands high.
In symbols
Depth 4 cm: lands about 22 cm out. Depth 16 cm: lands about 38 cm out.
The push of the air is air pressure. The air above us is very deep, so it pushes on everything. We do not feel it, because our bodies push out from inside just as hard.
Air pushes down on the water in the cup. When you drink, you lower the air pressure in the straw. The air outside pushes the water up.
When you drink through a straw, you lower the air pressure in your mouth and in the straw. The air pressing on the drink outside is now stronger. It pushes the drink up the straw. You do not pull the drink up.
Water also pushes up on anything in it. The water below a thing is deeper. It pushes up harder than the water or air above pushes down. The difference is the upthrust.
A boat with a mass of 0.5 kg weighs 5 N. It floats when the upthrust is also 5 N. The two forces are balanced.
A thing floats when the upthrust equals its weight. It settles into the water until the water pushes up as hard as gravity pulls down. If the water can never push up that hard, the thing sinks.
This stone weighs 6 N, but the water pushes up with only 2 N. The arrows are not equal, so the stone sinks. Arrows are drawn to the same scale as the boat.
Try it
Make a bottle with holes
You need: a tall plastic bottle, filled with water, a nail, and an adult to push it through the plastic, tape, a stool or box about 30 cm high, a string or ruler, and stones to mark where the water lands.
Be safe: An adult must push the nail through the plastic. Never heat the nail. Push away from your hand.
Ask an adult to push the nail through the plastic. Make three holes in a line down one side, well spaced. Put one long strip of tape over all three holes.
Stand the bottle on the stool near the edge, outside or over a sink. Face the holes out. Fill the bottle.
Pull the strip off in one go. Watch the jets.
Put a stone where each jet lands. Measure each distance from the bottle with the string.
Refill the bottle and stand it on the ground instead. Where does the lowest jet land now?
What you will see: The jet from the lowest hole leaves fastest and lands farthest. The top jet lands closest. Our marks were 22, 32, 38 cm, from top to bottom. Real jets land a little closer than a perfect jet would. They land closer still as the water runs down. Yours will differ. On the ground the lowest jet has little height to fall, so it no longer lands the farthest.
If you cannot try it, this is what happens.
A wrong idea many people have
When you drink through a straw, you pull the drink up with your mouth.
Why it feels right: It feels as if you are pulling, because your cheeks work to drink.
The test that breaks it: Ask an adult to snip a small hole near the top of a straw. Now try to drink with the straw in a cup. It is much harder, or does not work at all, because air comes in at the hole.
What is true: Your mouth lowers the air pressure in the straw. The air outside is at a higher pressure, and it pushes the drink up.
Key words
water pressure
the push of water in every direction. It grows with depth.
depth
how far a point is below the surface.
air pressure
the push of the air on everything it touches.
upthrust
the push up that water gives a thing in it.
Back to your guess
The answer is A: Deeper water has more water above it pushing down, so the pressure is bigger.
The deeper the water, the more water is above it. That makes the pressure bigger. A bigger pressure pushes the water out faster.
Check yourself
Try each one before you look. A wrong answer tells you where to go next.
A bottle of water stands on a tall block. It has three holes, 4, 10, 16 cm below the surface. Which hole's jet lands farthest from the bottle?
See the answer
The hole 16 cm down
Water pressure grows with depth. The lowest hole has the biggest pressure, so its jet leaves fastest and lands farthest.
If you chose A: The top hole has the least water above it, so the smallest pressure. Its jet lands closest. Go to S3.7.8.
If you chose C: The middle hole is deeper than the top one, but not as deep as the lowest. Look at which jet goes farthest. Go to S3.7.8.
Kofi swims down to the bottom of a deep pool. What does the water do to his body?
See the answer
It pushes on him from every side, and it pushes harder the deeper he goes.
Water pushes in every direction. The deeper he goes, the more water is above him, so the pressure grows.
If you chose A: Water pressure pushes in every direction, not just down. It pushes on his sides and from below too. Go to S3.7.8.
If you chose B: The deeper he goes, the more water is above him. That makes the pressure bigger, not the same. Go to S3.7.8.
If you chose C: It is the water above him that makes the pressure. More water above means more pressure, so it grows with depth. Go to S3.7.8.
You drink through a straw. What pushes the drink up the straw?
See the answer
The air pressing on the drink outside the straw.
You lower the air pressure in the straw. The air outside is at a higher pressure, so it pushes the drink up.
If you chose A: Your mouth only lowers the pressure inside the straw. Something else does the pushing. Go to S3.7.8.
If you chose B: A straw is only a tube. It does not squeeze. The push comes from the air. Go to S3.7.8.
A toy boat has a mass of 2 kg and floats at rest. How big is the upthrust on it?
See the answer
20 N
The weight is 2 × 10 = 20 N. The boat floats at rest, so the forces are balanced. The upthrust is also 20 N.
If you chose A: You used the mass in kilograms as if it were newtons. First find the weight: each kilogram weighs about 10 N. Go to S3.7.4.
If you chose B: A floating boat still has gravity pulling it down. The water pushes up to hold it, so the upthrust is not zero. Go to S3.7.8.
If you chose C: The upthrust does not need to be bigger. When it equals the weight, the forces are balanced and the boat stays at rest. Go to S3.7.3.
A stone weighs 30 N. The most the water can push up on it is 12 N. What is the overall force on the stone as you let it go in the water?
See the answer
18 N downwards. It sinks.
The forces are opposite, so take one from the other. 30 − 12 = 18 N. The weight is bigger, so the overall force points down and the stone sinks.
In the bottle test, why must the bottle stand high up?
See the answer
The water then has a long way to fall, so a faster jet lands farther out.
On the ground the lowest jet has hardly any height to fall, so it lands close. A tall stand lets the faster jet reach farther.
If you chose B: The pressure depends on depth, not on how high the bottle stands. The height gives the water room to fall and spread out. Go to S3.7.8.
If you chose C: The holes stay the same size. The height is about how far the water falls. Go to S3.7.8.
From an earlier chapter
Review: a force of 80 N pushes on an area of 20 cm². What is the pressure?
See the answer
4 N/cm²
Pressure = force ÷ area = 80 ÷ 20 = 4 N/cm².
If you chose A: You added the two numbers. Pressure is force divided by area. Go to S3.7.7.
If you chose C: You divided area by force. Put the force first: force ÷ area. Go to S3.7.7.
Keep it
Copy this into your exercise book.
Water pressure grows with depth and pushes in every direction. Air pressure pushes a drink up a straw. A thing floats when the upthrust equals its weight.
Draw: A bottle on a tall block with three holes. Draw the longest jet from the lowest hole.
