Lesson A10: Pressure

Key Ideas     Unit Checklist

  Video Lesson

Why do we need to measure air pressure when we inflate car or bicycle tires? Watch this video to learn more about how fluids exert pressure.

 
 

  Lesson A10: Pressure

Figure A.3.10.1 – Water jet cutters can slice through metal.
Figure A.3.10.2 – Water jet cutters can be used to cut out detailed tools or signs.

Powerful Water Pressure
Would it surprise you to discover that water can cut through metal? Water under very high pressure can exert enough force to slice through strong materials like metal and granite stone.


Water jet cutters are machines that release high pressure water in a thin stream. Water jet cutters are useful for cutting metal in situations when you don’t want the metal to be melted or warped by heat. Water jet cutters are good at cutting out detailed shapes, such as special airplane parts.
Reading and Materials for This Lesson

Science in Action 8
Reading: Pages 57–60

Materials:
2 identical balloons, 50 thumbtacks (the kind that lie flat on their backs), 3 drinking straws, drinking glass, water, modelling clay, empty plastic jar with plastic lid (like a peanut butter jar), hammer, nail, empty pop can, large stainless steel bowl, measuring spoons, stove burner, oven mitts, tongs.

 Watch More

The Power of Water Pressure

Watch this video to see some more examples of the huge force delivered by water under high pressure.

 
 

  Try It! 

Thumbtack Balloon Bed

Will a balloon pop if you press it down on a bed of thumbtacks? Try this simple experiment to find out! 

Materials: 
  • 2 identical balloons
  • 50 thumbtacks (the kind that lie flat on their backs)


Instructions:

  1. Blow up both balloons to exactly the same size.

  2. Place 50 thumbtacks on a flat surface with the sharp points of the thumbtacks facing up. The thumbtacks should be placed close together.

  3. Gently press 1 balloon down on the bed of thumbtacks. What do you observe?

  4. Try pressing the balloon down on the bed of thumbtacks a bit harder. What do you observe?

  5. Place 1 thumbtack on a flat surface with its sharp point facing up.

  6. Gently press the other balloon down on the single thumbtack. What do you observe?

  7. Try pressing the balloon down on the single thumbtack a bit harder. What do you observe?

  8. Watch this video to see this experiment and its results: 

 
 
 

Question: 

Think about the following question very carefully. Then, type or write your answer. After you have your answer, click the question for feedback.

The sharp point of the single thumbtack has a very small area. When force is exerted on the balloon over this small area, a large amount of pressure is applied. A large amount of pressure causes the balloon to pop.

The bed of thumbtacks has a large area to spread the applied force. When force is spread over a large area, the pressure is reduced. With less pressure, the balloon doesn’t pop.

 Watch More

Bed of Nails

A bed of nails is similar to a bed of thumbtacks. Do you think a person could lie down safely on a bed of sharp nails? Watch this video to find out!

 
 



Figure A.3.10.3 – The high atmosphere has a low air pressure.
Air Pressure

Air exerts a large amount of pressure, even though you don’t feel it pushing down on you. Most of the air particles in Earth’s atmosphere are located within 16 kilometers of the Earth’s surface. One way to visualize atmospheric air pressure is a 16 kilometer tall tower of air particles pushing down on the ground.

Atmospheric air pressure changes with height. Air pressure is lower at higher elevations. As you move higher in the atmosphere, there are less air particles above you. Less air particles exert less downward force, which reduces the air pressure. For this reason, airplanes flying at high altitudes need to increase the air pressure in the passenger cabin. Air outside the airplane has a very low pressure, which is good for flying, but not good for human comfort and breathing.

 Watch More

Under Pressure

This video explains more about pressure and how it is measured. You will also learn how to build a barometer to measure air pressure changes.

 
 
 
 
Air pushes on people with lots of pressure, so why don’t we get crushed by it? Watch this video to find out.

 
 

 
 
When an airplane ascends and descends, sometimes people’s ears pop. Ears pop because of air pressure differences on either side of a person’s eardrum. Watch this video to learn more.

 
 
 
 
Barometers are devices used to measure air pressure. Watch this video to learn how the barometer was invented.

 
 
Figure A.3.10.4 – Wind is created by the movement of air from high pressure to low pressure.
Figure A.3.10.5 – Warm and cold weather fronts have different pressures.


Figure A.3.10.6 – Barometers measure air pressure to predict weather.
Pressure Differences

Gases naturally move from high pressure areas to low pressure areas. High pressure air particles are crowded closely together. Gas particles like lots of space to move, which is why they naturally drift toward less tightly packed, low pressure areas.


Different weather patterns occur due to air pressure. Weather forecasters often talk about high and low pressure fronts. Weather fronts are boundaries between different air masses in the atmosphere. Warm fronts have fast moving particles with a low pressure. Cold fronts have slower moving particles with a higher pressure. When air moves from a cold front area of high pressure to a warm front area of low pressure, wind is created.

Air pressure changes usually indicate a change in weather. A barometer is a device that is used to measure changes in air pressure.

 Watch More

Weather Fronts

Watch this video to learn more about weather fronts.

 
 
Figure A.3.10.7 – Drinking straws work because of air pressure.
Figure A.3.10.8 – Aerosol cans work because of the pressure of a gas inside the can.

Drinking Straws and Aerosol Cans

Drinking straws require air pressure to work. When a straw sits in a glass, air pressure pushes down on the liquid in both the glass and the straw equally. This is why liquid doesn’t travel up a resting straw.

When you suck on a straw, you are removing air from the straw. This decreases the air pressure in the straw. The higher outer air pressure pushes down on the liquid in the glass as it tries to move to the lower pressure area in the straw. This pressure forces liquid up the straw.

Aerosol cans work in a similar way to straws. Aerosol cans contain a product liquid and a high pressure propellant gas. When you push the nozzle on an aerosol can, it opens a tube in the can to the lower atmospheric pressure outside the can. The higher pressure propellant gas inside the can moves toward the lower pressure area outside the can. In the process of moving, the propellant gas pushes down on the product liquid, forcing it up the tube and out the nozzle in tiny drops.

 Watch More

Using Pressure Every Day

Different substances have a limit of how far they can flow up a straw. This video shows an experiment of drinking through a 10 metre long straw.

 
 
 
 
Watch this video to learn more about how aerosol cans work.

 
 

  Try It! 

Straw Tricks

Try these fun straw tricks that can be explained by air pressure. 

Materials: 
  • 3 drinking straws
  • Drinking glass
  • Water
  • Modelling clay
  • Empty plastic jar with plastic lid (like a peanut butter jar)
  • Hammer
  • Nail
  
This activity involves using a hammer and nail to punch a hole in a plastic lid.
It must be completed with the supervision and assistance of an adult. DO NOT attempt this activity by yourself.
Two Straws Trick
Instructions:

    1. Fill a glass with water.

    2. Put the ends of 2 straws in your mouth.

    3. Put the other end of 1 straw inside the water and the other end of 1 straw outside the glass.

    4. Try drinking through the straw. What do you observe?

    5. Watch this video to see this experiment and its results:


 
 
Sealed Straw Trick
 
Instructions:

    1. With the nail, hammer a hole into the middle of the plastic lid.

    2. Insert a straw through the hole of the lid.

    3. Fill the plastic jar with water.

    4. Screw the plastic lid with the straw tightly on the jar. Adjust the straw so the bottom of the straw is in the middle of the liquid.

    5. Seal the hole in the lid around the straw with modelling clay, to make the jar airtight.

    6. Try drinking through the straw. What do you observe

    7. Watch this video to see this experiment and its results:

 
 

Questions: 

Think about the following questions very carefully. Then, type or write your answers. After you have your answers, click the questions for feedback.

Liquid cannot flow up the straw because there is not an area of low pressure in your mouth. The second straw connects your mouth to atmospheric air pressure. This is the same amount of pressure pushing down on the liquid in the glass. There is no movement from a high pressure area to a low pressure area, to make the straw work.

There is no outside air pressure working on the liquid in the glass. You can suck air out of the straw and create a lower air pressure in the straw, but there are no additional air particles entering the jar to press down on the liquid and force it up the straw.

Paper cups with lids are not airtight. There is a small amount of space open to the air at the straw hole.


Figure A.3.10.9 – Implosions occur when trapped low pressure causes a structure to collapse.
Figure A.3.10.10 – Implosions (left) are the opposite of explosions (right).
  
Implosions

Implosions occur when a structure crumples in on itself. Implosions happen as a result of pressure differences. If low pressure exists inside a container, higher pressure gas particles outside the container move towards the low pressure area. The higher pressure particles apply pressure to the walls of the container, pushing them inward.

 Watch More

Explode out, Implode in

How do pressure differences cause implosions? Watch this video to see how large metal containers are imploded.

 
 
 
 
This video shows the implosion of a massive oil tanker train car.

 
 

  Try It! 

Pop Can Crush

Try this simple experiment to implode a pop can.

Materials: 
  • Empty pop can
  • Water
  • Large stainless steel bowl
  • Measuring spoons
  • Stove burner
  • Oven mitts
  • Tongs
This activity involves hot fluids. It must be completed with the supervision and assistance of an adult.
DO NOT attempt this activity by yourself.

Hot fluids and metal can burn you or others if you are not careful.
Wear oven mitts and use tongs when handling the hot pop can.
Never leave a hot stove unattended.
Instructions:

  1. Fill the stainless steel bowl with cold water.

  2. Put 1 tablespoon of water into the empty pop can.

  3. Put the pop can on a stove burner and turn on the burner.

  4. Heat the pop can until you see steam coming out of the can. This means the water in the can is boiling. Let the water boil for 30 more seconds.

  5. Wearing oven mitts, use tongs to pick up the pop can.

  6. Holding the pop can with the tongs, dunk the can, upside down into the bowl of cold water. What do you observe?

  7. You can watch the experiment and results here:

 
 

Questions: 

Think about the following question very carefully. Then, type or write your answer. When you have your answer, click the question for feedback.

The pop can contained hot water vapour. The fast moving water vapour particles transferred energy to the air particles inside the can, which caused air particles to move out of the can. This left a low air pressure area inside the can.

When the can was dunked upside down in cold water, the water vapour inside the can suddenly cooled down and condensed back into liquid. However, no new air particles could enter the can because the opening was sealed by water. This kept the air pressure low inside the can. The higher pressure air outside the can applied force to the low pressure can and crushed it.




  Make sure you have understood everything in this lesson. Use the Self-Check below, and the Self-Check & Lesson Review Tips to guide your learning.

Unit A Lesson 10 Self-Check

Instructions


Complete the following 6 steps. Don't skip steps – if you do them in order, you will confirm your understanding of this lesson and create a study bank for the future.

  1. DOWNLOAD the self-check quiz by clicking here .

  2. ANSWER all the questions on the downloaded quiz in the spaces provided. Think carefully before typing your answers. Review this lesson if you need to. Save your quiz when you are done.

  3. COMPARE your answers with the suggested "Self-Check Quiz Answers" below. WAIT! You didn't skip step 2, did you? It's very important to carefully write out your own answers before checking the suggested answers.

  4. REVISE your quiz answers if you need to. If you answered all the questions correctly, you can skip this step. Revise means to change, fix, and add extra notes if you need to. This quiz is NOT FOR MARKS, so it is perfectly OK to correct any mistakes you made. This will make your self-check quiz an excellent study tool you can use later.

  5. SAVE your quiz to a folder on your computer, or to your Private Files. That way you will know where it is for later studying.

  6. CHECK with your teacher if you need to. If after completing all these steps you are still not sure about the questions or your answers, you should ask for more feedback from your teacher. To do this, post in the Course Questions Forum, or send your teacher an email. In either case, attach your completed quiz and ask; "Can you look at this quiz and give me some feedback please?" They will be happy to help you!

Be a Self-Check

Superhero!



Self-Check Quiz Answers

Click each of the suggested answers below, and carefully compare your answers to the suggested answers.

If you have not done the quiz yet – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

 
The bottom of high-heeled shoes have a smaller area than flat shoes. This means that high-heeled shoes apply force over a smaller area, creating larger pressure. A larger pressure makes the woman sink into the mud more, compared to flat shoes.
The spray nozzle reduces the area for the water to escape the hose. A smaller area with the same water force creates a higher water pressure. A higher pressure is more effective at delivering force to clean mud off the driveway.
The gas particles inside the aerosol can have a higher pressure than the air particles outside the can. Gas particles naturally move from higher to lower pressure. If the aerosol can is punctured, gas particles suddenly move out of the higher pressure can to the lower pressure area outside the can. The force of this movement is strong enough to break and explode the can.
The sharp tip on a nail reduces the amount of area on the nail that is in contact with the wood. A reduced area increases the pressure of the nail on the wood, when force is applied.
Snowshoes have more area than regular shoes. This reduces pressure from the body on the snow, so a person can walk without sinking.