Unit D Lesson D6 Gravity and Loads
Completion requirements
Lesson D6: Gravity and Loads
Video Lesson
When engineers design a skyscraper, they think about all the forces that will act on the tower. Watch this video to learn more about the various external forces that affect structures.
Lesson D6: Gravity and Loads
Figure D.2.6.1 β Double-decker buses are two storeys high.
Figure D.2.6.2 β Monster trucks look like pickup trucks with huge wheels.
Surprising Centres of Gravity
Although double-decker buses are used in various countries, most people associate double-decker buses with London, United Kingdom. Passengers climb a narrow staircase to sit on the top deck of the bus. Some have open upper decks for sight-seeing; others are fully covered with comfortable seating for travelling long distances. Edmonton and Calgary have some double-deckers, and they travel the highways to nearby suburbs, too.
Double-decker buses are tall and look as if they could tip easily, but they donβt! These vehicles have surprisingly low centres of gravity because their heavy engines and equipment are located very low to the ground. The low centre of gravity keeps the bus balanced and stable, even when an external wind force pushes against the bus.
Although double-decker buses are used in various countries, most people associate double-decker buses with London, United Kingdom. Passengers climb a narrow staircase to sit on the top deck of the bus. Some have open upper decks for sight-seeing; others are fully covered with comfortable seating for travelling long distances. Edmonton and Calgary have some double-deckers, and they travel the highways to nearby suburbs, too.
Double-decker buses are tall and look as if they could tip easily, but they donβt! These vehicles have surprisingly low centres of gravity because their heavy engines and equipment are located very low to the ground. The low centre of gravity keeps the bus balanced and stable, even when an external wind force pushes against the bus.
Reading and Materials for This Lesson
Science in Action 7
Materials:
Science in Action 7
Reading: Pages 285-289
Materials:
two
forks, small ball of modelling clay, toothpick, drinking glass
People race monster trucks through muddy obstacle courses for entertainment. Although monster trucks are large, they actually modified regular pickup trucks on huge wheels and are quite unstable. A monster truck has a high centre of gravity, which means
it tips easily when external forces are applied to it.
Watch More
Watch the following video of a monster truck crash due to the truck's high centre of gravity.
Daring Stunts and the Centre of Gravity
Some humans like to test their personal limits by doing daring stunts.
The first person to walk on a tightrope directly over Niagara Falls was Nik Wallenda in 2012. This was especially difficult because the mist from the waterfall made the cable slippery.
Some humans like to test their personal limits by doing daring stunts.
The first person to walk on a tightrope directly over Niagara Falls was Nik Wallenda in 2012. This was especially difficult because the mist from the waterfall made the cable slippery.
Figure D.2.6.3 β Niagara Falls are 51 metres tall, which is the height of a 15-storey building.
Watch More
Tightrope Walking across Niagara Falls
Watch this video to see Nik Wallenda's tightrope walk over Niagara Falls.
Watch this video to see Nik Wallenda's tightrope walk over Niagara Falls.
Watch this video to learn more about Nik Wallenda and his high-wire stunts.
To maintain balance, tightrope walkers must keep their centres of gravity directly over their feet.
Did you notice the long pole that Nik carries when he walks along a tightrope β and he seems to wave it constantly? The long pole helps him keep his centre of gravity over his feet. If Nik shifts his body weight slightly in one direction, he can tip the pole in the opposite direction. This balances his mass and keeps his centre of gravity over the wire. Did you notice that the ends of the long pole usually droop below the wire height? The lower pole ends make Nik's centre of gravity lower so he is more stable.
Cirque du Soleil is a travelling circus company that started in Quebec, Canada. Acrobats in Cirque du Soleil perform some fantastic stunts that focus on their centre of gravity.
If you look closely at acrobats, you will notice that their knees are slightly bent when they walk across high wires or perform balancing tricks. Bending the knees lowers the centre of gravity in the body and makes the person more stable.
Did you notice the long pole that Nik carries when he walks along a tightrope β and he seems to wave it constantly? The long pole helps him keep his centre of gravity over his feet. If Nik shifts his body weight slightly in one direction, he can tip the pole in the opposite direction. This balances his mass and keeps his centre of gravity over the wire. Did you notice that the ends of the long pole usually droop below the wire height? The lower pole ends make Nik's centre of gravity lower so he is more stable.
Cirque du Soleil is a travelling circus company that started in Quebec, Canada. Acrobats in Cirque du Soleil perform some fantastic stunts that focus on their centre of gravity.
If you look closely at acrobats, you will notice that their knees are slightly bent when they walk across high wires or perform balancing tricks. Bending the knees lowers the centre of gravity in the body and makes the person more stable.
Watch More
Cirque du Soleil
Watch this video to see a Cirque du Soleil high wire act.
Watch this video to see a Cirque du Soleil high wire act.
Try It!
The Incredible Balancing Forks
Try your own balancing stunt at home using simple materials. (We don't mean that Cirque du Soleil act β don't try it anywhere!)
Materials:
- two forks
- small ball of modelling clay
- toothpick
- drinking glass
Video: Don't be afraid to experiment and substitute materials!
Instructions:
- Stick the forks into the ball of modelling clay. The fork handles should be pointing slightly downwards in opposite directions.
- Stick a toothpick into the ball of modelling clay in a position that is perpendicular to the forks.
- Balance the edge of the toothpick on the edge of the drinking glass. You might need to move the toothpick back and forth on the glass edge to find where it balances.
Questions:
Think about the following questions very carefully; then, type or write your answers. After you have your answers, click the questions for feedback.
Think about the following questions very carefully; then, type or write your answers. After you have your answers, click the questions for feedback.
Pointing the forks downward lowers the centre of gravity on the structure.
The fork handles are lower than the toothpick is, which lowers the centre of gravity on the entire structure. Lowering the centre of gravity makes the structure stable and less likely to tip.
Connections
Connections: Math
>> Non-Perpendicular Buildings - On Purpose!
The Leaning Tower of Pisa was begun in 1173 to be perpendicular to the ground β more than 800 years ago. It began to tilt during its construction because the ground beneath was softer on one side than on the other. Various attempts were made to return it to the perpendicular, but a project in 2008 should keep it from collapsing.
In math, when two lines form a 90-degree angle, one is said to be perpendicular to the other in a figure that sometimes is called a square angle.
Most towers and buildings are designed to be perpendicular to the ground just as walls are usually perpendicular to floors. This is the easiest way to ensure the centre of gravity is located over the base of the building.
However, some creative architects have made towers and buildings that deliberately are not perpendicular to the ground. These structures are tilted on purpose! Non-perpendicular structures are designed carefully so that the centre of gravity is supported over the base.
The Capital Gate Tower in Abu Dhabi, United Arab Emirates, is the largest intentionally non-perpendicular tower in the world.
>> Non-Perpendicular Buildings - On Purpose!
The Leaning Tower of Pisa was begun in 1173 to be perpendicular to the ground β more than 800 years ago. It began to tilt during its construction because the ground beneath was softer on one side than on the other. Various attempts were made to return it to the perpendicular, but a project in 2008 should keep it from collapsing.
In math, when two lines form a 90-degree angle, one is said to be perpendicular to the other in a figure that sometimes is called a square angle.
Most towers and buildings are designed to be perpendicular to the ground just as walls are usually perpendicular to floors. This is the easiest way to ensure the centre of gravity is located over the base of the building.
However, some creative architects have made towers and buildings that deliberately are not perpendicular to the ground. These structures are tilted on purpose! Non-perpendicular structures are designed carefully so that the centre of gravity is supported over the base.
The Capital Gate Tower in Abu Dhabi, United Arab Emirates, is the largest intentionally non-perpendicular tower in the world.
Figure D.2.6.4 β Intersecting lines that make a 90Β° angle are called perpendicular.
Watch More
Capital Gate Tower
Watch this video to see the tilted Capital Gate Tower.
Watch this video to see the tilted Capital Gate Tower.
Career Connections
Connections: Careers
>> Architect Frank Gehry
An architect designs and plans buildings. Architecture is a career in which a personβs artistic creativity and scientific knowledge are combined to make aesthetically beautiful and functional buildings.
Frank Gehry is a famous architect born in Toronto, Canada. He is well-known for his unusual building designs, many of which are not perpendicular to the ground!
>> Architect Frank Gehry
An architect designs and plans buildings. Architecture is a career in which a personβs artistic creativity and scientific knowledge are combined to make aesthetically beautiful and functional buildings.
Frank Gehry is a famous architect born in Toronto, Canada. He is well-known for his unusual building designs, many of which are not perpendicular to the ground!
Figure D.2.6.6 β A group of tilted buildings designed by Frank Gehry in Dusseldorf, Germany.
Figure D.2.6.7 β A medical centre located in Las Vegas, Nevada, USA.
Figure D.2.6.8 β A museum in Spain.
Figure D.2.6.9 β An office building in New York City.
Figure D.2.6.10 β A pop culture and music museum in Seattle, USA.
Watch More
Interview with Frank Gehry
Watch this interview with Frank Gehry. You will see a model of one of his recent architectural designs
Watch this interview with Frank Gehry. You will see a model of one of his recent architectural designs
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 D Lesson 6 Self-Check
Instructions
Complete the following 6 steps.
Don't skip steps β if you do them in order, you will confirm your
understanding of this lesson and create a study bank for the future.
- DOWNLOAD the self-check quiz by clicking here.
- 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.
- 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.
-
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.
- 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.
- 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!
Self-Check Time!
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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 β STOP β and go back to step 1 above. Do not look at the answers without first trying the questions.
External forces are forces applied to a structure by something other than the structure. Some types of external forces that act on a structure are wind, earthquakes, people and machinery in the structure, and the weight of the structure.
Cars driving on a bridge contribute to the dynamic load. A dynamic load is a force on the structure that moves or changes. The number of cars on a bridge changes constantly, which is why it is a dynamic load.
The bridge deck contributes to the static load. A static load does not move. The bridgeβs deck does not move, so it is a static load.
The centre of gravity is the point in a structure where all the gravity pulls the structure toward the centre of the earth.
A compact car is more stable in strong winds. A compact car is lower to the ground than a semi-trailer is so its centre of gravity is lower. When the force of the wind is applied to the car, the car is not tipped easily even if it may be pushed
sideways. A semi-trailer has a higher centre of gravity; therefore, a strong wind can tip the semi-trailer more easily than it can tip a small car.