Lesson D12: Structural Joints and Friction

Key Ideas   Unit Checklist

  Video Lesson

Joints connect the various parts of a structure. Watch this video to learn more about various types of joints.

  Lesson D12: Structural Joints and Friction


Figure D.3.12.1 – Friction is needed to stop the wheels of a vehicle.

Figure D.3.12.2 – This close-up shows that friction causes vehicle brakes to get so hot they glow.

Discover Friction

Friction occurs when two surfaces rub against each other.

Friction between a surface and a moving object resists, or slows down, the movement. Rough surfaces produce more friction than smooth surfaces do.

Reading and Materials for This Lesson

Science in Action 7
Reading: Pages 313-318

Materials:
small toy car, masking tape, scissors, cardboard, sandpaper, waxed paper, bubble wrap, stack of books, smooth hard surface such as a shiny hardcover book or binder, ruler or measuring tape, 2 old books or phone books, 2 thin metal jar lids, small plastic jar lid, heavy textbook, glass marbles

  Try It!

Friction on Roads

Try this simple experiment at home to measure the effect of friction on road surfaces.

Materials:

  • small toy car
  • masking tape
  • scissors
  • cardboard
  • sandpaper
  • waxed paper
  • bubble wrap
  • stack of books
  • smooth hard surface such as a shiny hardcover book or binder
  • ruler or measuring tape
Take care with scissors. Do not cut yourself or anyone else!
Instructions:

  1. Cut 4 rectangles of cardboard, each measuring 10 cm by 75 cm

  2. Cover the surfaces of 3 rectangles with different materials: one with sandpaper, another with waxed paper, and a third with bubble wrap. Use masking tape to fasten the material to the cardboard. Leave the last rectangle as plain cardboard.

  3. Make a stack of books about 20 cm high.

  4. Make a ramp with your smooth hard surface, resting the top of the ramp on the book stack.

  5. Put your plain cardboard rectangle on the floor with the short 10 cm edge touching the bottom of the ramp.

  6. Hold the toy car at the top of the ramp. Let the car go. It will move down the ramp and along the piece of cardboard. Wait until the car stops on its own.

  7. With the ruler, measure the distance from the start of the cardboard road to the tip of the stopped car. Record your measurements in a table such as this:
    (if you like, you can download this table suitable for printing by clicking here)

    Road Surface Distance Travelled (cm)
    Cardboard
    Sandpaper
    Waxed paper
    Bubble wrap
    Carpet

  8. Repeat Steps 5 to 7 with the three other cardboard roads. Try the experiment on carpet, too.
Questions:

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

The car traveled the largest distance on the waxed paper road. The waxed paper is the smoothest surface of the materials, which produced the least friction between the road and the cars tires. Less friction means less resistance to slow the car, so it went further before stopping.
The car traveled the smallest distance on the bubble wrap road. The bubble wrap has a bumpy surface, which produced a lot of friction between the road and the cars tires. More friction means more resistance, which slowed the car and stopped it sooner.
Ice is a smooth surface, which means little friction is produced. Lower friction means that, when a driver hits the brakes, the car travels further than it does on a dry road before it stops. If drivers follow other cars too closely on icy roads, they might not have enough room to stop before hitting another vehicle.

Figure D.3.12.3 – This ancient Scottish house was built over 5000 years ago from stone. It even has stone furniture inside.

Strong Joints with Friction

Friction by itself can produce some amazingly strong joints! The Pont du Gard Bridge, seen in Figure D.3.12.4, has been standing for nearly 2000 years. No mortar cement was used to hold the bricks together on this bridge. The ancient Romans fitted the stones so perfectly that the bridge holds together only by friction!


Figure D.3.12.4 – The Pont du Gard is an ancient bridge in France. It was used to transport water.

Figure D.3.12.5 – In brick structures, the bricks are often joined together with mortar.

 Watch More

Overlapping Books

Watch this video to see two books connected with a strong interlocking joint that works by the force of friction.


  Try It!

Overlapping Book Trick

Were you amazed by the strength of the joint between the two phone books? Are you curious to see if this trick really works? Try it for yourself!

Materials:

  • two old books or phone books (Be sure to use books that can get wrecked accidentally!)

Instructions:
  1. Arrange the books on a table so that the pages are facing each other.

  2. Overlap the pages of the books, a couple pages at a time.

  3. Put your hands on the spines of the books. Try slowly and carefully to pull the books apart.

  4. Ask a family member or friend to try pulling apart the books!
Questions:

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

The overlapping pages show an example of a joint made with interlocking pieces. Where the pages are in contact with each other is a large area.
When you try to pull the books apart, the pages rub against each other. Because so much page area overlaps, a huge amount of friction is produced. The friction force from the pages is much stronger than the force you use to pull the books apart, which is why separating the books is nearly impossible.
Ball Bearings

When a wheel turns, it rubs against its axle. This rubbing is friction, which slows the spinning of the wheel.

If we want a wheel to turn very quickly, we must reduce friction between the wheel and the axle. Ball bearings, as seen in Figure D.3.12.7, are used in rotating parts to help the circling motion happen faster. Ball bearings are used in car wheels, train wheels, airplane wheels, and even skateboard wheels.


Figure D.3.12.6 – A wheel is a large circular turning structure. An axle is in the centre of a wheel.

Figure D.3.12.7 – Ball bearings for a wheel.

 Watch More

Ball Bearings

Watch this video to see an animation that explains how ball bearings work.


  Try It!

Make a Ball Bearing Spinner

Try this simple experiment to test how a ball bearing works.

Materials:
 
  • 2 thin metal jar lids (One lid should be wider and slightly thinner than the other is.)
  • 1 small plastic jar lid that fits inside one of the metal jar lids
  • heavy textbook
  • glass marbles

Instructions:

  1. Place the smaller lid inside the larger lid.

  1. Carefully place the heavy textbook on top of the lids. Gently turn the textbook clockwise and counterclockwise.
  2. Remove the textbook from the lids.
  3. Place the marbles around the smaller lid, in the space between the edges of the two lids.
  4. Try different sized lids if the marbles can't roll around the space between the edges of the two lids.
  5. Carefully place the heavy textbook on top of the marbles. Gently turn the textbook clockwise and counterclockwise.
Questions:

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

When the book was spun, the two lids rubbed together. This produced friction, which slowed the motion of the lids.
The marbles acted as ball bearings. The round and smooth glass marbles reduced friction, which allowed the book to spin easily without stopping.
Animal Glue

Did you know that glue used to be made from animals? When a horse became old, people used to say that it was going to the glue factory. The bones and hooves and hides of dead horses were boiled for days, which produced a sticky glue. Many modern glues are made from chemicals in crude oil that is pumped from the ground.


Figure D.3.12.8 – In the past, horse hooves were used to make glue.

Figure D.3.12.9 – Crude oil is a mixture of many chemicals used to make materials such as glues and plastics in addition to gasoline, diesel, and oil.

  Try It!

Natural Glues

Try this experiment to test glues made from various types of foods.

Materials:

  • 3 jars or plastic containers with lids
  • cooking pot
  • strainer
  • spoon
  • whisk
  • milk
  • white vinegar
  • baking soda
  • water
  • flour
  • corn syrup
  • cornstarch
  • 6 popsicle sticks
  • pen
Take care when heating things on the stove. Do not leave the stove unattended. Do not burn yourself or anyone else!
Instructions:

Part A: Make the Glues

  1. Milk Glue: Go to this website and follow the instructions to make glue from milk, vinegar, and baking soda.

  2. Flour Glue: Go to this website and follow the instructions to make glue from flour and water.

  3. Cornstarch Glue: Go to this website and follow the instructions to make glue from cornstarch, corn syrup, and vinegar.
Part B: Test the Glues

  1. Lay the end of one popsicle stick flat on top of the end of a second popsicle stick, overlapping the ends by 3 cm. Use the milk glue to glue the two popsicle sticks together to make a long straight line.

  2. Write milk glue on the popsicle sticks to identify the glue you used.

  3. Repeat Steps 1 and 2, using the flour glue and the cornstarch glue, being careful to label each.

  4. Let the glue dry overnight.

  5. The next day, try pulling apart the glued popsicle sticks. Which were the most difficult to separate? (Which glue made the strongest joint?)



  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 12 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!


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 – STOP – and go back to step 1 above. Do not look at the answers without first trying the questions.

Staples or thumbtacks are removed easily from a corkboard if someone wants to change the papers. Glue is an adhesive that might make a permanent joint between the paper and corkboard.
A movable joint on the side of eyeglass frames lets the frames fit a person’s head better. The joint allows the person to fold the frames, which protects them and makes them more portable.
Logs are round. Stacking logs without them rolling away is difficult. Fitting the logs together with interlocking notches is easier and perhaps stronger than fitting them using nails.
Friction from the stones touching each other holds the wall in place. The advantage of not using mortar is that no other material is needed and the walls can be moved if necessary.
The studs produce a rough surface on the bicycle tire. This increases friction when riding on icy roads, and the bicycle is less likely to slip.