Carts and Wheels

'A wheel is round because a circle rolls smoothly. No other shape rolls like a circle.'

1. What You Will Learn

• What makes a circle special
• Parts of a circle — centre, radius, diameter
• Drawing circles using a compass
• Round objects in everyday life
• Comparing circles of different sizes

2. Why Are Wheels Round?

Imagine a SQUARE wheel! The cart would JERK and JUMP with every turn.

A CIRCLE is the only shape that rolls SMOOTHLY. Every point on the edge is the SAME distance from the centre. That is why wheels, coins, and bangles are round.

Things That Are Round

3. Parts of a Circle

Centre

The FIXED point inside the circle. Every point on the circle is at the SAME distance from the centre.

Radius (Radii for plural)

The distance from the CENTRE to any point on the circle.

Diameter

The distance ACROSS the circle, passing through the centre.

• Diameter = 2 × Radius
• Radius = Diameter ÷ 2

In a Circle

      --------
    /          \
   |   ------   |
   |  | CENTRE|  |
   |   ------   |
    \          /
      --------

(The above is a simple representation of a circle with a centre.)

4. How to Draw a Circle with a Compass

A compass is a special tool for drawing circles.

Steps

1. Fix a pencil in the compass holder
2. Decide the radius (e.g., 3 cm)
3. Set the compass width to 3 cm using a ruler
4. Place the compass POINT on the paper (this is the centre)
5. Rotate the compass slowly — let the pencil drag along the paper
6. You have drawn a PERFECT circle!

Tips for a Good Circle

• Hold the compass from the TOP, not the legs
• Keep the compass POINT firmly on the paper
• Rotate SMOOTHLY without jerking
• Practice makes perfect!

5. Drawing Circles Without a Compass

Don't have a compass? Try these methods:

• Bangle method: Trace around a round bangle
• Bottle cap method: Trace around a bottle cap
• String method: Tie a pencil to one end of a string, hold the other end fixed, and swing the pencil around
• Freehand method: Draw a circle by hand — it takes practice!

6. Key Facts

• A CIRCLE is a closed curve with ALL points at the SAME distance from the centre
• Every circle has a CENTRE, RADIUS, and DIAMETER
• Diameter = 2 × Radius
• A COMPASS is used to draw accurate circles
• Wheels are round because circles roll SMOOTHLY
• The largest distance across a circle is the DIAMETER

7. Common Mistakes

'Do NOT confuse radius and diameter. Radius is HALF of diameter.' 'Do NOT lift the compass point while drawing — the circle will become wobbly.' 'Do NOT think the compass width is the diameter — it is the RADIUS.' 'Do NOT assume all round things are perfect circles — some are ovals.' 'Do NOT forget to label the centre, radius, and diameter on your drawing.'

8. Fun Activity

Circle Hunt Walk around your home or school. Find 15 round objects. List them and draw them in your notebook.

Measure Circles For 5 round objects (coin, plate, bangle, bottle cap, wheel):

1. Trace the circle on paper
2. Draw the centre (fold the paper in half twice — the intersection is the centre)
3. Measure the radius
4. Calculate the diameter (double the radius)

Make a Design with Circles Use your compass to draw overlapping circles of different sizes. Colour the design to make a PATTERN.

9. Self-Test

Q1. What is the radius of a circle with diameter 10 cm? Answer: Radius = 10 ÷ 2 = 5 cm

Q2. What is the diameter of a circle with radius 4 cm? Answer: Diameter = 2 × 4 = 8 cm

Q3. Name three things that are round and roll. Answer: Wheel, ball, coin (any three round objects)

Q4. Which part of the compass stays fixed on the paper? Answer: The POINT (the sharp end)

Q5. A circle has radius 7 cm. What is its diameter? Answer: Diameter = 2 × 7 = 14 cm

Q6. Why are wheels round instead of square? Answer: Because circles roll smoothly — a square wheel would jump and jerk.

Q7. Draw a circle with radius 3 cm. Label the centre, radius, and diameter. (Try it in your notebook!)

10. Key Vocabulary