Circle and disc
Circle and disc
A circle is a closed line whose points are all the same distance from one point – the centre ( in the drawing).
- Radius () – a segment from the centre to a point on the circle. All the radii of a circle are equally long.
- Chord – a segment joining two points on the circle ( in the drawing).
- Diameter () – a chord through the centre. It is made of two radii: .
A disc is a circle together with everything inside it.
- The circle is a line – like a hoop, or a loop of string.
- The disc is a flat region – like a plate, a coin or a pizza.
- A disc’s centre, radius and diameter are those of the circle around it.
A compass draws a circle because its pencil always stays the same distance from its point:
- Stick the point in the centre.
- The distance between the point and the pencil is the radius.
- Turn the compass: the pencil passes only through points at that distance from the centre – on the circle.
- The wider the compass is opened, the longer the radius and the bigger the circle.
- A big circle on a playground is drawn with a rope: one end tied to a peg in the centre, the other end marking all the way round, the rope kept tight.
Calculate, in cm.
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power
The drawing shows two arcs – parts of two circles.
Which arc belongs to the circle with the bigger radius?
Explain.
Draw a circle, and mark its centre, a radius, a diameter and two chords.
Which chord is the longest one that can be drawn in a circle?
Explain.
A box holds three tennis balls in a row, touching each other and the sides.
The drawing shows the box seen from above.
Each ball’s radius is 3.5 cm.
How long is the box? And how wide?
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – check and a new line · * – times · ^ – power, ↓ – end of the power
/ – fraction bar · Tab or the arrows – between numerator and denominator, then a note above or below · Enter – a new line · * – times · ^ – power, ↓ – end of the power