The same distance every way

About this video

A farmer ties a sheep to a post with a rope 4 metres long, and by evening it has eaten a patch of grass. This animation sweeps the rope round to show the shape of the patch: a circle, because every place on its edge is exactly 4 metres from the post. A circle is all the points that are the same distance from one point, its centre.

The video then names the parts. The radius is a line segment from the centre to the circle; the rope is a radius 4 m long, and as it turns all the way round it stays the same length, because every radius of a circle is equal. The diameter goes straight across through the centre. It is two radii end to end, so 4 + 4 = 8 m. The circumference is the length of the circle itself, all the way round: the circle's perimeter.

Finally, two wheels roll along the ground with the path of their axles drawn behind them. A round wheel's axle stays at the same height, because the ground is always one radius below it. A square wheel's corners are further from its centre than the middles of its sides, so its axle goes up and down and the ride is bumpy. That is why wheels are round. This video goes with the Circles lesson of Geometry Level 1 on Knaph.

Transcript

1. 0:00A sheep on a four metre rope. What shape is the patch it eats?
2. 0:07Every point on the edge is four metres from the post. That is a circle.
3. 0:15The point in the middle is the centre.
4. 0:21From the centre to the circle is a radius. Turn it all the way round. It never changes length.
5. 0:29Straight across, through the centre, is the diameter. Two radii. Four and four make eight.
6. 0:38The length all the way round is the circumference.
7. 0:45Every radius is equal, so a round wheel keeps its axle level. A square wheel's corners stick out further, so its axle bumps up and down.
8. 1:04A circle is every point the same distance from its centre.