# A2L Item 078

Goal: Hone the concepts of speed and velocity.

Source: UMPERG

The radius of the Earth is 6,400 km. The speed and direction would you
have to travel along the equator to make the sun appear fixed in the sky
is most nearly

1. 1680 km/hr, East
2. 840 km/hr, East
3. 533 km/hr, East
4. 267 km/hr, East
5. 267 km/hr, West
6. 533 km/hr, West
7. 840 km/hr, West
8. 1680 km/hr, West
9. Cannot be determined

### Commentary:

(8) You would attempt to remain underneath the sun as it traveled from
East to West. Some students may be confused by the tilt of the Earth’s
axis and think that the Sun could not remain fixed in the sky if you
were constrained to move along the equator. These students would likely

#### Background

Students should be able to determine the speed and direction even if
they do not yet have a solid grasp of velocity as a vector.

#### Questions to Reveal Student Reasoning

What is the circumference of the Earth? Does everyone on the Earth
travel at the same speed?

#### Suggestions

Build a simple model. Most students can readily grasp the result when
the Earth’s axis is perpendicular to the plane of the Earth’s orbit. A
model helps them understand that the tilt of the axis doesn’t matter.