A2L Item 078
- Description: Find the velocity along the equator to keep the sun fixed in the sky.
- Goal: Hone the concepts of speed and velocity.
- Source: UMPERG
- Keywords: Mechanics, Rotational Motion, Velocity
The question for students:
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
- 1680 km/hr, East
- 840 km/hr, East
- 533 km/hr, East
- 267 km/hr, East
- 267 km/hr, West
- 533 km/hr, West
- 840 km/hr, West
- 1680 km/hr, West
- Cannot be determined
Commentary for teachers:
Answer
(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 answer (9).
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.