Question

Rosa studies the position-time graph of two race cars.

A graph titled Position versus Time shows time in hours on the x axis, numbered 0 to 5, position in kilometers on the y axis, numbered 0 to 50. The graph has two straight lines, one from point (0, 20) up to (2.8, 50), the second line starts at the origin and goes to point (5, 20).
Rosa concludes that truck X has a greater velocity than truck Y, and that truck Y had a head start. She also concluded that the graph shows both trucks moving at a constant velocity and that neither truck changes direction.

Which statement best describes Rosa’s error?

Both trucks change direction after they start moving.
Truck X had a head start, not truck Y.
Truck Y has a greater velocity than truck X.
The trucks are not traveling at constant velocities.

Answer 1

"Truck X had a head start, not truck Y."

Explanation: at t = 0 one truck is at 20 km and the other at 0 km, so the truck that starts at 20 km has the head start. Its slope (velocity) = 30 km / 2.8 h ≈ 10.7 km/h, while the other’s = 20 km / 5 h = 4 km/h. Rosa mixed up which truck was ahead.