> College of Science > Physics Dept > Tatsu Takeuchi > Special Relativity > Practice Problems > Problem 10
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### The Offside Rule in Soccer

During a game of soccer, Matthew is initially in possession of the ball while his teammate Mark runs toward the goal being kept by Luke of the opposing team. Matthew kicks the ball toward Mark at point B on the spacetime diagram. Mark passes John of the opposing team and thereby enters the offside position.

1. At which spacetime point, labeled A through K, is Mark when the ball is kicked at B in Mark's frame of reference?
2. At which spacetime point, labeled A through K, is Mark when the ball is kicked at B in John's frame of reference?
3. In Mark's frame of reference, did Mark enter the offside position before the ball was kicked or after the ball was kicked? Explain, referring to the spacetime diagram.
4. In John's frame of reference, did Mark enter the offside position before the ball was kicked or after the ball was kicked? Explain, referring to the spacetime diagram.
5. What is the chronological order in which Luke, the goal keeper, sees the two events: the ball being kicked by Matthew, and Mark entering the offside position?