Representing motion as worldlines in a spacetime diagram.

Estimated Time: 20 minutes, including wrap-up

Students should be asked to answer the following questions on their whiteboards:
Draw a diagram representing yourself, standing still.
Draw a diagram representing your partner, moving right at 5 mph.
Draw diagrams representing each of you throwing a ball to the right at 10 mph.

None.

• Small whiteboards with markers

• Ask several students to present/defend their responses.
• Ask how fast each observer sees each thrown ball move.
• Ask students to explain how to answer this question from the diagram.
• Conclude that speed is (reciprocal) slope.