Truck B carries no card and is placed so that it is stationary between the light gates.
Truck A is set off towards truck B. As the card passes through the first gate it
records a time t1. Truck A then collides with truck B. They stick together and move
through the second gate which records the time t2.
Both trucks have the same mass. Explain why t2 = 2t1 if momentum is conserved.