The string connecting **P **and **Q **suddenly breaks after 3 s while force **F **is still being applied.

Learners draw the velocity-time graph for the motion of **P **and **Q **before and after the string breaks, as shown below.

Write down the time at which the string breaks. Justify your answer. (3)

Which portion (

**X**,**Y**or**Z**) of the graph represents the motion of box**Q**, after the string breaks? Use the information in the graph to fully support the answer. (4)