Vector loop or not? (exploring addition)

A set of vectors form a [u]loop[/u] when you can place them head-to-tail and the head of the last vector touches the tail of the first vector. [br][br]Here are some examples.
Examples of Loops
Here are some non-loops. Can you tell what is wrong? What makes them NOT vector loops?
Non-Loops
Drag the vectors in sets 1, 2, 3, 4. [br]Can any sets of vectors be made into loops? Which ones?
Loop sets or not?
Drag the vectors around in sets V, W, U, Z. Try to make loops.[br][list=1][*]Which sets [u]cannot[/u] make loops? [/*][*]What vector can you add to the set so that you [u]can[/u] make a loop?[/*][*]How does the new vector depend on the order you joined the original vectors head-to-tail?[/*][/list]
Loops possible or not?
Going further
Find the sums of the vectors in each set.[br]What do you notice?[br]What do you wonder?
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Information: Vector loop or not? (exploring addition)