Why is it mandatory to make the string taut/stretched for sending pulse/wave?

In order to send a pulse and to propagate it, the string must be under tension.$^\text{1}$

Why is the tension necessary? Why should the string be stretched/taut for the transmission of the pulse?

$^\text{1}$ Principles of Physics by Walker,Resnick,Halliday.

• You put "In order to send a pulse and to propagate it, the string must be under tension" in a box. Why did you do that? Is this a quotation from a book? – DanielSank Feb 5 '15 at 8:46
• Hint: vibrations are caused by elasticity. – Phoenix87 Feb 5 '15 at 9:09
• The tension in the rope provides a restoring force - in order for waves to propagate, each element of rope must experience a force from its neighbors bringing it back towards equilibrium when it is displaced, and a force away from equilibrium when its neighbors are displaced; that is the process by which a wave propagates. – Brionius Feb 5 '15 at 9:44
• @Brionius: Sir, I urge you to please answer this. – user36790 Feb 5 '15 at 11:14
• @Brionius: When the disturbed part provides tension to another undisturbed element, the net force is upwards but soon after restoring force starts to act to the previously undisturbed element thus decelerating it to zero when it reaches the highest amplitude. ,right? – user36790 Feb 8 '15 at 11:38