# Why do pulses get reflected by a fixed end?

Consider the case where a pulse hits a fixed end. I want to know the physics behind the following two phenomena

1) Why does the pulse get reflected?

2) Why does the pulse get inverted?

I somewhat understand the answer to the second question. The pulse exerts an upward force on the fixed end and by Newton's Third Law the fixed end exerts an equal and opposite force. Consequently, the particles on the rope will have a negative displacement. Regarding the first answer I think that it has to do with Principle of Superposition, but I can't quite verbalize an explanation.

• The wave is incident on the fixed end with some velocity value $u_1$. The initial momentum, therefore, is $P_i=m_1u_1$. After collision, with the fixed end, the final velocity of the fixed end is $0$ and the wave's final velocity is $v_1$. Thus, the final momentum is $P_f=m_1v_1$. Now according to Momentum conservation $P_i=P_f$, which implies that $v_1=u_1$. This does not prove that the final velocity is in the opposite direction of the initial velocity and therefore suggests that the wave must travel in the same direction. What mistake have I made? – model_checker Mar 13 '16 at 9:59