At the moment of interest, cart A has mass M and velocity "v" .
Find dv/dt the instantaneous acceleration of A .
In the drawing, we see two carts A and B; Aat the left and B at the right, a pipe as described , is vertical in B , and all the sand in B goes in the pipe from down (from cart B)then the pipe makes an right angle with its vertical, so it becomes horizontal, directed to the left towards A, and at the end of the pipe the sand leaves the pipes to fall in cart A. Finally the two carts are moving to the right , it means "u" and "v" directed horixontally from A and B to the right
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