當牽涉係碰撞既物件達致同一速度時...

Practically, colliding objects are not rigid, but elastic (thats how the term elastic collision appears). They would deform (change in shape) under a compressive force when colliding with one another. When the two objects are first in contact, the resistance force due to deformation causes the incident object (object A) to deccelerate and the object at rest (object B) to accelerate. This force continues to act after maximum compression between the two objects is reached and the objects return (rebound) to their original shapes. This is similar to the situation that a spring is first compressed and then released. In fact, you could imagine the collision as if there is a spring between the two colliding objects.

At the time when the velocities of the two objects are equal, they are still under a state of deformation and have not returned fully to their original shapes yet. As such, an elastic force exist and continues to accelerate object B (and decelerate object A too) until it completely returns to its original shape and leaves object A.

Regarding the case of hovercraft, the air-bags just lengthen the time of impact, hence reducing the impact force with the same change of momentum. It functions like the safety air-bag installed before the driver and the front-seat passenger in a motor car.

仲有就係點解登陸船既氣囊係著陸係陣變形就可以令碰撞時間加長?
因為登陸船的動能轉換成氣囊的彈性勢能(elastic potential energy)(俗稱彈弓勢力)
當有物件係高空跌落一個處於水平運動個陣，因為垂直的動量不會影響水平的動量
(因為cos90=0),所以其實果件物件的水平速度是零。佢跌落一個處於水平運動個陣，因為有慣性，所以兩件物件速度一樣。
彈性碰撞個陣,未必有相同速度，根據動量守恆,m1u1+m2u2=m1v1+m2v2
v1未必等於v2喎，沒有必然理由，你可以代下點數落去
碰撞個陣發生左D乜野事?其實在碰撞過程中，如果兩件質量不一樣，那麼它們所受的力不一樣，因為F=ma,大家既mass既然不一樣，咁兩個加速度都唔一樣，所以它們碰撞後是不會有共同速度