1. An object is thrown up 420 away from the ground and lands after 10 seconds, 2. The car with mass m moves forward on a smooth horizontal plane with speed V1. There is a man with mass m on it. He asks how fast the car is when the man jumps backward and horizontally from the speed V2 of the car, This involves the relative speed, I think people's speed should be (V1 - V2) But the answer is (v-v2). He sets V to make the speed of the car after jumping. I think the speed can't change suddenly. It should be v1. Why

1. An object is thrown up 420 away from the ground and lands after 10 seconds, 2. The car with mass m moves forward on a smooth horizontal plane with speed V1. There is a man with mass m on it. He asks how fast the car is when the man jumps backward and horizontally from the speed V2 of the car, This involves the relative speed, I think people's speed should be (V1 - V2) But the answer is (v-v2). He sets V to make the speed of the car after jumping. I think the speed can't change suddenly. It should be v1. Why

Although the process of jumping is not instantaneous, the change of car speed is not a sudden action, that is, a sudden change. It accelerates to V after the whole process (let v be the speed of the car relative to the ground after jumping). And does the sudden change not affect the whole problem? The conservation of momentum in horizontal direction