9. A 1800-kg car stopped at a traffic light is struck from the rear by a 900-kg car and the two become entangled (糾纏在一起). If the smaller car was moving at 20.0 m/s before the collision, what is the speed of the entangled cars after the collision? _______m/s.

4. A ballot box with mass m = 6.0 kg slides with speed v = 4.0 m/s across a frictionless floor in the positive direction of an x axis. The box explodes into two pieces. One piece, with mass m1 = 2.0 kg, moves in the positive direction of the x axis at v1 = 8.0 m/s. What is the velocity of the second piece, with mass m2 ? _______m/s.

9. A 1800-kg car stopped at a traffic light is struck from the rear by a 900-kg car and the two become entangled (糾纏在一起). If the smaller car was moving at 20.0 m/s before the collision, what is the speed of the entangled cars after the collision? _______m/s.

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