phy again

1. The question concerns about action and reaction forces. Patrick needs to exert a force to the left onto the skateboard in order to make him to jum to the right. Because of the left-ward force exerted by Patrick, the skateboard moves to the left. Therefore, the force exerted by Patrick onto the skateboard and the force pushing Patrick to the right form an action-reaction pair.

Gravity plays no role on this situation, as gravity is in the vertically downward direction. It only pulls Patrick downward, but not to the right

2. Velcoity of toy car B after collision = 1 x 0.5 m/s = 0.5 m/s
Use conservation of momentum,
total momentum before collision = 3 x 3 kg.m/s = 9 kg.m/s
total momentum after collision = (3v + 1 x 0.5) kg.m/s
where v is the velocity of toy car A after collision
Hence, 9 = 3v + 0.5
v = 2.83 m/s

Q: I don't understand why the accelerating period of car B(0.5s) is same
as that of car A.
A: The two toy cars accelerates or deccelerates when there is a collision force. This collision force only applies when the two cars are in contact. Because the contact time is common to both cars, hence car A shares the same accelartion time as that of car B.

Q: Friction increases as speed increases?If not,how can car move at a
constant velocity at high speed?
A: Generally, resistance force (which include friction and other retarding forces) increases with speed of the car. A car reaches a steady speed when the force delivered by the engine balances the resistance force experienced by the car.