impact

其實1同2係一樣
首先講左1，車頭較長，有變形區域使碰撞時間增長

你果兩個問題，係因為你覺得average net force一樣
但係而家個average net force係取決於m, v, u, t
F=(mv-mu)/t, 咁你要比較兩個碰撞, 首先v同u不變
咁而家個t越大，個F咪越細lor, 所以碰撞時間越長, average net force係越細

2012-03-05 15:13:01 補充：
同埋action and reaction 只係可以講部車撞落物件時，car act on object 同object act on car個力一樣，同average net force完全無關

2012-03-05 18:38:13 補充：
1.點解要neglect 個rebound speed?
因為你唔知個rebound speed, 題目係寫if the rebound speed is negligible, 係題目比你既條件，方便你計數。

2012-03-05 18:47:49 補充：
2. 因為佢喺一個短時間(0.1s)入面將佢既velocity由24.5 ms^-1(downward)變成0 ms^-2，呢個涉及一個較大既change of momentum。同埋你靜置個波喺地下，佢個Net Force=0，因為R=mg，用動量諗就係你放個ball落地果陣佢個velocity=0, 而你放完佢個velocity都係0，P=0, F=dP/dt=0

1. A person will be hurt if he is subjected to a great force. The seriousness of his injury depends on the magnitude of the force. Should the force be great enough, the person would have serious injury or even death.

Hence, it is essentially important in car design to reduce the impact force during accident to such a level that would not cause serious injury/death. One of such design is to lengthen the impact time as far as practically feasible. Alhough the change in momentum of the car and person is the same for a given car speed, lenghtening the impact time reduces the impact force proportionally.

2. No.
For a given car speed, say u, the change in momentum of the driver/passesnger during a collsion = mu (assume the car eventually comes to a stop after the collision), and m is the mass of the driver/passenger.

By F.t = mu
where F is the impact force acting on the driver/passenger anf t is the impact time
i.e. F = mu/t
Because mu is fixed for a given speed, lengthening the impact time t reduces the impact force F.
This is thus the effect of an air cushion that tends to lengthening the impact time.