緊急!!!!力學問題 (10分)

measure of mass reflect the information of inertia, therefore, whatever force(momentum) or energy approach is used, inertia is still involved.

force(momentum):
force equals to rate of change of momentum of a mass, m
F=ma=m(v-u)
to decelerate the driver from a spped, u, to stationary, v=0, an opposing force (normal reaction), F, should be exerted on the driver with a mass, m, by the seat belt.

energy:
as the driver is fastened by a seat belt, KE of the driver, m, is converted to the elastic PE of the seat belt.
as KE=0.5mu^2, when KE is lost, u decreases (decelerate).
by the way, the seat belt experiences a tension and is extended. elastic PE is stored in the seat belt and increased. where PE=0.5kx^2

as you see, both approaches involve the evaluation of mass, i.e. inertia.

2007-04-28 23:23:42 補充：
the equation should be:F=ma=m(v-u)/t

The driver is moving forward originally. If the car suddenly stops, the drive has the inertia to move forward due to its inertia of moving forward/ due to the the Newton's first law ( an object stay at in its state of uniform motion or state of at rest if there is not unbalanced force acting on it).