動能問題。10分

Ok.

Step one:
you need to calculate the average speed of the train first. Note that the speed of the passenger is the same because he is on the train.

average speed = distance / time = 1400 / (12 x 60) = 1.9444 m/s

Step two:
secondly you need to calculate the kinetic energy (K.E.) of the passenger by using the formula:

K.E. = 0.5 x mass x (speed^2) = 0.5 x 75 x (1.9444^2) = 141.78J

Answer: K.E. of the passenger = 142J

***Note that the units of the physical quantity:

unit of speed: [m/s]
unit of mass: [kg]
unit of energy: [J]

Thank you for reading. I hope it helps

2007-03-11 17:20:40 補充：
Please note that here the speed is AVERAGE speed, because no information about the journey is given. (such as acceleration or deceleration or stop when passengers getting off)Also note that 12(mins) have 12 x 60 = 720(seconds)

2007-03-12 09:08:20 補充：
It's a common question asked by my students too.Pls note that the v inthe 0.5mv^2 means SPEED, or magnitude of velocity.So, that's why we considered the distance when calculating the speed, instead of considering the displacement

2007-03-12 09:14:53 補充：
well, if the train runs around the park, the displacement can be zero. But it does not mean that the train didnt do work. Also note that the instantaneous velocity of the train is not zero.

2007-03-12 09:16:20 補充：
Displacement=0 simply tells you the train goes back to the starting point. If you use this zero value to calculate the velocity, that's an Average Velocity, which is very unfair and misleading

By applying V=s/t and KE=1/2*m*v(square)
V=1400/(12*60) then=1.9444ms per second
then 1/2mv(square)
KE=1/2*75*3.7808
=142J