12. a 15,000 kg train car is rolling at a speed of 3.2 m/s. compute the time required for a force
of 1200 n to stop the car.

40 s

Explanation:

By Newton's 2nd law,

The rate of change of momentum is directly proportional to the unbalance force applied on the object,

By that you can get the equation,

F = ma ⇒ a = F/m

but  a = (v-u)/t

So we get, t = (v-u)×m/F

where terms are in usual meaning

t =(3.2-0)×15000/1200  = 40 s

d) physical means can be used to separate the components of air.

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