Atrain approaching a crossing changes speed from 10 m/s to 25 m/s in 240 s. how can the train's acceleration be described the train will come to rest in 6 minutes.

the train's acceleration is positive.

the train is not accelerating.

the train's acceleration is negative.

The train's acceleration is positive (in the first part)

The train's acceleration is negative. (in the second part)

Explanation:

The acceleration of an object is given by:

where v is the final velocity, u is the initial velocity, t is the time taken.

In the first part, the train accelerates from u = 10 m/s to v = 25 m/s in t =240 s, so the acceleration is

and the sign is positive since the speed of the train is increasing.

In the second part, the train decelerates from u = 25 m/s to v = 0 (it comes to rest), in a time of t = 6 min = 360 s, so the acceleration is

and the sign is negative, since the speed of the train is decreasing.