If a subway train is moving to the left (has a negative velocity) and then comes to a stop, what is the direction of its acceleration? is the acceleration positive or negative?

The acceleration would be positive, pointing to the right.

Explanation:

The acceleration tells us how velocity changes every second. If the train stops, this means its velocity must go from a non-zero value to zero.

To reduce velocity, acceleration must point to the opposite direction. Since velocity is negative, acceleration must be positive.

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