A horizontal line means the object is moving at a constant speed. A downward sloping line means the object is slowing down.
An object moving in the negative direction and speeding up will be represented on a v-t graph by a sloped line located in the - velocity region. Since such an object has a - acceleration, the line will slope downwards (- acceleration).
Explain. If acceleration points in the same direction as the velocity, the object will be speeding up. And if the acceleration points in the opposite direction of the velocity, the object will be slowing down.
Deceleration has actually referred to the acceleration in a reverse way. We know that acceleration means the rate at which an object speeds up, that means deceleration means the rate at which an object slows down.