Deceleration in velocity vs time graph

[lazybutt26]

I know that in velocity vs time, slope is acceleration. But imagine that the slope is negative indicating deceleration. Deceleration can be either change in direction or reduction in velocity right? So how can you tell if you were given a velocity vs time graph with negative slope,

if the car is reversing(changing the direction) or reducing its speed?

Thanks for your help!

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[FaustK]

Easy question. It depends on the sign of the velocity.

If the sign of velocity is positive, the subject is going forward regardless of states of acceleration. If the sign of velocity is negative, it's going backward.
Now, if the sign goes from positive to negative, such as from "+3 m/s" to "-5 m/s", you can realize the subject changes the direction of movement.
On the other side, if the sign goes from "+10 m/s" to "+3 m/s", this just means the subject slows down. It's simply deceleration, but the subject doesn't change its direction.

If there's no sign indicated by the graph or so, it's a bad graph and you can not determine which kind of motion it is.

 

[aldol16]

I know that in velocity vs time, slope is acceleration. But imagine that the slope is negative indicating deceleration. Deceleration can be either change in direction or reduction in velocity right? So how can you tell if you were given a velocity vs time graph with negative slope,

At the moment it crosses the x-axis - that is, when the y-value changes from positive to negative or vice versa - velocity changes direction. So either an upward sloping graph or downward sloping graph could change direction - the x-intercept is the time at which the velocity is zero, or the instantaneous moment in time when direction changes.

 

[lazybutt26]

Cool! Thank you both of you for explaining this in great detail !