If acceleration of something $= – 10 \text{ m s}^{-2}$
And forwards is define as north.
Does that mean the object is getting slower (decelerating) or accelerating in the reverse direction (south)
How can you tell the difference?
[Physics] Acceleration vector – deceleration vs direction
vectors
Related Solutions
That quote is abit misleading, momentum is a vector, however a vector is neither negative nor positive, only its components can have this characteristic. The two objects you are describing does not have the same momentum, but they have the same magnitdue of momentum (length of vector).
This might be more of a math question. This is a peculiar thing about three-dimensional space. Note that in three dimensions, an area such as a plane is a two dimensional subspace. On a sheet of paper you only need two numbers to unambiguously denote a point.
Now imagine standing on the sheet of paper, the direction your head points to will always be a way to know how this plane is oriented in space. This is called the "normal" vector to this plane, it is at a right angle to the plane.
If you now choose the convention to have the length of this normal vector equal to the area of this surface, you get a complete description of the two dimensional plane, its orientation in three dimensional space (the vector part) and how big this plane is (the length of this vector).
Mathematically, you can express this by the "cross product" $$\vec c=\vec a\times\vec b$$ whose magnitude is defined as $|c| = |a||b|sin\theta$ which is equal to the area of the parallelogram that those two vectors (which really define a plane) span. To steal this picture from wikipedia's article on the cross product:
As I said in the beginning this is a very special thing for three dimensions, in higher dimensions, it doesn't work as neatly for various reasons. If you want to learn more about this topic a keyword would be "exterior algebra"
Update:
As for the physical significance of this concept, prominent examples are vector fields flowing through surfaces. Take a circular wire. This circle can be oriented in various ways in 3D. If you have an external magnetic field, you might know that this can induce an electric current, proportional to the rate of change of the amount flowing through the circle (think of this as how much the arrows perforate the area). If the magnetic field vectors are parallel to the circle (and thus orthogonal to its normal vector) they do not "perforate" the area at all, so the flow through this area is zero. On the other hand, if the field vectors are orthogonal to the plane (i.e. parallel to the normal), the maximally "perforate" this area and the flow is maximal.
if you change the orientation of between those two states you can get electrical current.
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Best Answer
It really doesn't matter. Basic kinematic formulas are designed to work just as well in either case, which is why physicists don't generally use the word "decelerating." It's just another kind of acceleration.
That being said, if you want to determine whether the object's speed is increasing or decreasing (which correspond to the popular meanings of "accelerating" and "decelerating" respectively), you can just look at the orientation of the acceleration with respect to the velocity. If the acceleration is parallel to the velocity, the object will be speeding up. If it's antiparallel, the object will be slowing down. You can see this mathematically by taking the derivative of the kinetic energy:
$$\frac{\mathrm{d}}{\mathrm{d}t}\biggl[\frac{1}{2}mv^2\biggr] = m\vec{v}\cdot\frac{\mathrm{d}\vec{v}}{\mathrm{d}t} = m\vec{v}\cdot\vec{a}$$
So the sign of the dot product $\vec{v}\cdot\vec{a}$ tells you whether the speed is increasing or decreasing.
Do note that velocity is reference frame-dependent. So two different inertial observers looking at the same object at the same time could have differing conclusions as to whether it is speeding up or slowing down. That's one big reason why the distinction is not important in physics.