Why is force a vector quantity and not scalar?

It's defined as a dot-product (or scalar product) of force and displacement, both of which are vectors.

A scalar product of two vectors gives a scalar result (aptly named!).

$$dW = \vec{F}\cdot\vec{S} = {\|F\|}{\|S\|}\cos\theta$$ ($\theta$ being the angle between the vectors).

No direction, only magnitude.

Thinking logically, what would be the direction of work, anyway? You may say, "In the direction of displacement!", but then why not in the direction of force? And if you say the direction of both, well then, it isn't always the same! A force can do work on a body even displacing at an angle to the direction of force ($\theta$!).

=>Note that when $\theta$ is $90^\circ$, the result will be zero ($\cos 90^\circ = 0$). When force and displacement are perpendicular, the force does no work on the body!

Edit: As said by @anna: Please also note that work is part of the energy in a system (work and energy) and energy is a scalar. If it were not so we would not be talking of "conservation of energy" as an experimental observation. Energy is a scalar.

No, force is a vector quantity, it is defined by its magnitude and direction.

Common vector quantities are displacement, velocity, acceleration and force.

Common scalar quantities are distance, speed, work and energy.

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