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# Physics/vectors

Question
Hi Sir,

When do we add vectors and when to subtract vectors...and what is the difference/significance of the result.

Thanks!!"

Hello yeeakc,

The vector algebra procedure for subtracting a vector is simply to make a 180 degree change in the direction of the vector to be subtracted and then find the sum. What operations require that? There may be others, but I can think of only one task where you would "subtract" vectors. To find a change in a vector that took place during some event.

Consider a collision -- you may need to find the change in velocity to study the collision. (The impulse the object received is equal to the change in its momentum. To determine the change in momentum, you need the change in velocity.) You have an initial velocity, Vi, and a final velocity, Vf. (Forgive me for not being able to produce the proper symbol to show that Vi and Vf are vectors.) Assume you know those velocities - their magnitudes and directions.

The change in velocity would be
deltaV = Vf - Vi

When you add vectors, you get the vector sum which is generally called the resultant or resultant vector. Assume you had 7 forces acting on an object and calculated the resultant. You could replace those 7 forces with one force that has the magnitude and direction of the resultant. The object would behave identically to when it had the 7 individual forces acting on it.

I hope this helps,
Steve
Questioner's Rating
 Rating(1-10) Knowledgeability = 10 Clarity of Response = 10 Politeness = 10 Comment Thanks Steve :)

Physics

Volunteer

#### Steve Johnson

##### Expertise

I would be delighted to help with questions up through the first year of college Physics. Particularly Electricity, Electronics and Newtonian Mechanics (motion, acceleration etc.). I decline questions on relativity and Atomic Physics. I also could discuss the Space Shuttle and space flight in general.

##### Experience

I have a BS in Physics and an MS in Electrical Engineering. I am retired now. My professional career was in Electrical Engineering with considerable time spent working with accelerometers, gyroscopes and flight dynamics (Physics related topics) while working on the Space Shuttle. I gave formal classroom lessons to technical co-workers periodically over a several year period.

Education/Credentials
BS Physics, North Dakota State University
MS Electrical Engineering, North Dakota State University