Math Is Fun Forum

paulb203

Member

Registered: 2023-02-24

Posts: 356

Vector quantities; direction

If the velocity of an object is -2m/s does the negative sign stand for direction? If so does that mean -2m/s is the complete answer, we don't need anything else such as North, South, Rightwards, Leftwards?

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Bob

Administrator

Registered: 2010-06-20

Posts: 10,654

Re: Vector quantities; direction

Yes it does.  A question ought to start by defining which direction is positive.  I think we had this in a previous post where an object was being thrown upwards.  You can take up or down as the positive direction but this needs to be consistent throughout the question.  eg. if 'up' is positive then gravity is -g.

Bob

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Children are not defined by school ...........The Fonz
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paulb203

Member

Registered: 2023-02-24

Posts: 356

Re: Vector quantities; direction

Ah, thanks, Bob, that does ring a bell. My memory is not what it... sorry, what were we talking about?

Also, if an object is shown to have moved in the negative direction could that mean either it turned around and headed back towards its origin; or, it could have went into reverse (if it was a vehicle; or, I suppose, a person, etc, like Spike Milligan Walking Backwards for Christmas, for example).

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KerimF

Member

From: Aleppo-Syria

Registered: 2018-08-10

Posts: 264

Re: Vector quantities; direction

An object is thrown upwards. Its initial velocity is +20 m/s.
Find its velocity at t=5 sec. (Assume the absolute value of g is 10 m/s2)

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paulb203

Member

Registered: 2023-02-24

Posts: 356

Re: Vector quantities; direction

An object is thrown upwards. Its initial velocity is +20 m/s.
Find its velocity at t=5 sec. (Assume the absolute value of g is 10 m/s2)

Thanks, KerimF

I had to use Google to get the formula 

v=u+at

v=+20m/s +(-10m/s^2)(5s)

v= +20m/s + -50m/s

v= +20m/s -50m/s

v= -30m/s

or;

v= 30m/s downwards

Is that correct?

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Bob

Administrator

Registered: 2010-06-20

Posts: 10,654

Re: Vector quantities; direction

Yes it is.

Wow! You've heard of Spike Milligan. My Dad had that record on  a bakelight 78 rpm disc.  Surely you're too young?

Bob

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Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei
Sometimes I deliberately make mistakes, just to test you!  …………….Bob

Phrzby Phil

Member

From: Richmond, VA

Registered: 2022-03-29

Posts: 59

Re: Vector quantities; direction

Be aware of an interesting result when calculating about projectiles.

The math will give you distance, velocity, etc., for any time, but since a projectile thrown up will eventually come back to, say, Earth, we know (ignoring bouncing) that it will then stop.  However, the calculation will give you other answers as if there were no Earth to come back to.

A good test for advanced math classes would expect the student to not blindly report the calculated answer, but to also determine when it does not match the physical situation.

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paulb203

Member

Registered: 2023-02-24

Posts: 356

Re: Vector quantities; direction

Thanks, Phil.

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paulb203

Member

Registered: 2023-02-24

Posts: 356

Re: Vector quantities; direction

Thanks, Bob.

I'm too young for bakelite records, and listening to the Goons on the radio, but we were taught On The Ning Nang Nong at school

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