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# Mechanics watch

1. A particle is moving with speed 10ms-1 in the direction 3i - 4j, find its velocity vector

I don't know how to link each variable?

The magnitude will be 5

s = distance/t
v = displacement/t

What do I do???

t = distance/speed
t = 5/10

????
2. (Original post by ps1265A)
A particle is moving with speed 10ms-1 in the direction 3i - 4j, find its velocity vector

I don't know how to link each variable?

The magnitude will be 5

s = distance/t
v = displacement/t

What do I do???

t = distance/speed
t = 5/10

????
Okay can you please post the original question, as your explanation is a bit unclear. The thing is the person who wants to help won't be able to, and might not even bother and then you won't receive any help.
3. (Original post by ps1265A)
A particle is moving with speed 10ms-1 in the direction 3i - 4j, find its velocity vector
Since the magnitude of the vector is 5, it represents a speed of 5ms-1, in the given direction.

Divide the vector by 5, and it will represent a speed of 1ms-1.

Then multiply it by 10, and it will represent a speed of 10ms-1. Done!
4. (Original post by gagafacea1)
Okay can you please post the original question, as your explanation is a bit unclear. The thing is the person who wants to help won't be able to, and might not even bother and then you won't receive any help.
A particle is moving with speed 10ms-1 in the direction 3i - 4j, find its velocity vector

5. (Original post by ghostwalker)
Since the magnitude of the vector is 5, it represents a speed of 5ms-1, in the given direction.

Divide the vector by 5, and it will represent a speed of 1ms-1.

Then multiply it by 10, and it will represent a speed of 10ms-1. Done!
Why does a magnitude of 5 represent a speed of 5?
6. (Original post by ps1265A)
A particle is moving with speed 10ms-1 in the direction 3i - 4j, find its velocity vector

No it was the part after that that confused me, because you said the speed is 10, which I assumed is the magnitude, but then you said the magnitude is 5, and I didn't know what you were talking about. It's probably my fault. Anyways, ghostwalker provided you with the answer.
7. (Original post by ps1265A)
Why does a magnitude of 5 represent a speed of 5?
Because what you want to do is to make the vector have a magnitude of 1, so that when you multiply by 10 it doesn't change the magnitude of the vector. Also you can ignore that part, it doesn't really matter what it represents; all you want to do is to normalize (like making a unit vector in the direction of) 3i-4j.
8. (Original post by gagafacea1)
Because what you want to do is to make the vector have a magnitude of 1, so that when you multiply by 10 it doesn't change the magnitude of the vector. Also you can ignore that part, it doesn't really matter what it represents; all you want to do is to normalize (like making a unit vector in the direction of) 3i-4j.
So speed changes depending on magnitude? I thought it stays the same... I still don't understand why it's 5ms-1
9. Why does the magnitude link with the speed??????/
10. And why are we considering VELOCITY when the SPEED is 10???
11. Haha, I just don't understand why we're doing what we're doing!
12. Bump!
13. (Original post by ps1265A)
Why does the magnitude link with the speed??????/
We consider the magnitude to be the distance moved in one time unit.

Note that the velocity vector will have units. It's not just ai + bj, but rather ai + bj, ms-1.
14. (Original post by ghostwalker)
We consider the magnitude to be the distance moved in one time unit.

Note that the velocity vector will have units. It's not just ai + bj, but rather ai + bj, ms-1.
Why can't we consider -3i + 4j as the displacement?
15. (Original post by ps1265A)
Why can't we consider -3i + 4j as the displacement?
The question doesn't tell you anything about displacement. It doesn't even mention it.
16. (Original post by ghostwalker)
The question doesn't tell you anything about displacement.
Hmmm okay!

So magnitude in general is distance in one unit time... so it's speed in one unit time?
17. (Original post by ps1265A)
Hmmm okay!

So magnitude in general is distance in one unit time... so it's speed in one unit time?
no no, the magnitude could have any unit you want. In this question, it's m/s, but that's TOTALLY irrelevant. What they want is a vector that has a magnitude of 10 and a direction the SAME as 3i-4j , that's it! No velocity, displacement and time equations or anything.
18. (Original post by gagafacea1)
no no, the magnitude could have any unit you want. In this question, it's m/s, but that's TOTALLY irrelevant. What they want is a vector that has a magnitude of 10 and a direction the SAME as 3i-4j , that's it! No velocity, displacement and time equations or anything.
You're saying magnitude of 10 because speed was 10... I'm just asking, how and why are these 2 components interlinked... Does speed = magnitude because the magnitude of a vector gives speed?
19. (Original post by ps1265A)
Hmmm okay!

So magnitude in general is distance in one unit time... so it's speed in one unit time?
Speed is rate of change of displacment, so it incorporates the "in one unit of time" already.

The magnitude of a vector represents the speed, in the appropriate units.

Consider a direction vector or 3i + 4j.

It has a magnitude of 5.

If I want it to represent 10000 ms^-1 in that direction, and I choose a magnitude of 1 to represent 1 ms^-1, then my vector needs to be multiplied by 10000/5, i.e. by 2000, to get 6000i+8000j ms-1.

We almost invariably choose a magnitude of 1 to represent a speed of 1 in whatever units. That's the link between the length of the vector and the speed.
20. (Original post by ghostwalker)
Speed is rate of change of displacment, so it incorporates the "in one unit of time" already.

The magnitude of a vector represents the speed, in the appropriate units.

Consider a direction vector or 3i + 4j.

It has a magnitude of 5.

If I want it to represent 10000 ms^-1 in that direction, and I choose a magnitude of 1 to represent 1 ms^-1, then my vector needs to be multiplied by 10000/5, i.e. by 2000, to get 6000i+8000j ms-1.

We almost invariably choose a magnitude of 1 to represent a speed of 1 in whatever units. That's the link between the length of the vector and the speed.
"I choose a magnitude of 1 to represent 1ms^-1" - Isn't it always in this ratio?

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