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# M1 Vectors Question watch

1. The question is:
At time t=0 a particle has position vector 4i + 7j and is moving with speed 15 ms^-1 in the direction 3i - 4j. Find its position vector after 2 seconds.

r = ro + vt however I'm a bit confused how to work out the velocity knowing just the speed.

I'd appreciate any help
2. (Original post by nm786)
change in speed/time will give you acceleration. Then use v=u+at to work out the velocity
Are you sure acceleration doesn't have to be change in velocity and can be just change in speed? I just wanted to clarify
Thanks for responding
3. (Original post by sarah.102)
The question is:
At time t=0 a particle has position vector 4i + 7j and is moving with speed 15 ms^-1 in the direction 3i - 4j. Find its position vector after 2 seconds.

r = ro + vt however I'm a bit confused how to work out the velocity knowing just the speed.

I'd appreciate any help
You need to find the unit vector first and then use the speed to find velocity

Do you know how to do this?
4. Travelling in the direction 3i - 4j, does a 3, 4, 5 triangle help ? Ooooh, 15 is divisible by 5....from which you can find the velocity......
Bric
5. (Original post by nm786)
no acceleration is change in speed/time.
Wait, this is example 14 from the book right? Yeah, that is the best way of doing it.
Yeah it is example 14 - good memory! I didn't actually understand the example...
I didn't understand where 1/5 came from if you have the book on you you'd know what I'm talking about haha
6. (Original post by .raiden.)
You need to find the unit vector first and then use the speed to find velocity

Do you know how to do this?
I'm not quite sure, how do you find the unit vector, could you please explain?
7. (Original post by nm786)
no acceleration is change in speed/time.
Wait, this is example 14 from the book right? Yeah, that is the best way of doing it.
Where did you get s/t = a from

The speed doesnt change anyway so its not even accelerating...
8. (Original post by sarah.102)
I'm not quite sure, how do you find the unit vector, could you please explain?
The unit vector is a unit in a given direction. You know that speed has no direction, unit vector is one unit in the direction so multiplying speed by the unit vector gives the velocity vector. The unit vector is is (1 divided by resultant of direction vector) multiplied by the direction vector.

Hope this is a good explanation, I find it quite hard to explain.
9. (Original post by .raiden.)
Where did you get s/t = a from

The speed doesnt change anyway so its not even accelerating...
here: http://www.bbc.co.uk/schools/gcsebit...tionrev1.shtml is it not right?
(Original post by sarah.102)
Yeah it is example 14 - good memory! I didn't actually understand the example...
I didn't understand where 1/5 came from if you have the book on you you'd know what I'm talking about haha
Here, watch this video on vectors - skip to 06:12 for unit vectors.
10. (Original post by nm786)
here: http://www.bbc.co.uk/schools/gcsebit...tionrev1.shtml is it not right?

Here, watch this video on vectors - skip to 06:12 for unit vectors.
GCSE was a lie. Change in velocity over time is acceleration.
11. (Original post by .raiden.)
GCSE was a lie. Change in velocity over time is acceleration.
Thanks for telling me, I'll remember that from now on.
12. (Original post by sarah.102)
Yeah it is example 14 - good memory! I didn't actually understand the example...
I didn't understand where 1/5 came from if you have the book on you you'd know what I'm talking about haha
Are you still stuck?

v = k(3i - 4j)

The magnitude of 3i-4j is 5 [Pythagoras]

So v = 15 gives v = 3(3i-4j) = 9i -12j

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