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M1 Vectors Question

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.

I would usually go about this using the formula
r = ro + vt however I'm a bit confused how to work out the velocity knowing just the speed.

I'd appreciate any help

Reply 1

sarah.102

Original post by nm786

change in speed/time will give you acceleration. Then use v=u+at to work out the velocity :smile:

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 :smile:

Reply 2

.raiden.

Original post by sarah.102

You need to find the unit vector first and then use the speed to find velocity

Do you know how to do this?

(edited 11 years ago)

Reply 3

Bric

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

Reply 4

sarah.102

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. :smile:

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

Reply 5

sarah.102

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?

Reply 6

.raiden.

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. :smile:

Where did you get s/t = a from :confused:

The speed doesnt change anyway so its not even accelerating...

Reply 7

.raiden.

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.

Reply 8

nm786

Original post by .raiden.

Where did you get s/t = a from :confused:

The speed doesnt change anyway so its not even accelerating...

here: http://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/forces/accelerationrev1.shtml is it not right? :colondollar:

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.

Reply 9

.raiden.

Original post by nm786

here: http://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/forces/accelerationrev1.shtml is it not right? :colondollar:

Here, watch this video on vectors - skip to 06:12 for unit vectors.

GCSE was a lie. Change in velocity over time is acceleration.

Reply 10

nm786

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. :biggrin:

Reply 11

TenOfThem

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