# Relative Motion

• May 11th 2008, 12:05 PM
bobak
Relative Motion
Quote:

Car A in traveling on a straight road on a bearing of 300 degrees, at a constant speed on 80 km/h. at 10 am it is 15 Km due east of a second car B which is also traveling at a constant speed. assuming that car B can travel in any direction

a) find the minimum speed of B if it is to intercept A.

Geomerty doesn't quiet cut this for me (Crying)

Thanks Bobak
• May 11th 2008, 12:32 PM
CaptainBlack
Quote:

Originally Posted by bobak
Geomerty doesn't quiet cut this for me (Crying)

Thanks Bobak

Draw a diagram.

The minimum speed to allow an intercept will involve B moving along a line that meets the course of A at right angles. Then you will have a 30, 60, 90 degree triangle, with one side 15km the one that B moves along 7.5 km and the third along which A mobves of length 7.5*sqrt(3).

RonL
• May 11th 2008, 12:47 PM
bobak
Quote:

Originally Posted by CaptainBlack
Draw a diagram.

The minimum speed to allow an intercept will involve B moving along a line that meets the course of A at right angles. Then you will have a 30, 60, 90 degree triangle, with one side 15km the one that B moves along 7.5 km and the third along which A mobves of length 7.5*sqrt(3).

RonL

I got $\displaystyle V_{min} = \frac{80}{\sqrt{3}} km/h$ but the books says the velocity is 40 km/h. Who's right me or them ???

Thanks Bobak
• May 11th 2008, 02:11 PM
CaptainBlack
Quote:

Originally Posted by bobak
I got $\displaystyle V_{min} = \frac{80}{\sqrt{3}} km/h$ but the books says the velocity is 40 km/h. Who's right me or them ???

Thanks Bobak

Opps, I think what I wrote earlier must be nonsense.

I'll have to think about the problem some more.

RonL
• May 11th 2008, 02:31 PM
CaptainBlack
Quote:

Originally Posted by bobak
I got $\displaystyle V_{min} = \frac{80}{\sqrt{3}} km/h$ but the books says the velocity is 40 km/h. Who's right me or them ???

Thanks Bobak

I am reasonably sure 40 km/hr can't be right.

RonL
• May 11th 2008, 02:42 PM
CaptainBlack
OK sorted out the diagram, see attachment, we use the sin rule and conclude the miminum speed for the pursuer occurs when sin(u)=1, and then v=sin(60)*80 km/hr.

RonL