- #1
junior_J
- 46
- 0
Question :
Two ships A and B leave port P at the same time . Ship A travels due north at a steady speed of 15km/h and ship B travels N 60 degree E at a steady speed of 10km/h. what is the distance and direction from A to B after 1 hour ? what is the velocity of B relative to A ?
Solution :
I couldn't get the velocity part right ...
I resolved the two velocities along North and East , since 15 km/hr is due north it doesn't have a component along the East ...the second velocity has components along both the directions so ...
v = [ (-15 + 10 cos60)^2 + (10 sin60)^2 ]^0.5 = 13.22 km/hr
1).what i can't get is why don't i get this answer when i use the cosine rule ... is it because I am using the relative velocity of A which BTW seems to be negative ?
2). I get the rigt answer if i changed the '-' part in the cosine rule to a '+'
?
Two ships A and B leave port P at the same time . Ship A travels due north at a steady speed of 15km/h and ship B travels N 60 degree E at a steady speed of 10km/h. what is the distance and direction from A to B after 1 hour ? what is the velocity of B relative to A ?
Solution :
I couldn't get the velocity part right ...
I resolved the two velocities along North and East , since 15 km/hr is due north it doesn't have a component along the East ...the second velocity has components along both the directions so ...
v = [ (-15 + 10 cos60)^2 + (10 sin60)^2 ]^0.5 = 13.22 km/hr
1).what i can't get is why don't i get this answer when i use the cosine rule ... is it because I am using the relative velocity of A which BTW seems to be negative ?
2). I get the rigt answer if i changed the '-' part in the cosine rule to a '+'
?