Vector addition and Relative motion

[nadong2dong]

Homework Statement

1) A boy is floating on a raft in a river. THe water flows at a speed of 12m/s east. The boy's feet kick the water,causing a velocity of 2m/s at an angle of 43 degrees north of east. Using Vector components, calculate the magnitude of the raft's velocity.

2)One vector has a magnitude of 20m/s and acts an angle of 25 degrees. The next vector has a magnitude of 12m/s and acts at an angle of 120 degrees. The final vector acts at an angle of 70 degrees and has a magnitude of 14m/s. Calculate the magnitude and the angle of the resultant vector.

Homework Equations

1) 12xtan(43)= 11.19m/s


2)20m/s + 12m/s+ 14m/s = 46m/s?
25degree+ 120+ 70= 215degree...

The Attempt at a Solution

1) 11.19m/s
2)46m/s and 215 degree?

 

[lewando]

Hello nadong2dong, welcome to PF! For part 1 (assuming east is +x and north is +y), the x-component of the river is clearly 12 m/s, and the y-component of the river is clearly 0 m/s. What x and y components can you come up with for the vector caused by the boy's kicking?

 

[nadong2dong]

uhhh i don't quite understand what you are talking about

 

[nadong2dong]

I get that x is 12 and y is 0, but what should i do to foind components caused by the boy's kicking
?

 

[nadong2dong]

2sin(43)
2cos(43)?

 

[lewando]

When the question asks:

Using Vector components, calculate the magnitude of the raft's velocity.

Do you get what the term "vector components" means?

 

[lewando]

2sin(43)
2cos(43)?

Sure, but which is the x component? Which is the y component??

 

[nadong2dong]

cos is the x component
and sin is the y?

 

[nadong2dong]

It is very hard for me to picture the question in my mind.

 

[lewando]

It is very hard for me to picture the question in my mind.

Why don't you draw a vector diagram of what is going on?

 

[nadong2dong]

that is the problem, i can't really draw a vector diagram for this question

 

[lewando]

cos is the x component
and sin is the y?

Are you sure? Why the question mark? You happen to be right. Make sure you understand why you are right.

 

[nadong2dong]

ok i got 13.53 m/s

Is that right?

 

[lewando]

ok i got 13.53 m/s

Is that right?

That looks right, how did you get there?

 

[nadong2dong]

square root of (2sin43)^2 + (2cos43 +12)^2

 

[lewando]

Alright, good work by you!

that is the problem, i can't really draw a vector diagram for this question

Well, [drawing vector diagrams] are fundamental to the solution of part 2. You should understand this this as soon as possible.

If your textbook does not cover this, get another textbook or search for online resources such as:


I'm done for the night. If someone else does not jump in, let's continue this later.

 

[nadong2dong]

Thank You