What is the final speed of the boat?

  • Thread starter Westin
  • Start date
  • #1
87
0

Homework Statement



Jack and Jill are maneuvering a 3570kg boat near a dock. Initially the boat's position is [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png2,0,9[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngm [Broken] and its speed is 4.6m[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmmi10/alpha/144/char3D.pngs. [Broken] As the boat moves to position [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png12,0,0[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngm, [Broken] Jack exerts a force of [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png-80,0,290[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngN, [Broken] and Jill exerts a force [PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char68.png0,240,0[PLAIN]https://s3.lite.msu.edu/adm/jsMath/fonts/cmsy10/alpha/144/char69.pngN. [Broken]

What is the final speed of the boat?[/B]


Homework Equations



W=Fd
Kfinal = Kinitial +W
KE=(1/2)mv^2[/B]


The Attempt at a Solution



1) Final total work

Jack Work = (-80*10)+(0*0)+(290*-9) = -3410
Jill Work = 0

(It says my Work is correct)

2) Kfinal = Kinitial + W
Kinitial = (1/2)(3570)(4.6)^2 = 37770.6

Kfinal = 37770.6+(-3410)
Kfinal = 34360.6

3) Final Velocity = KE/m
34360.6/3570 = 9.6248m/s

Not sure where my error is... any help would be appreciated!



[/B]
 
Last edited by a moderator:

Answers and Replies

  • #2
TSny
Homework Helper
Gold Member
12,792
3,150
3) Final Velocity = KE/m
This is not the correct relation between velocity and KE.
 
  • #3
87
0
I was able to get the worst answer, I just plugged final KE back into K= 1/2mv^2 and solved for v.

The second part of the question asks... Which person exerted a force perpendicular to the displacement of the boat?
a) Neither Jack nor Jill.
b) Both Jack and Jill.
c) Jill
d) Jack.

I thought it was Jack since Jack is the only person exerting Work. But then I thought Jill because she's exerting force, but just in the wrong direction (perpendicular?) so her work is 0. I only have one shot at this, is my reasoning ok for saying its Jill?
 
  • #4
TSny
Homework Helper
Gold Member
12,792
3,150
Have you covered the "dot product" or "scalar product" of two vectors? If so, you probably covered how to use the dot product to test if two vectors are perpendicular.

If you are not familiar with the dot product, then can you sketch the two force vectors and the displacement vector on a x,y, z coordinate system?
 

Related Threads on What is the final speed of the boat?

  • Last Post
Replies
2
Views
7K
Replies
6
Views
1K
  • Last Post
Replies
1
Views
6K
Replies
6
Views
2K
  • Last Post
Replies
6
Views
1K
Replies
12
Views
4K
Replies
2
Views
7K
Replies
4
Views
1K
Replies
3
Views
5K
Top