How to Tackle Difficult Online Homework Assignments

[mkwok]

Homework Statement

A force F = (5 i - 1 j) N acts on a particle that undergoes a displacement r = (2 i + j) m.
(a) Find the work done by the force on the particle. (Calculate all numerical answers to three significant figures.)

(b) What is the angle between F and r?

* the problem required us to put our answer in 4 sig figs

Homework Equations

W=F*R
cross product |F||R|cos\theta

The Attempt at a Solution

part A,
since W=F*R, so I did: 5*2 + 1(-1) = 9.000 (4 sig figs)

Part B,
cross product |F||R|cos\theta
|F|=\sqrt{26}
|R| = \sqrt{3}
9=\sqrt{26}\sqrt{3}cos\theta
\theta=cos-1(9/(\sqrt{26}\sqrt{3})) = 37.87 degreesMy homework website tells me its wrong, what did I do wrong then?

 

[robphy]

check your definitions again...

define [and distinguish]: cross product, dot product
define: work

 

[Dick]

That's what I get. But i) it's the dot product, not the cross product and ii) the length of the second vector is sqrt(5) not sqrt(3). My question is how did you get the correct angle?

 

[Bill Foster]

http://en.wikipedia.org/wiki/Cross_product

 

[mkwok]

oops. I am so sorry
I really meant the dot product...
the angle is still incorrect though

 

[Dick]

oops. I am so sorry
I really meant the dot product...
the angle is still incorrect though

No, it's not. It's correct. Maybe you are supposed to give it in radians? Maybe sig figs? These automated homework checkers should be destroyed.

 

[Bill Foster]

No, it's not. It's correct. Maybe you are supposed to give it in radians? Maybe sig figs? These automated homework checkers should be destroyed.

Automatic homework checker? Don't they have TAs anymore?

What the hell are they teaching in colleges these days?

 

[robphy]

Automatic homework checker? Don't they have TAs anymore?

What the hell are they teaching in colleges these days?

Most undergraduate colleges don't have TAs.
Online homework assignments [however imperfect] can encourage students to work outside of class meeting times.
In addition, online homework assignments can be customized and randomized for each student.

 

[Bill Foster]

Most undergraduate colleges don't have TAs.
Online homework assignments [however imperfect] can encourage students to work outside of class meeting times.
In addition, online homework assignments can be customized and randomized for each student.

You know how they encouraged us to work outside class meeting times when I was in college? They made us turn the homework in, gave us pop quizzes, and/or frequent exams.

 

[robphy]

You know how they encouraged us to work outside class meeting times when I was in college? They made us turn the homework in, gave us pop quizzes, and/or frequent exams.

Yep... me, too.
But then again... I really really wanted to learn it... possibly unlike the generic student.

 

[Dick]

Most undergraduate colleges don't have TAs.
Online homework assignments [however imperfect] can encourage students to work outside of class meeting times.
In addition, online homework assignments can be customized and randomized for each student.

True. But they can be pretty difficult to reason with.