# Work from a constant force

1. Feb 16, 2008

[SOLVED] work from a constant force

1. The problem statement, all variables and given/known data

i cant figure out the answer to hint 2

2. Relevant equations

w = f deltar

w = F cos theta (deltar)

is this img right?

Last edited: Feb 16, 2008
2. Feb 16, 2008

### Staff: Mentor

Your diagram looks good to me. Now how will you make use of it to find the dot product $\vec{F}\cdot\vec{L}$?

3. Feb 16, 2008

thats what i have been trying to figure out

i have tried to relate angles and got

sin (theta*PI/180)* tan (theta*PI/180)

4. Feb 16, 2008

### Staff: Mentor

5. Feb 16, 2008

well the only reason i didnt say it was

cos ( theta * PI /180)

nor it is like cos ( (theta - 180) * PI /180)

because that is the wrong answer so i dont know wth they want..

and yes thats in my text i have been looking through it the whole time, not examples seem to be like this nor does my book explain this well at all

6. Feb 16, 2008

### Staff: Mentor

$$W = F L \cos \phi$$

Express this in terms of $\theta$.

7. Feb 16, 2008

I know how to set it up thats is not what i am trying to figure out,

im trying to express PHI in terms of THETA, u see my IMG where it says FIND THIS i cant find that angle, i know how my FINAL answer is gonna be expressed.

i cant figure out the how to answer the HINT 2: question

it will be cos ( of some stuff in here * pi /180)

8. Feb 16, 2008

### Staff: Mentor

Hint: $\phi + \theta =$ ?

(Note: The angles are already in radians.)

9. Feb 16, 2008

180..

i have tried

cos ( (180 - theta) * (PI /180))

still wrong

10. Feb 16, 2008

### Staff: Mentor

The angles are in radians, so no need to convert to radians:

$$\phi + \theta = \pi$$

Now find $\cos\phi = \cos(\pi - \theta)$

(Review your trig identities if you need to.)

11. Feb 16, 2008

in rads it would = PI..

12. Feb 16, 2008

phi = pi - theta

i have never seen that before in my life..

13. Feb 16, 2008

thanks for your help

14. Feb 16, 2008

### Staff: Mentor

15. Feb 16, 2008

L*F*cos(PI-THETA)

what threw me off is how they switch between rads and degrees and they are liek dont forget to convert even whe nyou dont need to convert... and whe nyou haev to convert they dotn tell you to convert and when u submit the wrong answer they say, oh ya dont forget to convert.. so i assumed it was in degrees and well it was in rads go figure, threw off my answer big time, and when i went to submit my answer in the hint they told me to "not forget to switch to radians" screwey system

Last edited: Feb 16, 2008
16. Feb 16, 2008

### Staff: Mentor

OK, but you can simplify it a bit further using a trig identity. (And get rid of that $\pi$.)

17. Feb 16, 2008

-cos(theta)

18. Feb 16, 2008

Exactly!