Why Is My Calculation of Resultant Force Incorrect?

[myelevatorbeat]

Two forces are applied to a car in an effort to move it, as shown in Figure P4.12.
Here's Figure P4.12: <img src="http://www.webassign.net/sercp/p4-12.gif"/>

(a) What is the resultant of these two forces?

Magnitude: N
Direction: Degrees (measured to the right from the forward direction)

(b) If the car has a mass of 3000 kg, what magnitude acceleration does it have? Ignore friction.


For part A, I found F1 = (400cos30)i + (400sin30)j
F2 = (450cos10)i + (450sin10)j

___+______________________________
Fnet=[346i + 443i] + [200j + 78.1j]
Fnet= 789i + 278.1j

I then used the Pythagoreon Thereom to figure out the resultant force. I got 837 N. However, webassign.net says this is wrong and I've been re-doing the problem over and over and get the same answer.

I used tan-1 [278/789] for the angle and got 19.4 degrees but webassign.net also tells me this is the wrong answer.

Finally, for part B, I used F=ma and used 837=3000a and solved for a. I got a=.279 m/s^2 but again webassign.net tells me this wrong.

So, basically, I was hoping someone could look through my work and tell me if you can figure out what I'm doing wrong cause I could swear I did this correctly.

 

[learningphysics]

Your force equations aren't right... take i in the positive x direction, and j in the positive y direction...

write out the equations again... note that one force has a positive x component, the other force has a negative x component...

 

[Doc Al]

For part A, I found F1 = (400cos30)i + (400sin30)j
F2 = (450cos10)i + (450sin10)j

Define the i & j directions. Are they horizontal and vertical? Or the reverse?

Be careful with signs.

 

[myelevatorbeat]

So, instead of using 450 I should be using -450?

 

[learningphysics]

So, instead of using 450 I should be using -450?

sketch the forces and their components... what is the direction of the x-component of the 450N force... what is direction of the y-component of the 450N force

 

[myelevatorbeat]

Ok, so now I've changed my Equations so they are:

Fnet=[346i+ -443i] + [200j + 78.1j]

For a total of -97.0i + 278j

Am I doing any better at this or I'm still doing it wrong??

 

[learningphysics]

Ok, so now I've changed my Equations so they are:

Fnet=[346i+ -443i] + [200j + 78.1j]

For a total of -97.0i + 278j

Am I doing any better at this or I'm still doing it wrong??

No, it's still wrong... can you define your directions? I'm confused by what directions i and j are...

 

[myelevatorbeat]

i is in the horizontal (x) direction and j is in the vertical (y) direction.

At this point I'm about to just scrap all my work and start over. I'm thinking that might be easier than going back and making corrections.

 

[learningphysics]

i is in the horizontal (x) direction and j is in the vertical (y) direction.

At this point I'm about to just scrap all my work and start over. I'm thinking that might be easier than going back and making corrections.

I agree, start fresh... and be careful with directions...