Acceleration from 2 Force Vectors

[zcabral]

Homework Statement

Two forces, F1 and F2, act on a 2.00 kg object where F1 = 30.0 N (on the x-axis) and F2 = 10.0 N. A 60 degree angle is between them. Find the magnitude and direction of the acceleration.

http://www.webassign.net/pse/p5-15.gif (ignore figure a)

Homework Equations

F=ma
a=sqrt. (ax^2 + ay^2)

The Attempt at a Solution

I figured out that the magnitude is a=sqrt. (ax^2 + ay^2)
which is equal to 17.67

however i can't get the correct direction! i thought it would be theta = arctan (2.5 /17.5)= 8.13 but it keeps saying its not the right answer! help

 

[Doc Al]

The angle should be arctan (a_y/a_x).

 

[TVP45]

General rules for these problems:

Use units throughout.
Combine the two forces into a single one.
Show all work.

 

[zcabral]

i did that and it didnt come out right

 

[Doc Al]

i thought it would be theta = arctan (2.5 /17.5)= 8.13 but it keeps saying its not the right answer! help

Where did you get the numbers 2.5 and 17.5? What are they?

 

[zcabral]

2.5= ay
17.5= ax

together they are the acceleration = 17.67 (im 100% sure)

i got them by using this technique
Fx= 30cos(0)+10cos(60)= 35
A=F/m A= 35/2 A= 17.5

Fy= 30sin(0)+10cos(60)= 5
A=F/m A= 5/2 A= 2.5

 

[Doc Al]

2.5= ay
17.5= ax

together they are the acceleration = 17.67 (im 100% sure)

i got them by using this technique
Fx= 30cos(0)+10cos(60)= 35
A=F/m A= 35/2 A= 17.5

Fy= 30sin(0)+10cos(60)= 5
A=F/m A= 5/2 A= 2.5

Redo your calculation of Fy.

 

[zcabral]

oops no i used sine just didnt type it out right here

 

[Doc Al]

oops no i used sine just didnt type it out right here

But 10 \sin(60) \ne 5

 

[portillj]

why is it the cos(0) and sin(0)

 

[Doc Al]

why is it the cos(0) and sin(0)

Because the angle that the 30 N force makes with the x-axis is 0. (That force is along the x-axis.)

 

[portillj]

how do i find out the angle is i have the magnitudes??