# Finding the angle in which the resultant force points

• alexi_b
In summary, the magnitude of the acceleration of a 3.93kg mass under the forces of 11.8N north, 19.2N east, and 15.9N south on an air table is 5.0 m/s^2 and the direction of the acceleration is -12.0 degrees from the positive x-axis.

## Homework Statement

Forces of 11.8N north, 19.2N east, and 15.9N south are simultaneously applied to a 3.93kg mass as it rests on an air table. What is the magnitude of its acceleration?
What is the direction of the acceleration in degrees? (Take east to be 0 degrees and counterclockwise to be positive. Enter an angle between -180 degrees and +180 degrees.)

## The Attempt at a Solution

I was able to find out the answer to the first part of the equation which was 5.0m/s^2 but the angle i Have no idea to get. I just used the tan inverse of my components and got 14.3 degrees but it wasnt right. I also tried -14.3 but no luck there either.

Any help would be appreciated![/B]

alexi_b said:

## Homework Statement

Forces of 11.8N north, 19.2N east, and 15.9N south are simultaneously applied to a 3.93kg mass as it rests on an air table. What is the magnitude of its acceleration?
What is the direction of the acceleration in degrees? (Take east to be 0 degrees and counterclockwise to be positive. Enter an angle between -180 degrees and +180 degrees.)

## The Attempt at a Solution

I was able to find out the answer to the first part of the equation which was 5.0m/s^2 but the angle i Have no idea to get. I just used the tan inverse of my components and got 14.3 degrees but it wasnt right. I also tried -14.3 but no luck there either.

Any help would be appreciated![/B]
What are your components? If they are not in the first or the fourth quadrant, taking the inverse tan will not give the correct angle (assuming they want the angle relative to the positive x axis)

nrqed said:
What are your components? If they are not in the first or the fourth quadrant, taking the inverse tan will not give the correct angle (assuming they want the angle relative to the positive x axis)

Well i just redid it and I got 19.2 deg (east) and 4.1 deg (south) so it is in the fourth quadrant still. How do you propose I fix this?

alexi_b said:
Well i just redid it and I got 19.2 deg (east) and 4.1 deg (south) so it is in the fourth quadrant still. How do you propose I fix this?
I am not sure what you mean by "19.2 deg (east) and 4.1 deg (south)". You mean 19.2 degrees east of south?? And 4.1 degrees south of east?? But these two angles should add up to 90 degrees so I am confused. What are your components?

nrqed said:
I am not sure what you mean by "19.2 deg (east) and 4.1 deg (south)". You mean 19.2 degrees east of south?? And 4.1 degrees south of east?? But these two angles should add up to 90 degrees so I am confused. What are your components?
im terribly sorry, its late where I am, what i meant to say was 19.2 N (east) and 4.1 N (south)

alexi_b said:
im terribly sorry, its late where I am, what i meant to say was 19.2 N (east) and 4.1 N (south)
No problem! We must both be on the east coast, it is quite late here too :-)

Ok, your components are good. But recalculate the angle, it does not give -14.3 degrees. Did you use an arctan?

nrqed said:
No problem! We must both be on the east coast, it is quite late here too :-)

Ok, your components are good. But recalculate the angle, it does not give -14.3 degrees. Did you use an arctan?
Yeah I did recalulate and it gives me 12.05 or 12.0 because of significant digits. And i used tan^-1 to find my angles

alexi_b said:
Yeah I did recalulate and it gives me 12.05 or 12.0 because of significant digits. And i used tan^-1 to find my angles
Good. I get this too. So the answer should be -12.0 degrees.

• alexi_b
nrqed said:
Good. I get this too. So the answer should be -12.0 degrees.
thank you!