# Homework Help: Forces and acceleration on frictionless table?

1. Jun 10, 2014

### syavian1019

The two forces F1 and F2 act on a 27.0-kg object on a frictionless tabletop. If F1=10.2 N and F2=16.0 N find the net force on the object and its acceleration for cases (a) and (b).

(a) is 90 degrees, in the 3rd quadrant
(b) is 120 degrees, from the positive y-axis and moving clockwise

To calculate the angles at which the resultant forces act in this problem, please start at the positive x axis and travel counter-clockwise.

1. The net force in figure (a) (above) is?

2. The angle from the positive x axis at which the net force in figure (a) acts is?

3. The magnitude of the acceleration of the object in figure (a) is?

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For 1, I don't know which formula I'm supposed to use. I can't find it in my textbook or anywhere else... but I think for 3 it's: a = (net force)/(mass in kg)

For 2, is this how you do it?: from the positive x-axis, move counterclockwise until you hit the angle.

Thanks for all help!

Last edited: Jun 10, 2014
2. Jun 10, 2014

### Nathanael

The link to the picture doesn't work (you need an account on that website)

3. Jun 10, 2014

### BiGyElLoWhAt

This is why the forum rules say to attach the picture to the post and not to host it externally...

4. Jun 10, 2014

### BiGyElLoWhAt

Here's a general way to find net forces for non parallel/perpendicular forces.

$R^2 = R_{x}^2 + R_{y}^2$

$R_{x} = R\text{cos}(\theta)$

$R_{y} = R\text{sin}(\theta)$

For 2D motion:

$\vec{R} = <R_{x},R_{y}>$

if you decompose all your forces down into this vector form, then you can add all your vectors together simply by adding the x components and y components.

There's numerous trigonometric relationships that can be used to solve for your componts, and it's really as simple as using trig.

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5. Jun 10, 2014

### syavian1019

@ BiGyElLoWhAt
does R= F?

6. Jun 10, 2014

### syavian1019

would it be 10.2 cos 180?

7. Jun 10, 2014

### Nathanael

R is an arbitrary vector, it can be Force or anything else (so yes)

Edit:
Nevermind, I didn't notice you edited your original post

This is unclear.

You can't draw a picture or something?

8. Jun 10, 2014

### BiGyElLoWhAt

R is a general vector of length |R| and direction theta.

F can be equal to R, if you're seeking to break F down into components. If so, use the trigonometric relations I posted.

And as for 10.2 cos(180)

I have no clue, as I haven't seen the picture. I'll let you be the judge of that. ;)

9. Jun 10, 2014

### BiGyElLoWhAt

What do you mean by 90 degrees in the third quadrant?

II I
III IV

do you mean <-
or
|
v
?

(those are arrows btw haha)