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Force Problem x-axis

  • Thread starter Cursed
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[SOLVED] Force Problem

1. Homework Statement
Only two forces act on an object (mass = 3.00 kg), as in the drawing (below). Find the magnitude and direction (relative to the x-axis) of the acceleration of the object.

http://img148.imageshack.us/img148/6302/imgfbic2.png [Broken]

2. Homework Equations

F=ma

3. The Attempt at a Solution

F=ma
40N=(3.00kg)(a)
13.33m/s/s=a

Correct answer: 30.9m/s/s, 27.2 degrees above the +x axis.
 
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Answers and Replies

there are two forces acting on it so you would have to find the resultant force because you have to find the force net in order to use the F=ma equation. Finding the resultant by using vector addition method will give you the Force net and then you can plug in the mass to solve.
 
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there are two forces acting on it so you would have to find the resultant force because you have to find the force net in order to use the F=ma equation. Finding the resultant by using vector addition method will give you the Force net and then you can plug in the mass to solve.
I understand what you're saying, but I'm confused. I thought I knew how to calculate the resultant vector, but the way I'm calculating it leads me to the wrong answer.

Can you please show me how to do it? I would greatly appreciate it. ;)
 
Doc Al
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Show how you calculated the resultant.
 
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Show how you calculated the resultant.
Okay.

I tried THREE ways:

1. I averaged the two vectors: .5(60N+40N) = 50N... Then I divided by 3 (since the weight is 3.00kg and F=ma). That equaled 16 and 2/3 m/s/s. Wrong.

2. I found the y component via Pythagorean theorem. It was √2000 = 44.7214N. Then I divided by 3 for the same reasons as #1. That equaled 14.97 m/s/s. Wrong.

3. I did R = r- r0. (R being the resultant vector I am trying to find.). R = 40N. I divided it by 3 and got 13 and 2/3 m/s/s. Wrong.
 
Doc Al
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Why in the world would you (attempt to) take the average?

In any case, do this: Find the x & y components of each vector. Add up the x components: that will be the x component of the resultant. Do the same with y components to find the y component of the resultant.

You can then find the magnitude of the resultant via the Pythagorean theorem.
 
you have to split the 60 N into two different vector quantities since it has a horizontal and vertical magnitude. the 40 N only has a horizontal quantity. then you would add the horizontal component of 60 and 40 to get the total force in the x direction.
 
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Why in the world would you (attempt to) take the average?
Haha, because none of the other ways I did it worked. :rofl:

In any case, do this: Find the x & y components of each vector. Add up the x components: that will be the x component of the resultant. Do the same with y components to find the y component of the resultant.

You can then find the magnitude of the resultant via the Pythagorean theorem.
Thank you. I finally got the answer. :]

you have to split the 60 N into two different vector quantities since it has a horizontal and vertical magnitude. the 40 N only has a horizontal quantity. then you would add the horizontal component of 60 and 40 to get the total force in the x direction.
Thank you for your help too. :]
 

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