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Simpe Force Question

  • Thread starter Sheneron
  • Start date
  • #1
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[SOLVED] Simpe Force Question

Homework Statement


A 2.00 kg object placed on a frictionless, horizontal table is connected to a string that passes over a pulley and then is fastened to a hanging 7.00 kg object, as in Figure P5.24. Find the magnitude of the acceleration of the two objects and the tension in the string.

The Attempt at a Solution


I have a specific question about this problem that I came upon at the end. Here is how I solved it.

[tex]T = m_1a[/tex]
[tex]-T + m_2g = m_2a[/tex]
So two equations to uknowns, and here is what I got.
[tex]-m_1a + m_2g = m_2a[/tex]
[tex]a = \frac{m_2g}{(m_1+m_2)}[/tex]

So solving for a I got -7.62. The thing I have a question about is the negative on the acceleration. At the beginning of the problem I defined rightward and downward direction as positive and upward direction as negative. So if acceleration is a negative value does that not mean it is traveling up, since I defined up as negative? I know that it doesn't not, but I am confused it can be this way.
 

Answers and Replies

  • #2
alphysicist
Homework Helper
2,238
1
Hi Sheneron,

How did you get a negative answer? All the values (masses and g) are positive numbers.

Did you use g=-9.8? The value of g is a magnitude and therefore always a positive number; if we were talking about an object in free fall we would say that its acceleration is

[tex]
a_y = -g
[/tex]

just to keep g a positive number.
 
  • #3
360
0
Ah ok I see. thanks
 

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