Why is the acceleration positive?

  • Thread starter DrunkApple
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Homework Statement


Two blocks attached by a string slide down a 17 incline. The lower block has a mass of 0.2 kg and a coefficient of kinetic friction of 0.2. The upper block has a mass of 0.9 kg and a coefficient of kinetic friction of 0.9. Find the magnitude of the acceleration of the blocks. The acceleration of gravity is 9.81 m/s2 .

Homework Equations


uk = kinetic coefficient
Fx1 = m1a = m1gsin 17 - T - uk1 * m1gcos 17
Fx2 = m2a = m2gsin 17 + T - uk2 * m2gcos17

The Attempt at a Solution


a(m1+m2) = (m1 + m2)gsin 17 - uk1 * m1gcos 17 - uk2 * m2gcos 17
a = -4.3765924 m/s^2

However, the answer was a = 4.3765924 m/s^2.
It seems like the equation was:
Fx1 = m1a = T + uk1 * m1gcos 17 - m1gsin 17
Fx2 = m2a = uk2 * m2gcos 17 - m2gsin 17 - T
The equation was flipped, but I do not understand why. Since blocks are sliding down, shouldn't friction have the negative value because it is going against acceleration??
 

Answers and Replies

  • #2
cepheid
Staff Emeritus
Science Advisor
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Find the magnitude of the acceleration of the blocks.
Regardless of whether you've set up your equations correctly or not, (which I haven't checked), this word in red above is important. Your answer is going to be positive, because the question asks you to state the magnitude of the acceleration vector only, not its direction.
 

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