(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

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 .

2. Relevant equations

uk = kinetic coefficient

Fx1 = m1a = m1gsin 17 - T - uk1 * m1gcos 17

Fx2 = m2a = m2gsin 17 + T - uk2 * m2gcos17

3. 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??

**Physics Forums | Science Articles, Homework Help, Discussion**

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Why is the acceleration positive?

**Physics Forums | Science Articles, Homework Help, Discussion**