# Dynamics High School Question [closed]

Alright so, my book says that the answer for the acceleration is $3.0 \;\mathrm{m\, s^{-2}}$ but I got $1.6 \;\mathrm{m\, s^{-2}}$

Let the directions of acceleration be the positive directions

Equation 1: \begin{align} F_{net,A} &= F_t - F_g \iff \\ m_1 a &= F_t - m_1 g \end{align}

Equation 2: $$F_{net,B,x} = F_{g,x}+F_f - F_t,$$ this is what I think is correct, but my book says this equation is actually $F_{net\,B,x} = F_{g,x} - F_f - F_t$, which I don't understand because the direction of these forces are different for block $B$? Shouldn't friction and the direction gravity in the $x$-plane be added together? Why am I subtracting when I think I should be adding? This is the only part I need help with, thanks in advance.

• You may be interested in the fledgling offshoot site Physics Problems Q&A which is more geared towards this type of question. Dec 1, 2016 at 22:47

Part of your problem seems to arise because you assume the positive direction to be the direction of acceleration and then treat down for block A as the negative direction. And assuming the directions as you note the force on block B should be $$F_{net}B_{x} = F_{tension} - F_{friction} - F_{gx},$$ that is the net force is the force due to tension minus the force due to friction of the box on the slope and then subtract the force due to gravity "down" the box.