# My question is about the ommision of pseudo forces when we draw the free body diagram from an inertial frame of reference

I have this question, in my physics book where a bob attached to a string is in a slanting position as it is inside a cart moving forward with an acceleration. I understand that if we draw the free body diagram of the bob from inside the cart, we have to include a pseudo force as we are applying newtons laws from a non-inertial frame of reference. My question is, can we draw the same free body diagram from an inertial frame of reference, i.e. the road? if we can draw it, can someone tell me what the forces acting on the bob are? are we are supposed to get the same free body diagram as the previous case? THis is confusing me for some reason!

• Is it a sort of pendulum? You will only get the weight and the tension then. Commented Nov 21, 2017 at 8:10

• @GouthamSwaminathan $\sum F_x = T\sin(\theta) = ma_x$, so $a_x = \frac{T\sin(\theta)}{m}$. I'm not sure what else you're looking for. Commented Nov 21, 2017 at 8:51
• @GouthamSwaminathan In the non inertial frame you have the equation $T\sin b -ma_x =0$ while in the inertial frame you have the equation $T\sin b = ma_x$, where the lhs is the forces acting on the bob in each respective frame. So the equations are of course mathematically equivalent, it’s just their physical interpretation that’s different.