Non-uniform circular motion question

[neshepard]

Homework Statement

An object of mass m=.500kg is suspended from the ceiling of an accelerating truck. Taking a=3.00m/s^2 find the angle θ that the string makes with the vertical and the tension T in the string.

Homework Equations

The Attempt at a Solution

mgsinθ=ma => sinθ=1.5/4.9 =>θ=17.8

mgcosθ=T => T=4.67N

But alas this is incorrect. Where did I fail?

 

[Chewy0087]

Be careful with mg & T.

For example, in your second equation, you're implying that mg is bigger than T, think about why that can't be true.

Your resolving is fine though, just think about the mg & T, if you need more help just ask.

Edit: I'm also not sure what this has to do with circular motion.

 

[neshepard]

As to why circular motion...got me. It's in that chapter in the book.

With T and mg, wouldn't mg be bigger since due to acceleration? If I reverse my equation and have T bigger, is it mg-Tcosθ? and why?

 

[Chewy0087]

The reverse is correct. Draw yourself a diagram and label all of the forces.

The string has to both support the weight of the mass (mg) in the upward direction, but it also acts against the acceleration of the lorry which pushes the mass towards the back of the lorry. Because the net force on it is 0, right? Anyway, I think the diagram should help.