What is the angle of a hanging dice during acceleration?

[bpw91284]

Homework Statement

A pair of fuzzy dice is hanging by a string from your rearview mirror. While you are accelerating from a stoplight to 29 m/s in
5.7 s, what angle θ does the string make with the vertical?

Homework Equations

v_f=v_0+a*t
x_f=x_0+v_0*t+0.5a*t^2
v_f^2=v_0^2+2*a(x_f-x_0)
x-x_0=[(v_0-v+v_f)/2]*2

And just sum of forces=0
F=ma

Note: v_f^2 is final velocity squared

The Attempt at a Solution

I'm lost because it seems to me that the mass of the dice must be known to solve?

 

[P3X-018]

This is an accelerated frame of reference. You know the direction of the acceleration that the frame has. There is a fictious force and the natural force (gravity), what is the vectorial sum of these two?

 

[turdferguson]

Forget about ficticious forces. Draw a picture and a free body diagram. First, solve for acceleration. The tension force acts at an angle, which means a component of tension accelerates the dice. If you do things correctly, mass should cancel out of the equation

 

[AngeloG]

A good idea would be to draw a force diagram outside and inside of the car =). To get a frame of reference for which way the dice are accelerating.

They will be accelerating positively or negatively as the speed of the car =).

Also, split this up into two vectors x and y. Once you start to do all the calculations, you'll find that the mass cancels out in the end =).