The force responsible for sliding the block down along the ramp is the component of the weight parallel to the ramp.
The weight of the block is
[tex]W=mg=(80 kg)(9.8 m/s^2)=784 N[/tex]
The component of the weight parallel to the ramp is
[tex]W_{par} = W sin 30^{\circ}= (784 N)( sin 30^{\circ} )=392 N[/tex]
And now we can find the acceleration of the box by using Newton's second law:
[tex]a=\frac{F}{m}=\frac{392 N}{80 kg}=4.9 m/s^2[/tex]
So, the correct answer is
-4.9 m/s2
(the negative just means we have taken "upward along the ramp" as positive direction)
