physics

hello! I have been having a lot of trouble with one physics problem that I have for homework: any help would be greatly appreciated!

'A block of mass 12 kg starts from rest and slides a distance of 8 m down an inclined plane making an angle of 40 with the horizontal. The coefficient of sliding friction between the block and the plane is 0.4.
the acceleration of gravity is 9.8 m/s/s.
What is the net force on the block along the incline? answer in units of N.'

I think I am supposed to use acceleration-Fnet/mass. I just can't find the acceleration? I don't think it would be 9.8 because that's gravity and this object is not join straight down.

thanks!

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  1. You cant answer this unless you know the acceleration. If it slid at constant velocity, or you knew the time it took to go down, that would allow you to work it.

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    bobpursley
  2. M*g = 12 * 9.8 = 117.6 N. = Wt. of block.

    Fp = 117.6*sin40 = 75.59 N. = Force parallel with incline.

    Fn = 117.6*cos40 = 90.09 N. = Normal force.

    Fk =[ u*Fn = 0.4 * 90.09 = 36.0 N. = Force of kinetic friction.

    Fnet = Fp-Fk = 75.59 - 36.0 = 39.6 N.

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