Physics Problem Center Of mass HELP

[FahimP]

Homework Statement

The problem is asking to find the Acceleration vector before jack reaches the less steep section.
the Ans from back of book says : (3.95)i + ( -2.29 ) j

My answer does not match at all : ANY HELP PLZZZZ

anyone know how to get this ?

Two skiers, Annie and Jack, start skiing from rest at different points on a hill at the same time. Jack, with mass 85 kg, skis from the top of the hill down a steeper section with an angle of inclination of 35°. Annie, with mass 70 kg, starts from a lower point and skis a less steep section, with an angle of inclination of 20°. The length of the steeper section is 175 m.



Here is a pic :
http://s172.photobucket.com/albums/w30/afghanplayr/?action=view&current=8-p-032-alt.gif
Category

Homework Equations

The Attempt at a Solution

This is how i tried to find the acceleration vector :

F = mg
F = 85kg( 9.81 ) = 568.98 N

568.98sin(35) = 326.35
568.98cos(35) = 466.05

a = Fnet/m
326.35/85 = 3.83
466.05/85 = 5.48

a= ( 3.83)i + ( 5.48 ) j

 

[cepheid]

I suspect that one problem is that the components into which you have resolved the gravitational acceleration vector g are components that are parallel and perpendicular to the incline. (Draw the vector decomposition and you'll see this is true). The problem is that your x and y coordinate axes are not aligned with these parallel and perpendicular directions (look at the figure).

Another problem is that Jack's weight is not the only force that acts on him (Hint: he never accelerates in the direction perpendicular to the incline, right? So something other than just gravity must be present). So, computing the net force in the x-direction and the net force in the y-direction requires a little bit more work and thought than what you've done here.

Draw a free body diagram for Jack.