Dynamics Question: Finding Fa using mass and acceleration

[Apollinaria]

Hello again forum I am still studying for that unit test and am still having problems with dynamics... I've attached the problem with the calculations/solutions. Basically, I don't understand why I'm getting the wrong answer. Probably b/c idk what the heck I'm doing

Homework Statement

15kg object thrown into the air. Initial acceleration is 8.8m/s^2. Find applied force.

Homework Equations

Please see attachment.

The Attempt at a Solution

Please see attachment. Btw, I realize Fa needs to be positive.

Please advise regarding correct way of solving?.. Thank you for your time once again

EDIT: I found my mistake. However, I still don't understand why "that" is the correct solution.
My Fg value had to be negative;
Fa= (15kg x 8.8m/s^2) - (-147.15)
Fa= 279.15

 

[CWatters]

The force available to accelerate the object (Fnet) is equal to the sum of all the forces acting on the object.

F_net = F_A + (-F_g)
or
F_net = F_A - F_g

F_g is negative because it's in the oposite direction to F_A.

Rearrange..

F_A = F_net + F_g

= ma + mg
= m (a+g)
= 15 (8.8 + 9.81)
= 279 N

 

[Saitama]

You have done everything right, you simply made a mistake while solving the equations.
You wrote:
F_A-F_g=ma
F_A=ma-F_g. <--- This is wrong. It should be F_A=ma+F_g.

 

[CWatters]

Well spotted. I looked at his working and didn't see that error.

 

[Apollinaria]

The force available to accelerate the object (Fnet) is equal to the sum of all the forces acting on the object.

F_net = F_A + (-F_g)
or
F_net = F_A - F_g

F_g is negative because it's in the oposite direction to F_A.

Rearrange..

F_A = F_net + F_g

= ma + mg
= m (a+g)
= 15 (8.8 + 9.81)
= 279 N

Thank you Watters That organizes it a little better lol. In regard to your last post, I am very much female by the way

 

[Apollinaria]

You have done everything right, you simply made a mistake while solving the equations.
You wrote:
F_A-F_g=ma
F_A=ma-F_g. <--- This is wrong. It should be F_A=ma+F_g.

Eek! So sad that I didn't notice that... I'm sorry you had to point it out but thank you once again