Homework Help: Dynamics Question: Finding Fa using mass and acceleration

1. Jul 21, 2012

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

1. The problem statement, all variables and given/known data

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

2. Relevant equations

3. The attempt at a solution

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

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

Attached Files:

• Dynamics.jpg
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2. Jul 22, 2012

CWatters

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

Fnet = FA + (-Fg)
or
Fnet = FA - Fg

Fg is negative because it's in the oposite direction to FA.

Rearrange..

FA = Fnet + Fg

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

3. Jul 22, 2012

Saitama

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

4. Jul 22, 2012

CWatters

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

5. Jul 22, 2012

Apollinaria

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

6. Jul 22, 2012

Apollinaria

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