Why Was FNetx Changed to ma in the Physics Problem?

[aprilrenee]

Homework Statement

In 1988, a very large telephone was constructed in the NEtherlands. In order to move the phone, a 7980 N horizontal force was applied that result in an acceleration of 1.33 m/s^2 along level ground.

Homework Equations

I received this question on a practice test for my exam tomorrow. I also have the work (shown below.)

- I do not understand how/why FNetx was changed to ma.
Why was this done?

The Attempt at a Solution

FNety = Fw- W (0 because not moving vertically.)
FNetx = Fa-Fk
ma = Fa - (mu)mg
ma+ (mu)k mg = Fa
m (a+(mu)k g) = Fa
m = Fa / a+ (mu)k g
m = 7890 / [(1.33m/s^2) + (.180)(9.80m/s^2)]
m = 2579.18552 kg
m = 2580 kg

... So on and so forth.

 

[collinsmark]

- I do not understand how/why FNetx was changed to ma.
Why was this done?

Here's a hint. What is Isaac Newton's second law of motion?