# Mary applies a force of 71 N to push a box with an acceleration of 0.54 m/s2. When she increases the pushing force to 80 N, the box's acceleration changes to 0.69 m/s2. (a) What is the mass of the box?

## There is a constant friction force present between the floor and the box.

Mar 13, 2017

I got $60 k g$

#### Explanation:

We can use Newton's second law where the resultant of all forces is equal to mass times acceleration.
$71 - f = m \cdot 0.54$
$80 - f = m \cdot 0.69$
Where $f$ is the constant friction.
From the first equation we have:
$f = 71 - 0.54 m$
Substitute into the second equation:
$80 - 71 + 0.54 m = 0.69 m$
$0.15 m = 9$
$m = \frac{9}{0.15} = 60 k g$