(a) When the applied force is 84 N, the friction force on the box is 36.02 N.
(b) When the applied force is 94 N, the friction force on the box is 46.02 N.
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Frictional force on the box</h3>
The frictional force on the box when it is pushed with a certain force is calculated as follows;
Ff = μFₙ
F - Ff = ma
Mass of the box is calculated as follows;
W = mg
m = W/g
m = 120/9.8
m = 12.24 kg
Acceleration of the box is calculated as follows;
a = μg
a = 0.4(9.8)
a = 3.92 m/s²
When the applied force is 84 N,
F - Ff = ma
F - ma = Ff
84 - 12.24(3.92) = Ff
36.02 N = Ff
When the applied force is 94 N,
F - Ff = ma
F - ma = Ff
94 - 12.24(3.92) = Ff
46.02 N = Ff
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