The magnitude of the frictional force is 5 N according to Newton's second law of motion, the net force acting on an object is equal to the product of its mass and acceleration.
Option D is correct
The force exerted by the boy on the box is 40.0 N.
F(net) = m*a
where F(net) = the net force,
m = the mass of the object, and a is its acceleration.
In this case, the net force acting on the box is the force applied by the boy minus the frictional force:
F(net) = F(applied) - F(friction)
substituting the values:
F(applied) = 40.0 N
m = 5 kg
a = 7 m/s²
we then use frictional force:
F(friction) = F(applied) - F(net)
F(friction) = F(applied) - ma
= 40.0 N - (5 kg)(7 m/s²)
= 5 N
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