Friction problem (pushing a box)

[Alyona]

Homework Statement

A person is pushing a box. The net external force on the 60 kg box is stated to be 90 N. If the force of friction opposing the motion is 30 N, what is the acceleration of the box?

Homework Equations

This seems to be a pretty easy, straightforward problem, but I'm confused. Is the first or second solution correct?

The Attempt at a Solution

Fnet = F - f = ma
90 N = F - 30 N = (60 kg)a
a = 90 N / 60 kg = 1.5 m/s2

Fnet= F - f = ma
90 N = F - 30 N
F = 120 N
F = ma
120 N = (60 kg)a
a = 120 N / 60 kg = 2.0 m/s2

 

[tnich]

Homework Statement

A person is pushing a box. The net external force on the 60 kg box is stated to be 90 N. If the force of friction opposing the motion is 30 N, what is the acceleration of the box?

Homework Equations

This seems to be a pretty easy, straightforward problem, but I'm confused. Is the first or second solution correct?

The Attempt at a Solution

Fnet = F - f = ma
90 N = F - 30 N = (60 kg)a
a = 90 N / 60 kg = 1.5 m/s2

Fnet= F - f = ma
90 N = F - 30 N
F = 120 N
F = ma
120 N = (60 kg)a
a = 120 N / 60 kg = 2.0 m/s2

I am confused, too. The problem statement says the net external force is 90N and then introduces another external force - friction. Friction certainly an external force - it is a force the surface exerts on the box. So I think your first answer is correct given the way the problem is stated, but I am not sure that was the intent of the person who wrote the problem.
Edit: I do not think your second answer is correct - that answer ignores the force of friction on box.

 

[Alyona]

Yes, it's confusing. If you don't need the force of friction to solve the problem, since the net force was given, why put it in the problem then? I think the first solution is correct, but I'm not sure.

 

[Alyona]

Edit: I do not think your second answer is correct - that answer ignores the force of friction on box.

Ohh, I get it now, thank you for your help!