# Friction problem (pushing a box)

• Alyona
In summary, the net external force on a 60 kg box is 90 N and the force of friction opposing the motion is 30 N. Using the equation Fnet=ma, the acceleration of the box is calculated to be 1.5 m/s2. The second solution provided, which ignores the force of friction, is not correct.
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

Alyona said:

## 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.

jim mcnamara and 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.

tnich said:
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!

## 1. What is friction?

Friction is a force that resists motion between two surfaces that are in contact with each other.

## 2. Why does friction occur when pushing a box?

When pushing a box, the force you apply creates a normal force between the box and the ground. This normal force, combined with microscopic roughness on the surfaces, creates friction.

## 3. How does the weight of the box affect the amount of friction?

The weight of the box affects the normal force between the box and the ground, which in turn affects the amount of friction. A heavier box will have a greater normal force and therefore more friction.

## 4. Can friction be reduced or eliminated?

Friction can be reduced by using lubricants or by making the surfaces smoother. It cannot be completely eliminated, as it is a natural force that is necessary for many everyday activities.

## 5. How does the angle of pushing affect the friction on a box?

The angle of pushing can affect the normal force and therefore the amount of friction. Pushing at an angle can reduce the normal force and friction, while pushing in a straight line will result in the maximum amount of friction.

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