# Friction; the very basics

1. Aug 18, 2015

### Cliff Hanley

1. When we walk we push backwards against the ground with our feet and the opposing force pushes us forwards, is this opposing force friction? Or is it the Earth pushing us in response to us pushing it (with our foot) re Newton’s 3rd Law of Motion? Is there one pair of interaction forces occuring here or are there two pairs?

2 . Re the above question; If I push a door, the door pushes me; but there is no friction involved, yes? Is there just one pair of interaction forces involved here?

3. If I push a book across a table the book pushes me (but there is also the opposing frictional force between the bottom of the book and the table, yes?). Are there two pairs of interaction forces at work here?

2. Aug 18, 2015

### A.T.

Yes.

Yes.

One

Depends on how you push it. Friction is the component parallel to the surface.

Friction is an interaction force too.

Yes

3. Aug 18, 2015

### Qwertywerty

You must always mention your system .
Yes , it is . The earth pushes back on us , via friction . One - between the earth and us .
Yes , it pushes back on us . Friction is not involved here .
Yes , but only one pair is related to us .

Hope this helps ,
Qwertywerty .

4. Aug 18, 2015

### Cliff Hanley

So the Earth pushing back on me involves friction (my foot and the Earth rubbing over each other, yes?). And this is one pair of interaction forces. Would we say that it's one pair of interaction frictional forces?

“Depends on how you push it [the door]”

With a flat palm, square on to the door, no rubbing across the surface involved.

“Friction is an interaction force too.”

Are all forces interaction forces given what Newton’s 3rd Law of Motion states?

And, you said, yes, there are two pairs of interaction forces at work when a book is pushed across a table; does this differ from when I push a door (one pair of interaction forces) in that there is no rubbing of two surfaces when I push the door?

5. Aug 18, 2015

### Cliff Hanley

“You must always mention your system.”

What does that mean?

“Yes , but only one pair [one of the two pairs of interaction forces at work when a book is pushed across a table] is related to us.”

This is presumably the I-push-the-book-the-book-pushes-me pair, yes? Why is the other pair (the frictional pair between the surfaces of the book and the table) not related to us?

“Hope this helps.”

It does (as ever). Thank you.

6. Aug 18, 2015

### A.T.

If the contact force is perpendicular to the surface, no friction is involved.

Inertial forces do not obey Newton's 3rd Law

Depending on how you define the bodies, you can have as many interactions as you want.

7. Aug 18, 2015

### Qwertywerty

Yes - to the former .

To the latter - Consider an FBD for us . Which forces would we take into account ? Do forces acting between other objects enter it ?
It means that you should always specify what exactly you are talking about . For example , if you took yourself and the book as one system , then the force acting on you + book - and the table , would be the pair which would be talked about .

While , at the same time , internal ( between you and book ) would not be counted for Newton's third law pair of the system ( as you are considering them as one ) .

Hmmm ... But , I think this was unnecessary . Please ignore .

8. Aug 24, 2015

### Cliff Hanley

"Inertial forces do not obey Newton's 3rd Law"

What is an inertial force? Is this when an object is, for example, at rest and there are two forces acting on the object but are balanced, ie, they don't cause the object to accelerate? If so, and if you and I were at the opposite ends of, say, a car, and we both pushed with equal force, the object would remain at rest, yes? But wouldn't there still be two pairs of interaction forces at either end each obeying the 3rd law (me pushing the car and being pushed by it; you pushing the car and you being pushed by it)?

9. Aug 24, 2015

### A.T.

10. Aug 24, 2015

### Cliff Hanley

"Consider an FBD for us . Which forces would we take into account ? Do forces acting between other objects enter it ?"

Thanks. I'll need to learn a bit more about free body diagrams and which forces are relevant etc.

11. Aug 24, 2015