# Relationship between adhesion and friction

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1. Oct 20, 2014

### Jazz

Maybe this question is pretty simple to tackle, but I’m really confused with it. Perhaps I’m just overthinking.

The question is:

The glue on a piece of tape can exert forces. Can these forces be a type of simple friction? Explain, considering especially that tape can stick to vertical walls and even to ceilings.

By simple friction means the force that opposes the motion of an object resting in a horizontal surface. So, simple friction is proportional to the normal force (source: http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@8.9:32/College_Physics).

I really don’t get that concept of ‘simple friction’. After looking at Wikipedia I found that static and kinetic friction are a type of friction called Dry Friction, which resists lateral motion. Thus dry and simple friction would be the same.

According to what I understand, simple (dry) friction is:

- Directly proportional to the normal force.

- Independent of the area of contact.

The forces exerted by adhesive tapes, I think, are exactly the opposite: are inversely proportional to the weight of the object being stuck. If I stick a pencil to the wall, then the piece of tape will do the job but if I try to do the same with a metal bar (that weighs more), it will end unsticking. In order to avoid this I would need to put more tape, therefore increasing the area of contact.

Are these assumptions correct? If that were the case, what kind of forces does an adhesive tape exert?

Is the concept of ‘simple friction’ really used?

Thanks!!

2. Oct 20, 2014

### Staff: Mentor

You are right about the need to use more tape, but in the bolded text above is backwards. The force needed to support the object is directly proportional to its weight while the force exerted by a given piece of tape is constant. There's no inverse proportionality going on; the amount of tape required is directly proportional to the weight.

They are electrical if you dig down to the level of interactions between individual atoms at the surface of the tape.

Yes, all the time, but not for things like tape which are sticky.

3. Oct 20, 2014

### Jazz

So just to be sure, what would keep the pencil from falling is the electrical force, right? Would it have to do with chemical bonding?

4. Oct 20, 2014

### Staff: Mentor

Generally we reserve the term "chemical bonding" for the electrical forces that cause atoms to form molecules and use "inter-molecular forces" for stickiness, cold welding, the way that solids retain their shape. All of these forces are ultimately electromagnetic in origin - they come from interactions between the charged particles that make up atoms - so as long as you understand the origin of the forces I wouldn't get too hung up on the terminology.

5. Oct 20, 2014

### Jazz

Excellent.

Thanks again !!