# Torque and Friction

1. Feb 10, 2006

### killerdoxie

can friction cause torque?

2. Feb 10, 2006

### SpaceTiger

Staff Emeritus
Do you have any ideas? If I fall off my bicycle and roll down a paved street, what slows down my spinning (and gives me all those bruises)?

3. Feb 10, 2006

### killerdoxie

well, the question was on a test, and we were having a debate in class today about it.

I think no, that friction cannot cause torque, because friction is a reaction, something has to be applied for it to happen. A book sitting by itself does not have friction, but as soon as I push against the book there is friction. The same is true for normal force, I believe, because if i am standing up, the floor pushes back up on me (normal force), but the space two feet in front of me has no normal force since nothing is pushing down on it.

4. Feb 10, 2006

### z-component

But think about the definition of torque. $$\tau = F \times r$$ so under what circumstances could a force cause torque? You're right, forces come in pairs (per Newton's third law) but what makes friction different from any other "applied force" like something pushing against a book?

Last edited: Feb 10, 2006
5. Feb 10, 2006

### killerdoxie

well, since friction is a reactionary force, it doesn't cause the displacement, the other force does, so if r=0, then how can it cause torque?

6. Feb 10, 2006

### z-component

Sorry, but you are incorrect. In SpaceTiger's example, positive torque is undergone while rolling down the hill but the kinetic friction by the ground also causes a torque to counter the positive torque. The torque by friction is negative (positive/negative is ambiguous, but the two torques are in the opposite directions), causing the rotating SpaceTiger's angular acceleration, and therefore velocity, to decrease and eventually become zero.

7. Feb 11, 2006

### Staff: Mentor

Both involve forces, but they are completely separate manifestations of mechanics.

Friction is an undesirable dissipative force encountered where two masses interact, generally by virture of shear forces, i.e. the force is applied parallel with the surface of a solid. Friction occurs in liquids and gases by virtue of atomic collisions and/or adhesion.

See z-components example for torque. Torque is a force applied with a moment.

A net (nonzero) force causes acceleration (or deceleration), while a zero force, i.e. a couple of forces of equal magntide by opposite direction would not provide acceleration, but would provide tensile or compressive loading (and pressure, which = force/area). Similarly, a net nonzero torque produces angular acceleration, while a net zero torque would provide some internal shearing forces (stresses) in a solid object.

As for torque and friction, consider the example of a shaft in a hole (bore). If the static friction exceeds the applied torque, the shaft will not move. Only when the applied torque exceeds the friction between the surfaces of the shaft and bore, will it turn. So in that sense, friction increases the 'required' torque, but does not 'cause' it. The force applied at some distance (moment arm) causes the torque.