Friction force and constant velocity

1. Dec 3, 2003

m_wu

Does the speed of a block of wood being pulled across a surface go twice as fast if you pull twice as hard? If yes/no, why?

Thanks to anyone who will answer that!

2. Dec 3, 2003

Staff: Mentor

The net (horizontal) force on the block will determine the block's acceleration, the rate at which its speed changes. If you double the net force, the acceleration is doubled. You need more info to find the speed, since it changes.

If the net force is zero, the speed will remain constant.

3. Dec 3, 2003

m_wu

Would the force of friction double if you double the constant velocity of the wooden block? If yes, why?

Thank you again.

4. Dec 3, 2003

krab

No. It requires more force to accelerate up to high speed, but once at high speed, the force needed to maintain that speed is the same as the force needed to maintain a lower speed. Subjectively (I mean if you are using your own muscles to do the pushing), it feels harder to go faster, but that is because the power expended is higher. Power = Force times speed, so you get tired faster.

5. Dec 5, 2003

turin

In the simplified freshman physics model, no. The kinetic friction is independent of velocity. In the real world, if you're just talking surface contact, I would imagine that the friction would actually decrease with velocity.

6. Dec 5, 2003

Stingray

Fluid friction depends strongly on velocity. Normal sliding friction usually doesn't depend on velocity much at all.

7. Dec 6, 2003

turin

By this, do you mean "viscosity" or something else. I've heard an old man call "viscosity" "pipe friction" (and then an argument ensued. Arguing with old men can be quite frustrating if you care to convence them).

8. Dec 6, 2003

Stingray

I meant viscosity as well as what would more precisely be called "pressure or inertial drag" (the main component of aerodynamic drag that's due to fluid being forced out of the way from an oncoming object).

9. Dec 6, 2003

turin

I didn't think this was the same thing as viscosity, but is was imprecise for me to say "viscosity" for force, when it is a coefficient, not a force. I thought that there could be drag without viscosity, though.

10. Dec 6, 2003

Stingray

Yes, you're right. My "as well as" meant that I was mentioning two different things.

I think it is common practice to call a force viscosity, isn't it?

11. Dec 6, 2003

turin

Oh. I misunderstood. Sorry.

I don't know. I don't ever talk about fluid dynamics with anyone. My first year phys book speaks of viscosity as a coefficient, an makes a point of not calling it a force.

12. Dec 8, 2003

pallidin

A 1 ounce marble pulled by a connected string with your hand moving at 1 mph will go no faster or slower than the same marble pulled by an 80 ton hydraulic jack whoes piston moves at 1 mph.

13. Dec 8, 2003

pallidin

The acceleration TO the 1 mph speed is much greater with the 80 ton hydraulic jack, however.

14. Dec 8, 2003

HallsofIvy

Especially if they are right! "Pipe friction" doesn't cover everything, but it's a good off-the-cuff representation.

No, it is not. "viscosity" is more like the "coefficient of friction" that determines the force than the force itself.

15. Dec 8, 2003

turin

I was trying to explain to him that my "viscosity" was his "pipe friction." It was a disaster. He just wasn't havin' any of that newfangled technical jargon.