Help in Homework -- difference b/w kinematics friction and static friction?

1. Apr 27, 2015

*.*.*AnUm*.*.*

Hi,

Please explain what is the difference b/w kinematics friction and static friction ?

Please explain, what is rolling friction ?
Is it when we are pulling a weight with rope, and rope is rolling on a wheel ?
for example in past days, people used rope to get water from well.

Thanks.

2. Apr 27, 2015

Staff: Mentor

What do you think the difference is between static friction and dynamic friction? What do you find when you use Wikipedia or Google?

3. Apr 27, 2015

*.*.*AnUm*.*.*

I think static friction when any force causes friction
dynamic friction when a body is moving, right ?

4. Apr 27, 2015

Staff: Mentor

Yes. When the object is sliding, that is dynamic friction. When it hasn't broken loose yet to start sliding, that is static friction.

http://en.wikipedia.org/wiki/Friction

5. Apr 27, 2015

*.*.*AnUm*.*.*

Thanks :)

6. Apr 27, 2015

*.*.*AnUm*.*.*

In other words,

• static friction between non-moving surfaces, and kinetic friction between moving surfaces, right ?

7. Apr 27, 2015

*.*.*AnUm*.*.*

Is this explanation correct:

"Please explain, what is rolling friction ?
Is it when we are pulling a weight with rope, and rope is rolling on a wheel ?
for example in past days, people used rope to get water from well.
"

8. Apr 27, 2015

Staff: Mentor

Yep!

9. Apr 27, 2015

Staff: Mentor

I think of rolling friction more like between a tire and the road. But I suppose it would also apply to a rope over a pulley too.

10. Apr 27, 2015

*.*.*AnUm*.*.*

Okey, thanks :)

11. Apr 27, 2015

*.*.*AnUm*.*.*

And I think friction shouldn't be constant, is it correct ?

12. Apr 27, 2015

haruspex

Static friction can never be less than kinetic friction, but I know of no rule that says it has to be more.
In reality, friction is rather more complicated than is taught in school. But generally you are safe to say that each of the two coefficients of friction of a given combination of surfaces is constant, not varying according to normal force nor, in the case of kinetic friction, according to relative speed of the two surfaces.

13. Apr 27, 2015