# The force behind Rolling

1. Jun 21, 2015

If I make a force on a wheel on its center of mass,the wheel will do a combination of translation and rotation and that what is called "Rolling".
which one of the two statements below is correct?
⇒The wheel rotates because making a force on the center of mass of the wheel is like making a force on its rim and this force will let the wheel rotate.

⇒the wheel will rotate because of the frictional force, and if there is no frictional force the wheel will just translate.

why when I make a force on the center of mass of the wheel,the wheel will have a tendency to do slipping?

Last edited: Jun 21, 2015
2. Jun 21, 2015

### mathman

Without friction it will not rotate. Think of a wheel moving in space - it will not rotate unless you apply a force to the rim.

3. Jun 21, 2015

### Drakkith

Staff Emeritus
The wheel rotates because of the frictional force between the rim and the ground. If you pick up a bicycle and move it around, the wheels aren't rotating because there's no frictional force.

What do you mean? Unless the friction between the wheel and the axle is larger than the friction between the ground and the wheel, the wheel will rotate, not slip. (unless I'm misunderstanding what 'slip' means)

4. Jun 22, 2015

### AYPHY

Rotation, rolling and sliping are different when you apply force on centre of wheel radius from center is zero and hence there is no torque as torque= r X F. and if r is 0 hence no torque and no angular acceleration means no change in angular velocity hence no rotation. But it starts rotating because of friction which acts on the point of contact rotates the body here r is also present with force also hence the body rotates.

Rolling means pure rolling where v is equal to rω and velocity of point of contact is 0.
Rolling with forward slipping means v>rω
and with backward skipping means v< rω.

5. Jun 22, 2015

I was really reading the book 'fundamentals of physics'by Halliday,in chapter 11 section 11.4,which is talking about :The forces of Rolling.
But I get confused and that's why I wrote the first post,so I just need more explanation about it.
The first paragraph states:
"If a wheel rolls at constant speed,as in fig 11-3,it has no tendency to slide at the point of contact p,and thus no frictional forces acts there(I didn't get why there is no frictional forces acting in this case).However,if a net force acts on the rolling wheel to speed it up or to slow it,then the net force causes acceleration a→com of the center of mass along the direction of travel,it also cases the wheel to rotate faster and slower,which means it causes an acceleration α.These accelerations tend to make the wheel slide at p(I didn't get this).Thus,a frictional force must act on the wheel at to oppose that tendency(I didn't get this either :p).
If the wheel does not slide,the force is a static frictional force f
s and the motion is smooth rolling.(I didn't get why)
If the wheel does slide when the net force acts on it ,the frictional force is a kinetic frictional force fk. fig 11-3 and another figure are shown below.

6. Jun 22, 2015

### A.T.

7. Jun 22, 2015

### Mixander

The first is explain about newton's 1st law. The second is explaining about Newton's 2nd law. The third is explaining about newton's 3rd law.

8. Jun 22, 2015

### mathman

There appears to be two contradictory statements The first sentence says if the wheel rolls, there is no frictional force. The next to last statement says if the wheel does not slide, there is a static frictional force. I believe most of us would agree with the second (next to last) sentence.

9. Jun 22, 2015

### Mixander

The first sentence is the first situation that happening in the wheel, that is the wheel is rolling and move with constant speed without any force (newton's 1st law) and that's means no frictional force, but in the second, there is force in the wheel that makes the wheel move more slower or faster, then that "rolling" wheel will got frictional force.

10. Jun 23, 2015

### AYPHY

see there are two types of rolling one is accelerated and other is non accelerated in accelerated a=rα and other is v=rw.

in v=rw relative velocity of pt.of contact with ground is 0 hence no friction force. but force is acting then balance a=rα.

Want to read a book then i will suggest you to read mechanics part-2 d.c pandey its of indian author. really nice book it will clear all your concepts

11. Jun 23, 2015