# Homework Help: Rolling with slipping

1. Jun 17, 2013

### andyrk

What do we mean by the statement "Rolling with slipping"? I couldn't imagine this situation as a real life scenario.. I mean to say that if the body (say a solid sphere) is slipping/skidding then how can it rotate? If it will be rotating it won't be slipping/skidding. How can the two happen together?

2. Jun 17, 2013

### consciousness

When
v≠Rω,v≠0,ω≠0.

3. Jun 17, 2013

### Redbelly98

Staff Emeritus
Imagine a car "peeling out". Shortly after the car has started moving (the wheels are rolling), the wheels are slipping, spinning a lot faster than a hypothetical wheel moving at the same speed without slipping.

4. Jun 17, 2013

### andyrk

How is the wheel slipping then? As soon as the car is is set into motion or say even when the car is not moving the wheels are rolling and not slipping/skidding. Won't slipping/skidding mean that the wheels are just moving forward without rotating/rolling, just like when we move a block?

5. Jun 17, 2013

### voko

Ever seen or driven a car on ice or in mud?

6. Jun 17, 2013

### andyrk

Yes, but then the wheel is just rammed into the mud, so it is stationary in translatory motion. So it is not slipping is it?

7. Jun 17, 2013

### voko

I do not understand what you are saying.

8. Jun 17, 2013

### andyrk

Maybe I am inferring slipping/skidding/sliding incorrectly.(Can I use slipping/skidding/sliding interchangeably and same question for using rolling and rotating?) According to me, slipping of a body means it need to translate just as we say that a block is sliding on the horizontal surface. So according to your example of a car in mud, there can be 2 cases, namely: Either the wheel is totally stuck in the mud so it cannot move/translate ahead or the wheel does move ahead but slowly due to large friction by mud. In the former case, the wheel is rotating but the wheel as a whole is not translating and hence not slipping/sliding. In the latter case the wheel is rotating and translating both but still I don't think this should be considered as rolling with slipping. This is because of the following example: A cyclist is cycling on his bicycle. The wheel of the bicycle are rolling without slipping. THe bicycle brake system is very powerful. Suddenly a pedestrian comes in the view of the cyclist and he applies the brakes. As soon as the brakes are applied the wheels stop rotating instantaneously and the then skid with the point on the ground that was in contact with the ground when the brakes were applied remaining in contact with the ground all along the way while skidding. So the bicycle stops after some time due friction by road. This whole process when the wheels stopped rotating and started skidding upto the point when they stopped is what I consider skidding but clearly this is without rolling/rotating. Going on with this I don't understand how could the wheels of the bicycle would have rotated/rolled while simultaneously skidding also?

9. Jun 17, 2013

### voko

You obviously have never seen a car in mud or on ice, which is a pity. In mud and ice, a car may be going (translating) at some speed, and its wheels can be spinning so fast that the tangential velocity at the point of contact with the surface is much greater than the translational velocity. Many cars these days are equipped with "traction control" systems which apply brakes to wheels when this condition is detected.

It is more difficult to visualize that a wheel may even be slipping "in the opposite direction" to motion, but this can easily happen when a car suddenly plunges into mud or ice, so it may spin about a vertical axis and end up going "backwards", with its wheels "counter-rotating".

10. Jun 17, 2013

### Staff: Mentor

Have you ever been bowling? When you toss the bowling ball down the lane, it is initially slipping (not rotating down the lane), but friction soon gets the ball rolling in the direction of the lane, and often the ball is no longer slipping when it hits the pins at the end of the lane.

11. Jun 17, 2013

### andyrk

So why do we say "Rolling with slipping"? As you said the ball is initially not rolling but slipping. And also for the friction part you are saying does the friction change from kinetic friction to static friction as the ball starts rolling?

12. Jun 17, 2013

### andyrk

This message is posted by mistake.

13. Jun 17, 2013

### Staff: Mentor

No, the friction is kinetic until near the end of the lane. The ball is rolling more slowly than if it were completely rolling without slipping. Search YouTube to see what it looks like.

14. Jun 17, 2013

### voko

I am not sure how else I can explain that it is perfectly possible that an object can be translating with some velocity, and it may also be spinning at the same time, and its tangential velocity at the contact with the underlying surface my be different in magnitude and direction from the translational velocity. This is known as rolling with slipping.