Direction of Friction for Rolling Motion

In summary: What makes the ball start to roll if the force is applied through the center? If static friction points in the direction of motion, what is supplying a torque in the opposite direction to make the ball roll properly?The static friction is proportional to the tension because that is how the equation works. There is a force (tension) that needs to be greater than the static friction in order for the ball to start rolling.
  • #1
better361
24
1
A ball is attached with a string on the side. When the string is pulled up, static friction accelerates the ball forward while the tension rotates the ball.
When a ball is placed on a incline with friction, is kinetic friction opposing the motion and giving the ball torque while gravity accelerates the ball down? Is there also static friction is this case?

Also, does the ball lose energy in either case due to friction?
 
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  • #2
hi better361! :smile:
better361 said:
When a ball is placed on a incline with friction, is kinetic friction opposing the motion and giving the ball torque while gravity accelerates the ball down? Is there also static friction is this case?

Also, does the ball lose energy in either case due to friction?

in rolling (without slipping), there is only static friction (plus "rolling resistance" due to deformation)

does static friction ever do work? (hint: what is the definition of work done?)
A ball is attached with a string on the side. When the string is pulled up, static friction accelerates the ball forward while the tension rotates the ball.

what is the question? :confused:
 
  • #3
So would the direction of static friction be pointing to up on the incline then if there is no kinetic friction?

I would say static friction doesn't do any work because it only applies to stationary objects, and the ball can be treated as one at a particular instant. Am I right?

Ohh, I was supposed to ask the similar questions on that problem as the incline one. I thought that the only way for the ball to accelerate on the level plane was from static friction, because the other two forces, the tension in the string and gravity, are added with the normal force so that there is no net force in the upwards direction.
So, would the tension need to overcome the torque from the static friction to start rolling or would the ball roll as soon as I apply a bit of tension to the string?

Also, if a uniform electric field were to act on a ball with some charge, and the ball is on a surface with friction, what direction is the friction when I let the ball go?
 
  • #4
hi better361! :smile:

(i'm sorry for taking so long to reply :redface:)
better361 said:
I would say static friction doesn't do any work because it only applies to stationary objects, and the ball can be treated as one at a particular instant. Am I right?

yes, static friction from a stationary surface doesn't do work :smile:

work done, usually written as force "dot" displacement, is technically ∫ force "dot" velocity dt

the velocity is of the point of application of the force, so if (as here) that is stationary, the work done is zero

of course, if the surface the ball is on is moving, then the point of application is moving, and so the static friction does do work!

(eg a box on an accelerating trailer has increasing kinetic energy, supplied entirely by the static friction :wink:)​
Ohh, I was supposed to ask the similar questions on that problem as the incline one. I thought that the only way for the ball to accelerate on the level plane was from static friction, because the other two forces, the tension in the string and gravity, are added with the normal force so that there is no net force in the upwards direction.
So, would the tension need to overcome the torque from the static friction to start rolling or would the ball roll as soon as I apply a bit of tension to the string?

static friction is not fixed at µN, it is less than µN, and is proportional to the tension

so yes, it starts moving (rolling) even with a tiny amount of tension
Also, if a uniform electric field were to act on a ball with some charge, and the ball is on a surface with friction, what direction is the friction when I let the ball go?

the total electric force will act through the centre of the ball, so the result is the same as for any mechanical force (or torque) applied at the centre :wink:
 
  • #5
tiny-tim said:
the total electric force will act through the centre of the ball, so the result is the same as for any mechanical force (or torque) applied at the centre :wink:

What makes the ball start to roll if the force is applied through the center? If static friction points in the direction of motion, what is supplying a torque in the opposite direction to make the ball roll properly?

tiny-tim said:
static friction is not fixed at µN, it is less than µN, and is proportional to the tension

so yes, it starts moving (rolling) even with a tiny amount of tension

How exactly is the static friction proportional to friction? If the equation is r(T-F)=Iα, where F is the force from friction, wouldn't T, tension, need to be greater than friction for rolling to occur?

Also, why does the direction of friction depend on the radius at which I apply the torque?(new scenario)
 
Last edited:
  • #6
bump on questions from my last post.
 
  • #7
If there is force applied to the ball through the center, then the static friction exerted onto the surface of the ball is in the opposite direction of acceleration, resulting in angular acceleration or angular decleration of the ball.
 
  • #8
Too much to say about this, you may get more precise answers if you have more precised question. However, I will try to help you with some of the above discussed issues:

1) The force could NOT be considered at the center unless the object behaves as a particle (no rotation, no deformation). The rotating ball is definitely not a particle, so the force could NOT be considered at the center.

2) If the ball is rolling upward on an inclined plane, then the static friction is also upward resisting the rotating motion of the ball.

3) The ball can roll at constant speed if T = f. If T > f, the ball will accelerate. But at the start, T should be greater than f to make the ball starts to move.

4) static friction does not do any work because its point of application (point of contact between the ball and the surface) does not undergo displacement; each new instant a new point of application exists with no displacement.
 

What is the difference between static and kinetic friction?

The main difference between static and kinetic friction is that static friction occurs when two surfaces are in contact but not moving relative to each other, while kinetic friction occurs when two surfaces are in contact and moving relative to each other. Static friction is generally stronger than kinetic friction.

How does the direction of friction affect rolling motion?

The direction of friction affects rolling motion by providing a force that opposes the motion of the rolling object. This force is known as rolling friction or rolling resistance, and it acts in the opposite direction of the object's motion. The direction of rolling friction depends on the direction in which the object is rolling and the direction in which the surface is resisting the motion.

What factors influence the direction of friction for rolling motion?

The direction of friction for rolling motion is influenced by several factors, including the shape and size of the rolling object, the nature and texture of the surface it is rolling on, and the speed and direction of the rolling motion. The surface roughness, weight distribution, and air resistance can also affect the direction of friction.

How can the direction of friction for rolling motion be calculated?

The direction of friction for rolling motion can be calculated using the principles of physics, such as Newton's laws of motion and the concept of torque. The direction and magnitude of the rolling friction force can be determined by considering the type of contact between the rolling object and the surface, the angle of the surface, and the mass and velocity of the object.

Why is understanding the direction of friction important in engineering and design?

Understanding the direction of friction is important in engineering and design because it helps in predicting and controlling the motion of rolling objects. Knowing the direction of friction can also help in designing efficient and safe systems that involve rolling objects, such as wheels, gears, and bearings. It is also crucial in minimizing wear and tear and improving the overall performance and efficiency of machines and devices.

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