Motion of a Sphere: No Friction vs With Friction

In summary, when there is no friction, a force parallel to the surface passing through the center of the sphere will cause purely translational motion. However, when there is friction, it will also cause rotational motion due to the torque created by the friction on the bottom of the sphere. The friction acts as an invisible torque, rotating the sphere equivalent to the product of the frictional force and the sphere's radius. The book's statement that there can be no rotational motion under any conditions is incorrect, as friction can still cause rotational motion.
  • #1
chaoseverlasting
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Homework Statement


A sphere is resting on a horizontal surface. A force parallel to the surface passing through the center of the sphere is applied.

Describe the motion of the sphere
1. Without any friction
2. With friction.



The Attempt at a Solution



When there's no friction, then the motion is purely translatory. But, if there is friction, it will apply a torque which will force rotation as well as translation. The second part isn't right though. Why?
 
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  • #2
It cannot apply any torque, because the force goes right through the center of the sphere. What happens is that the friction touching the ball below is the culprit in creating this invisible torque.
 
  • #3
The force initially applied does not have any torques but it is the friction itself that acts as a torque that rotates the ball equivalent to (frictional force*radius of the sphere)
 
  • #4
Yeah, that's what I thought, but the book says that there can be no rotational motion under any conditions. I thought the book was wrong but maybe I'd missed something.
 

1. What is the difference between motion of a sphere with no friction and with friction?

The main difference between these two types of motion is that a sphere with no friction will continue to move with a constant velocity and direction, while a sphere with friction will eventually come to a stop due to the opposing force of friction.

2. How does the mass of the sphere affect its motion with and without friction?

The mass of the sphere will affect its motion in both cases. A sphere with no friction will have a constant velocity regardless of its mass. However, a sphere with friction will experience a greater force of friction with a larger mass, leading to a quicker deceleration and shorter distance traveled.

3. Can the motion of a sphere with friction be described using Newton's laws of motion?

Yes, the motion of a sphere with friction can be described using Newton's laws of motion. Newton's first law states that an object will remain at rest or in motion with a constant velocity unless acted upon by an external force, which applies to the sphere with no friction. Newton's second law states that the force on an object is equal to its mass multiplied by its acceleration, which applies to the sphere with friction.

4. How does the surface on which the sphere is rolling affect its motion with friction?

The surface on which the sphere is rolling can greatly affect its motion with friction. A rough surface will provide more resistance and increase the force of friction, leading to a shorter distance traveled. A smooth surface will provide less resistance and decrease the force of friction, allowing the sphere to travel a greater distance before coming to a stop.

5. What are some real-life examples of motion of a sphere with and without friction?

Examples of motion of a sphere with no friction include a hockey puck sliding on ice or a ball rolling on a smooth surface. Examples of motion of a sphere with friction include a car rolling to a stop on a road or a ball rolling on a rough surface.

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