How Does Friction Affect Rotational Motion in Physics Problems?

In summary, the rotational motion of a ball is when a ball spins or rotates on its axis. This motion can be affected by factors such as the force applied to the ball, the surface it is rolling on, and any external forces acting on it. The direction and speed of the rotation can also change depending on these factors. Understanding rotational motion is important in various fields, including sports, physics, and engineering.
  • #1
vu10758
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These problems have diagrams with them so I posted those pictures are at
http://viewmorepics.myspace.com/index.cfm?fuseaction=viewImage&friendID=21023924&imageID=1420239256

1) A spherical ball rolls up an incline. Determine the acceleration of the ball and the magnitude and direction of the friction force.

Based on the diagram, I know that friction is pushing the ball up the incline. I know that the ball is sphere so I = 2/5 MR^2. I drew free body diagrams with N pointing diagonally up to the left, mg pointing straight down, and friction point diagonally right and along the incline.

so

y: mgcos(theta) - N = 0
x : mu*mg*cos(theta) - mgsin(theta) = ma

a = mu*g*cos(theta) - gsin(theta)

I don't know mu and friction so I cannot solve for a. What am I missing to allow me to solve this problem? The correct answers are a = (5/7)g*sin(theta) and f = (2/7) mg *sin(theta). I don't know what I am missing.

2)

A string unwinds from a disk and the disk rolls smoothly. Explain why the direction of friction is unknown in the problem. The picture for this is at the link above.

I don't know why. Is it because of the fact that the disk will be pulled to the right, and it is uncertain whether it would roll. If it rolls to the right, wouldn't friction resist to the left. But then again, there must be friction pushing it to the right to make it roll. Why don't we know the direction of the friction. Am I on the right track explaining it?

Thanks for your help.
 
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  • #2
For the second question, in physics world once a wheel is moving with no acceleration (forget how it got moving), there is no net force acting on it. You might think in order for the wheel to move there has to be friction, but think of a wheel that began to spin on a road and then hit a patch of ice. It would continue to spin because no friction acts on it
 
  • #3
Thanks very much.
 

Related to How Does Friction Affect Rotational Motion in Physics Problems?

1. What is rotational motion?

Rotational motion is the movement of an object around an axis or center point. This type of motion is often seen in circular or spherical objects, such as a ball.

2. How is rotational motion different from linear motion?

Unlike linear motion, which involves movement along a straight line, rotational motion involves movement around a fixed point. In rotational motion, the distance traveled by a point on the object is measured by its angle of rotation, rather than its linear displacement.

3. What factors affect the rotational motion of a ball?

The rotational motion of a ball is affected by several factors, including the mass of the ball, the radius of the ball, the force applied to the ball, and the moment of inertia of the ball.

4. How is rotational motion measured?

Rotational motion can be measured using angular velocity, which is the rate of change of the angle of rotation over time, and angular acceleration, which is the rate of change of angular velocity over time. These measurements can be taken using specialized tools such as an accelerometer or a gyroscope.

5. What is the relationship between rotational motion and energy?

Rotational motion involves the transfer of kinetic energy from the object's linear motion to its rotational motion. This is known as the principle of conservation of angular momentum, which states that the total angular momentum of a system remains constant unless acted upon by an external torque.

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