# Solving a Physics Problem with Friction: Directions of Forces and Torques

• sapz
In summary, the conversation is about a question involving a ball in a bowl with different forces and torques acting on it. The person is having trouble understanding the solution and is asking for clarification on the direction of the forces and torques. There is also a discussion about the correct torque equation and its effect on the answers.
sapz

## Homework Statement

Hi

Im trying to solve this question, and I have the solution of it in front of me, but I really can't understand why its true.

The question is this: A bowl with radius R is given, it can't move. In it there's a ball with radius r << R, and a mass m.
Find... (lots of subclauses for the question)
A picture is attached.

My question is this:
Am I mixing the directions of the forces/torques?

F is the friction force. I think the torque equation should be I*w_dot=F*r.
and the equation for the tangential direction should be ma=F-mgs*sin(theta).

However, in the solution I see here the torque equation appears with a minus sign: I*w_dot=-F*r, which obviously changes the answers of the things I need to find.

Should it be with a minus? Or without? Thank you.

## The Attempt at a Solution

#### Attachments

• rolling.png
14.8 KB · Views: 459
sapz said:
F is the friction force. I think the torque equation should be I*w_dot=F*r.
This doesn't look right to me at all. If we take moments about the centre of the ball, there is indeed a torque Fr anticlockwise, and it does tend to reduce theta. But ##\ddot{\theta}## is not the angular acceleration of the ball.
There should be another angle in the picture - the angle through which the ball has rotated. It is related to theta by a simple formula.

## 1. What is friction and how does it affect a physics problem?

Friction is a force that opposes motion between two surfaces in contact. It can affect a physics problem by changing the direction and magnitude of forces and torques, making the problem more complex to solve.

## 2. How do I determine the direction of friction in a physics problem?

The direction of friction can be determined by considering the direction of motion of the object and the surface it is in contact with. Friction always acts in the opposite direction of motion or potential motion.

## 3. How do I incorporate friction into my free body diagram?

To incorporate friction into a free body diagram, you must include a force vector in the opposite direction of motion or potential motion. This will account for the frictional force acting on the object.

## 4. How does friction affect the calculation of torque in a physics problem?

Friction can create a torque that opposes the torque created by other forces. This can change the overall magnitude and direction of torque in a problem, requiring careful consideration and calculation.

## 5. Can friction ever be a beneficial force in a physics problem?

Yes, friction can provide stability and prevent slipping or sliding in certain situations. For example, friction between the tires and the road allows a car to maintain traction and prevent skidding.

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