# Frictional force in a free-body diagram

• zeralda21
In summary, the friction force in the top diagram is in the downward direction to oppose the motion of the 50 kg block.
zeralda21

## Homework Statement

I am having trouble understanding the direction of a friction force in a sample problem from Engineering Mechanics, Meriam & Kraige. I will attach a picture for no confusion.

http://i.imgur.com/0EdCofi.jpg

## The Attempt at a Solution

Consider only the top free-body diagram. Why is the friction force in that direction?

"The friction forces are assigned in the directions to oppose the relative motion which would occur if no friction were present." So where would the object move if there was no friction? Well, it would not since it is attached to a wire. If not attached, it would(or tendency) to move downward and therefore an frictional force in the opposite direction drawn. So, from my own conclusion I would have chosen:

1) Upward force or 2) No friction force at all since it would not move if there was no friction.

Why am I wrong?

The object held by the wire would not move. But the object just below would.

voko said:
The object held by the wire would not move. But the object just below would.

Let's say that the 50kg-block would move. Then I assume it would move downward and therefore there is a friction force F_1 in the upward direction, opposite the motion, exactly as in the SECOND free-body diagram. But that doesn't explain why the friction force in the TOP f-b diagram is in the downward direction. Right?

Apply Newton's third law.

voko said:
Apply Newton's third law.

Alright, I think I see it now. Thanks voko.

zeralda21 said:
Let's say that the 50kg-block would move. Then I assume it would move downward and therefore there is a friction force F_1 in the upward direction, opposite the motion, exactly as in the SECOND free-body diagram. But that doesn't explain why the friction force in the TOP f-b diagram is in the downward direction. Right?

No.

The friction opposes relative motion between the surfaces in contact. If the 50 kg block moved it would leave the top block behind without friction. So the force of friction between the blocks tends to move the top block together with the 50 kg one downward.

ehild

## 1. What is frictional force in a free-body diagram?

Frictional force is a force that opposes the motion of an object when it is in contact with another surface. It is represented by the symbol "Ff" in a free-body diagram.

## 2. How is frictional force represented in a free-body diagram?

Frictional force is represented by an arrow pointing in the opposite direction of the object's motion, in contact with the surface it is interacting with. The length of the arrow is proportional to the magnitude of the frictional force.

## 3. What factors affect the magnitude of frictional force?

The magnitude of frictional force is affected by the type of surfaces in contact, the force pressing the surfaces together, and the roughness of the surfaces.

## 4. How does frictional force impact the motion of an object?

Frictional force acts in the opposite direction of the object's motion, causing it to slow down or come to a stop. It also helps to maintain stability and prevent slipping or sliding.

## 5. Can frictional force be eliminated?

No, frictional force cannot be eliminated completely. However, it can be reduced by using lubricants or by using materials with less roughness in contact with each other.

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