Frictional force in a free-body diagram

AI Thread Summary
The discussion centers on understanding the direction of frictional force in a free-body diagram involving a 50 kg block. The friction force is directed downward because it opposes the relative motion that would occur if the block were to move downward, pulling the top block with it. If the 50 kg block were to move, the top block would tend to remain in place due to inertia, creating a scenario where friction acts downward to keep both blocks moving together. The key takeaway is that friction always acts to oppose the relative motion between surfaces in contact. This clarification resolves the confusion regarding the direction of the friction force in the top free-body diagram.
zeralda21
Messages
119
Reaction score
1

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?
 
Physics news on Phys.org
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
 

Thread 'Rolling without slipping on a curved surface'

Kindly see the attached pdf. My attempt to solve it, is in it. I'm wondering if my solution is right. My idea is this: At any point of time, the ball may be assumed to be at an incline which is at an angle of θ(kindly see both the pics in the pdf file). The value of θ will continuously change and so will the value of friction. I'm not able to figure out, why my solution is wrong, if it is wrong .
View full post »
Thread 'Voltmeter readings for this circuit with switches'

Thread 'Voltmeter readings for this circuit with switches'

TL;DR Summary: I would like to know the voltmeter readings on the two resistors separately in the picture in the following cases , When one of the keys is closed When both of them are opened (Knowing that the battery has negligible internal resistance) My thoughts for the first case , one of them must be 12 volt while the other is 0 The second case we'll I think both voltmeter readings should be 12 volt since they are both parallel to the battery and they involve the key within what the...
View full post »
Thread 'Trying to understand the logic behind adding vectors with an angle between them'

Thread 'Trying to understand the logic behind adding vectors with an angle between them'

My initial calculation was to subtract V1 from V2 to show that from the perspective of the second aircraft the first one is -300km/h. So i checked with ChatGPT and it said I cant just subtract them because I have an angle between them. So I dont understand the reasoning of it. Like why should a velocity be dependent on an angle? I was thinking about how it would look like if the planes where parallel to each other, and then how it look like if one is turning away and I dont see it. Since...
View full post »

Similar threads

Is it friction or tension acting on the person hanging on a rope?
Replies
4
Views
1K
Relation between Friction, Velocity, and Acceleration
Replies
16
Views
1K
Coefficient of kinetic friction of a block sliding across the ceiling
Replies
7
Views
1K
Why is the direction of kinetic/static friction up a ramp?
Replies
7
Views
1K
Free body Diagram of a Plank resting on two Cylinders
Replies
14
Views
3K
Why Is Average Thermal Power Not Negative in This Friction Scenario?
Replies
1
Views
1K
How friction acts on a block moving down a slope moving side to side
Replies
2
Views
903
For the friction formula when to use Fn(normal force)×mu and when F×mu
Replies
17
Views
2K
Vertical Mass Spring System | Analyzing work
Replies
18
Views
1K
Find the direction and the magnitude of Frictional force
Replies
3
Views
807

Hot Threads

Recent Insights

Back
Top