What is a free body diagram and how can it be used to analyze forces on a body?

Click For Summary

Discussion Overview

The discussion revolves around the concept of free body diagrams and their application in analyzing forces acting on a body. Participants explore the requirements for such diagrams, including the number of forces and specific scenarios, while also considering examples from various contexts.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant requests a system diagram with at least five forces acting on a body at rest.
  • Another suggests a simple block with five arrows, but this is challenged by a participant who specifies the need for a human figure.
  • Examples of forces acting on an airplane in level flight are provided, noting that it typically has four forces, with a fifth potentially being lift due to buoyancy.
  • A creative scenario is proposed involving a person being pulled by five ropes, each representing a different force, including gravity.
  • Questions arise about the possibility of drawing force arrows pointing inwards, with some participants agreeing that inward arrows can represent forces effectively.
  • A participant describes a block connected to another block via a rod being pushed up a ramp, identifying five forces acting on the system.
  • Discussions about the nature of forces such as pushing and tension lead to clarifications about their roles in the context of the block's movement.
  • Concerns are raised about the variable nature of friction when analyzing forces, especially when considering static scenarios.
  • One participant inquires about the placement of force arrows in relation to specific gear on a character in a diagram, seeking guidance on how to represent these forces accurately.
  • A response suggests placing forces where they act on the body and breaking them into components for analysis.

Areas of Agreement / Disagreement

Participants express differing views on the requirements for free body diagrams, the nature of forces, and the specifics of force representation. There is no consensus on a single approach or example, and multiple competing views remain throughout the discussion.

Contextual Notes

Participants mention various scenarios and examples, but limitations exist regarding the clarity of force directions, the complexity of friction, and the assumptions made about static versus dynamic conditions. The discussion does not resolve these complexities.

razor_charles
Messages
4
Reaction score
0
Hi i just wanted to know if anyone can help me located a system diagram with atleast 5+ forces acting on it? P.S. the diagram it self should be in rest position.
 
Physics news on Phys.org
Why not just draw a block with five arrows on it?

Note: any irregular pentagon will give you the correct lengths and directions for the arrows.
 
no it has to be like a person with 5 or more forces acting on him/her.
 
It is difficult to find examples with more than 3 forces, let alone 5!
An airplane in level flight has 4 Obvious forces...thrust, drag, weight and lift.
If you felt it was OK to include lift due to buoyancy I suppose you have 5 forces!
 
OK - draw a picture of a person with five arrows pointing off them.

Maybe the person has been lassoed by five other people and they all pull in a different direction to hold the captive still?

Tie a rope to each limb, also tied to a horse each ... there's 5 forces: tension in each rope, and gravity.

Perhaps the person is seated at a computer, and there is a cat sitting on his/her lap with all claws slightly extended in affection - that's five forces: gravity/chair and one for each set of claws :)Arctic wolves count?
Diagram shows 5 arctic wolves pulling on a carcass.
[PLAIN]http://www.whitwellhigh.com/jcantrell/pwc/www.physicsclassroom.com/Class/vectors/u3l3a5.gif
... what is this for anyway, that you cannot google "five force statics"?
 
Last edited by a moderator:
A block connected to another block via a rigid rod being pushed up a ramp with friction has 5 forces. Gravity, normal force, friction force, pushing force and tension force from the rod.
 
@razor_charles: yes it is. An inwards force arrow is the same as an outwards one with the same magnitude and direction. Usually we do it when the force is a push rather than a pull.

@Matterwave: nice example - aren't the tension-force and pushing-force the same?
 
No, otherwise, the block wouldn't go up the incline. The pushing force must exceed both the friction and the "tension" and the gravitational force in the parallel direction (perhaps tension is not the best words, it's the "push back" from the rigid rod).
 
  • #10
Ah - it's the "bottom" block you are considering. (You are right, "tension" gave me entirely the wrong picture.)

it gets the push-back from pushing the top block, drag from friction, weight, normal-force, and the actual push. That's 5. Good call.

To fit the criteria - the block should be held stationary or moving at a constant speed, of course ;)

There's quite a lot - for eg. an aircraft could get thrust from more than one engine, giving more than 5 forces; a car parked on a slope has 4 normal forces, and four frictions (each tire), which would be especially important if there were an uneven mass distribution. Also take into account the springs in the suspension and it gets as complicated as you like.
 
  • #11
If the thing has to be at rest, perhaps my example is not the simplest because the friction force is variable (keeping the thing from moving), so it's not easy to tell which direction it points right off the bat, depending on how strong the pushing force is.
 
  • #12
You have static friction - no worries.
In analysis you just pick a direction and if you guessed wrong, the friction force will just come out negative.
(There would be two static solutions, one where you push against friction and the other where you let friction take some of the weight.)
... of course stationary, the blocks could probably just get treated as one.
 
Last edited:
  • #14
You'd put the forces where they would act on the body - so shoulderpads press the shoulders, thigh gear pull on the thighs, the shield pulls the arm etc.

You then break these forces into components through the center of mass of your barbarian warrior in that pose, and torques about the center of mass. From that, you get the free body diagram.
 

Similar threads

High School Elevator Question -- What does a downward acceleration mean?
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad How can you make a free body diagram for someone running then sliding?
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad What is the free-body diagram for a massless pulley in free fall?
  • · Replies 31 ·
2
Replies
31
Views
5K
Undergrad Calculating Free Body Diagram Forces
  • · Replies 4 ·
Replies
4
Views
3K
High School Can you me understand free body diagrams
  • · Replies 2 ·
Replies
2
Views
2K
Graduate How can I draw a free body diagram of a rotating wheel?
  • · Replies 3 ·
Replies
3
Views
4K
Undergrad How do I use a free body diagram in this case?
  • · Replies 3 ·
Replies
3
Views
2K
Graduate Free body diagrams, coordinate systems origin/orientation
  • · Replies 14 ·
Replies
14
Views
3K
Undergrad What Are the Rules for Drawing a Free Body Diagram?
  • · Replies 5 ·
Replies
5
Views
8K
High School Why is the 3 body problem unsolvable? And what will happen if someone solves it?
  • · Replies 26 ·
Replies
26
Views
2K