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

[razor_charles]

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.

 

[Simon Bridge]

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.

 

[razor_charles]

no it has to be like a person with 5 or more forces acting on him/her.

 

[technician]

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!

 

[Simon Bridge]

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"?

 

[razor_charles]

one other question is that, is it possible to draw force arrows pointing inwards and not outwards? like for this diagram http://www.google.ca/imgres?q=barba...6&tbnw=68&start=0&ndsp=32&ved=1t:429,r:12,s:0

 

[Matterwave]

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.

 

[Simon Bridge]

@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?

 

[Matterwave]

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).

 

[Simon Bridge]

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.

 

[Matterwave]

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.

 

[Simon Bridge]

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.

 

[razor_charles]

my main question is ... in this diagagram : http://www.google.ca/imgres?q=barba...9&tbnw=74&start=0&ndsp=70&ved=1t:429,r:12,s:0


where will the force of the shoulder pads and thigh gear be placed? like will the arrow be located in the thigh? and shoulderS? please ANSWER QUICKLY , THX in ADVANCE

 

[Simon Bridge]

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.