Free Body Diagram of a incline

  • Thread starter thonwer
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Homework Statement



Draw a free body diagram of the incline. Build the free body diagram of the man. Which is the force which makes the man to go up the incline?

http://imagizer.imageshack.us/v2/800x600q90/46/t92m.png [Broken]

Homework Equations



Newton's 2º Law

The Attempt at a Solution



For the incline I've put a reaction from the floor, a reaction from the van, weight from the man and the box and the normal reaction from the incline to the weights of the man and the box.

For the man I've put, the weight, the normal reaction from the incline to the weight, and the friction force going to the top of the incline.

I suspect I am missing some force for the incline free body diagram, can you help me please?
 
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Answers and Replies

  • #2
.Scott
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Is there force between the man and the box?
 
  • #3
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Yes, do I have to draw it too?
 
  • #4
.Scott
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Yes, do I have to draw it too?
It's asking for the free body diagram of the man.
(So, yes)
 
  • #5
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My question is about the free body diagram of the incline not the man.
 
  • #6
.Scott
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My question is about the free body diagram of the incline not the man.
In the case of the incline, you could treat the man and the box as a unit.
If it was my homework assignment, I would treat them separately. It wouldn't make a difference until the box started to move or the man started to slip. At that point, you would have a mixture of inertial and kinetic friction.
 
  • #7
tiny-tim
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hi thonwer! :smile:
For the incline I've put a reaction from the floor, a reaction from the van, weight from the man and the box and the normal reaction from the incline to the weights of the man and the box.
no, you're not applying the basic rule of a free body diagram:

only include the forces on the body

the weight of the box (or the man) only acts on the box (or the man), it does not act on the incline, and so it does not go on the free body diagram

the reaction from the box (or the man) does act on the incline, and so it does go on the free body diagram

(also you've missed out the friction, though it would be easier if you simply said "reaction" (not "normal reaction"), since that includes the friction)
 
  • #8
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hi thonwer! :smile:


no, you're not applying the basic rule of a free body diagram:

only include the forces on the body

the weight of the box (or the man) only acts on the box (or the man), it does not act on the incline, and so it does not go on the free body diagram

the reaction from the box (or the man) does act on the incline, and so it does go on the free body diagram

(also you've missed out the friction, though it would be easier if you simply said "reaction" (not "normal reaction"), since that includes the friction)

Ok, so I have the floor and the van forces, but I don't really understand how do the block and the man affect the incline, I know there is a friction force but I don't know its direction in the incline but I know it for the man :confused:
 
  • #9
tiny-tim
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between the man and the incline:​

there's a normal force, just like the normal force when the man is ion the ground

and there's a friction force

you can combine both of them into the total reaction force

between the incline and the man:​

exactly the same forces, equal in magnitude but in the opposite direction
 

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