# FBD Description

1. Apr 7, 2014

### tmobilerocks

1. The problem statement, all variables and given/known data

Describe a real situation that would give rise to the FBD of the block. N is normal, T is tension, W weight, f static friction

2. Relevant equations
Fnet = ma

3. The attempt at a solution
A block glued to a wall, while someone pulling a string down on it.

2. Apr 7, 2014

### SammyS

Staff Emeritus
The glue's effect probably won't count as friction.

Where would the normal force come from ?

3. Apr 7, 2014

### tmobilerocks

The normal force would come from the surface, say a wall

4. Apr 7, 2014

### SammyS

Staff Emeritus
Under what condition(s) would the wall be exerting a horizontal force on the block ?

5. Apr 7, 2014

### tmobilerocks

If you push with a horizontal force, canceling the normal force. Problem is I don't think you can add your own force.

6. Apr 7, 2014

### SammyS

Staff Emeritus
How about if the wall pushes with a constant force ? (In what direction would that have to be?)

What situation would require such a force? -- At least the magnitude of the force might be constant.

7. Apr 7, 2014

### tmobilerocks

Block at rest on a rough wall. Man pulls the string attached to block to balance with static friction?

8. Apr 7, 2014

### SammyS

Staff Emeritus
You need a normal force for there to be friction.

The normal force is generally a "reaction" force, so if the wall exerts a force of N (to the left) on the block, the block is exerting a force of magnitude N on the wall (to the right.)

It looks like there is only one horizontal force exerted on the block, namely N, which must be unbalanced if the magnitude of N is not zero. What would Newton say about that?

9. Apr 8, 2014

### tmobilerocks

Will accelerate in normal force's direction

10. Apr 8, 2014

### tmobilerocks

Person just lets go off block originally held by hand

11. Apr 8, 2014

### SammyS

Staff Emeritus
Then there is no longer any normal force -- once the person lets go.

12. Apr 8, 2014

### SammyS

Staff Emeritus
Yes.

13. Apr 8, 2014

### tmobilerocks

So what is an example of a real situation?

14. Apr 8, 2014

### SammyS

Staff Emeritus
It seems that you are to come up with the answer.

15. Apr 8, 2014

### tmobilerocks

Two blocks on table. Second block accelerating to left, exerting a normal force on first block. Person pulls down string on block.

16. Apr 8, 2014

### tmobilerocks

Anyone else willing to offer more tips/guidelines to solving this problem?

17. Apr 8, 2014

### PhanthomJay

18. Apr 9, 2014

### tmobilerocks

Man still not getting it... can someone please offer specific advice? I understand the principles. It must be accelerating to the left, but I can't think of a real life example.

19. Apr 9, 2014

### SammyS

Staff Emeritus
Wow!

Jay gave an excellent suggestion -- to think about (rides) at an amusement park.

Initially, you might not have the tension involved.

20. Apr 9, 2014

### tmobilerocks

How about a larger block (m3) moving to the left which exerts a normal force on m2. m2 is connected to m1 via a string. m2 and m1 have the right values so they do not move vertically; instead they move at the same acceleration as m3.