Find distance for gravity to do only -4 J of work

[P-Illiterate]

Homework Statement

[PLAIN]http://img137.imageshack.us/img137/9280/43448180.jpg
find distance for gravity to do only -4 J of work

Homework Equations

[URL]http://buphy.bu.edu/~duffy/PY105/12m.GIF[/URL]
Wg = mgX

The Attempt at a Solution

Can anyone help me from the first stepp

 

[P-Illiterate]

its urgenttt:(

 

[tiny-tim]

find distance for gravity to do only -4 J of work

The spring, and the speed, have nothing to do with it …

the question only asks about the https://www.physicsforums.com/library.php?do=view_item&itemid=75" by gravity.

 

[P-Illiterate]

I know this is wrong because there's no way the answer can be this simple ...

W=F.d
-4 = mgsin30 * d
d = -4/20sin30
d = -.4

.4 to the left

is that even righttt?

 

[iRaid]

There's no force of friction?

 

[The legend]

There's no force of friction?

Even if there is, it won't matter since the question is asked only based on gravity.

 

[noleguy33]

How are you getting 20sin30?

In particular, the sin30...

 

[The legend]

You should use the concept of Work-Energy to solve this, since loss in energy is work.

 

[PhanthomJay]

I know this is wrong because there's no way the answer can be this simple ...

W=F.d
-4 = mgsin30 * d
d = -4/20sin30
d = -.4

.4 to the left

is that even righttt?

To the left? Not so simple...if the work done by a force is negative, what does that imply?

 

[P-Illiterate]

To the left? Not so simple...if the work done by a force is negative, what does that imply?

so the mass is going to the opposite direction of force ...

 

[P-Illiterate]

How are you getting 20sin30?

In particular, the sin30...

the weight component that acting on the mass

 

[P-Illiterate]

can someone help meee:((

 

[noleguy33]

the weight component that acting on the mass

and why are you using sin?

 

[P-Illiterate]

and why are you using sin?

the component will be sin?? should it be?

 

[tiny-tim]

Hi P-Illiterate!

(just got up :zzz: …)

I know this is wrong because there's no way the answer can be this simple ...

W=F.d
-4 = mgsin30 * d
d = -4/20sin30
d = -.4

.4 to the left

is that even righttt?

Yes! (assuming the 20 is correct)

except it's to the right … gravity acts down (try it! ), so https://www.physicsforums.com/library.php?do=view_item&itemid=75" is positive if you go down, negative if you go up!

 

[P-Illiterate]

Thank you guys :D
I'll be back with another problem soon ... hahahah xD ^-^