# Vector force& and Optic question

1. Apr 15, 2012

### lionely

Log question

The diagram shows a log being pulled up an earth slope from X to y.

How much work is done against friction, the answer I get is not among the 4 choices..

also the part messed up by the scanner says 1000N force due to friction

The diagram shows a boy doing homework. He has a difficulty reading his book under the small light bulb.

Draw a ray diagram to show the shadow produced by his head on the table. What difference would it make to the shadow if the light bulb were replaced by a long fluroescent bulb? Draw a diagram to illustrate this case.

my attempt...

the darker part is the umbra

and the strokes are the penumbra does that drawing make any sense?

2. Apr 15, 2012

### PeterO

"Log question

The diagram shows a log being pulled up an earth slope from X to y.

How much work is done against friction, the answer I get is not among the 4 choices..

also the part messed up by the scanner says 1000N force due to friction
"

The question also says the log was dragged 50m

If you oppose a 1000N force over a distance of 50m you certainly get one of the options.
Perhaps you were working out a total energy change brought about by dragging up the hill as well.

3. Apr 15, 2012

### lionely

is it 50,000 J?

4. Apr 15, 2012

### PeterO

In short, the drawing does not make any sense.

Better look up umbra and Penumbra

http://en.wikipedia.org/wiki/Umbra

is a start, though most Physics texts have great diagrams.

5. Apr 15, 2012

### PeterO

That sounds logical to me.

6. Apr 15, 2012

### lionely

Okay thanks sir I'll try to redraw the diagram, I drew the diagram in a rush.

7. Apr 15, 2012

### PeterO

Don't forget they said "replaced by a long fluroescent bulb".

That means it behaves like an array of globes perhaps 1m long.

8. Apr 15, 2012

### lionely

I'm sorry I don't follow, what would be 1m long the shadow?

9. Apr 15, 2012

### PeterO

The light source.

Have you seen the long flourescent tubes they use in many businesses, schools and many home garages / basements - instead of a normal Globe light fitting.

http://en.wikipedia.org/wiki/Fluorescent_lamp