# Homework Help: Another question(s)

1. Oct 16, 2005

### PhysicBeginner

Hi everyone, I have another question which troubles me. It says
" A 5g spider is lowering itself to the floor. Its web has a tensile strength of 0.03N. What is the minimal acceleration with which the spider can lower itself?"

Well my question is what do they mean by minimal acceleration and what equation can I use to find it. Just to let you guys know I'm only in Gr. 11 physics so I won't understand any complicated equations that are used in Universities or higher grades. Thanks.

Last edited: Oct 16, 2005
2. Oct 16, 2005

### Päällikkö

No complicated equations are needed, this is more like a conceptual thing.

Draw a diagram.
Why can't the spider stay still on the web? What is N, the force exerted by the web, if the spider does not move?

3. Oct 16, 2005

### PhysicBeginner

Well what I'm trying to find is the minimal acceleration of the spider so I think the spider have to move to have acceleration?

4. Oct 16, 2005

### Päällikkö

True, that's the info given in the problem. If the spider would, however, stay still, what would happen? Why?

5. Oct 16, 2005

### PhysicBeginner

could you give me a little more hint at what your getting at?

6. Oct 16, 2005

### Päällikkö

The web would snap, as it can only hold 0,03 N and the spider weighs 0,005 kg * 9,81 m/s^2.

If you already haven't, draw a diagram with all the forces that are acting on the spider.

7. Oct 16, 2005

### PhysicBeginner

so what you are saying is that the force of gravity acting on the spider is 0.05N which is 0.02 N more then the web's capacity which in term causing it to snap?

So is this a trick question in which I can't find an actual minimal acceleration?

8. Oct 16, 2005

### Päällikkö

Yes.
No.

Please, draw a diagram. The forces acting on the spider are N (which I explained earlier) and G, the force caused by gravity (plus air resistance, but that doesn't count). We know that F (the net force) = ma (Newton II). What is the net force?

9. Oct 16, 2005

### Janus

Staff Emeritus
No. The web will snap if the spider sets still, but the Spider is accelerating downwards and this will reduce the tension on the web. (example: If the Spder were to accelerate down the thread at 9.8m/sec2, then this would be the same as if he were freefalling, and he would put zero tension on the thread.)

The question is asking what the least downward acceleration he must have in order to reduce the tension on the thread to below its breaking point.

10. Oct 16, 2005

### PhysicBeginner

so the acceleration would be smaller than 9.8m/s^2 right?
So I just have to find the acceleration which gives a Force smaller than that of the thread so it wouldn't snap?

like
0.29=0.0005*a
a=5.8m/s^2 ?

Last edited: Oct 16, 2005
11. Oct 16, 2005

### Päällikkö

No. If you set a = g, tension should be 0.

As I've said earlier: draw a diagram.
Can you figure out the net force acting on the spider?

12. Oct 16, 2005

### PhysicBeginner

The net force is 0.02N[Down]=0.05N[Down] (The Force of gravity acting on the spider)-0.03N[up] (The Force of the tread pulling the spider up) ?

13. Oct 16, 2005

### Päällikkö

Yep, that looks more like it.

(For greater accuracy: Instead of using 0,05 N, use 0,005 kg * 9,81 m/s^2).

14. Oct 16, 2005

### PhysicBeginner

So now that I've found the net force acting on the spider do I then find the acceleration with the net force like this?

0.02N=0.005*a
a=4m/s^2
so the minimal acceleration is 4m/s^2 which eliminates the extra 0.02N acting on the thread?

15. Oct 16, 2005

### Päällikkö

Correct. (Actually 3,81 m/s^2, if you use 9,81 instead of 10 for gravity, but that rounds to 4 m/s^2.)

16. Oct 16, 2005

### PhysicBeginner

Yes I got it at last. Thanks Paallikko for all your patience and time. Thank you soooooooo much!!