# Forces on an Accelerating Object

1. Dec 11, 2005

### sweet877

A 2.1 X 10^-4 kg spider is suspended from a thin strand of spider web. The greatest tension the strand can withstand without breaking is 2.0 X 10^-3 N. What is the maximum acceleration with which the spider can safely climb up the strand?

Fnet = ma = (2.1 X 10^-4)(a) < 2.0 X 10^-3
a < 9.52 m/s^2?

2. Dec 11, 2005

### PhY_InTelLecT

Is the spider hanging vertically? Is there no gravity? If there is, then the spider cant even climb up the web. As you can see, gravity alr require a greater acceleration than the ans that u have found. If there is gravity, the web will break under the spider's own weight...

Last edited: Dec 11, 2005
3. Dec 11, 2005

### sweet877

There is gravity...hmm...
So the force the spider is exerting w/ 0 acceleration is mg = 2.1 X 10^-4 (9.8) = 2.06 X 10^-3
Fnet = ma = (2.1 X 10^-4)(a)

4. Dec 11, 2005

### PhY_InTelLecT

ya.. The Weight of the spider as calculated by u, 2.06 X 10^-3N, is already more than the web can actually support.. Unless the qn requires you to assume that gravity is not present, then your previous solution is right..

5. Dec 11, 2005

### sweet877

6. Dec 11, 2005

### PhY_InTelLecT

If gravity is present.. Your working muz be this
Fnet= mg + ma < 2.0 X 10^-3..
By taking g as 9.81ms^2, you will see that , a will be negative. which means that the spider must decelerate..ya?

Last edited: Dec 11, 2005
7. Dec 11, 2005

### PhY_InTelLecT

oh, no problem at all.

8. Dec 11, 2005

### sweet877

Wait...wouldn't gravity be negative though?

9. Dec 12, 2005

### PhY_InTelLecT

The -ve sign of gravity actually defines the direction of the force in which it is acting. However, when you encounter qns like this, it all comes down only to the absolute value, magnitude. Since both the g and a are in the same direction, the -ve sign does not matters anymore.
What you will get is just a negative value for Fnet, but note: the negative doesn't represent the value!, It just shows that the Fnet that u found is at an opposite direction to tension. Tension is upwards, thus +ve, while Fnet is downwards, thus -ve. And when T-Fnet>0 in order not for web to break, T > Fnet.

Last edited: Dec 12, 2005
10. Dec 12, 2005

### sweet877

Oh OK...I get it now. Thanks!

11. Dec 12, 2005