man0005's physics thread :)

[man0005]

hey guys
any help would be appreciated ^^

first question: Why are graphs of Hooke's Law experiments usually plotted as force vs extension graphs, when the extension appears to be a consequence of the weigh force applied, that is, the dependent varuable, which is usually plotted on the vertical axis?

my guess is that the force is dependent on the mass being used, while the extension is related to the spring constant? not really sure :S

[xZero]

I'm guessing that f=kx, if you plot force vs extension, you can find k by finding the gradient of the graph

[man0005]

ah okay
how does Newton's Third Law relate to Hooke's Law experiment?

[schnappy]

The force of weight on the string is equal and opposite to the force applied to the mass by the string?

[Whatlol]

well firstly what does f=-kx actually signify... its saying there is a restoring force proportional to the displacement of the spring and that the restoring force is in the opposite direction of the displacement.

hehe well i just read schnappys answer so im removing mine XD

[evaever]

the force required to stretch/compress depends on the extension/compression

[Whatlol]

The force of weight on the string is equal and opposite to the force applied to the mass by the string?

or is the set of reaction forces, the weight force on the mass and the restoring force on the spring

[man0005]

hmm there are 4 options
A - F on spring by mass of Earth
B - F on spring by hanging mass
C - F on hanging mass by spring
D- F on mass of Earth by hanging mass

and just one more question, if there are two springs in a system and a 150g mass attached to the bottom
then the force applied by Spring 2 to Spring 1 would just be 1.5 N yeah?

[Aurelian]

first question: Why are graphs of Hooke's Law experiments usually plotted as force vs extension graphs, when the extension appears to be a consequence of the weigh force applied, that is, the dependent varuable, which is usually plotted on the vertical axis?

I think it's fairly easy to argue the opposite - springs, after all, aren't just stretched by applying some weights onto one end and letting them hang...

Rather, you stretch a spring until you get a desired extension - you don't say "I'm going to apply x amount of force". After all, is it even humanly possible to be able to apply exactly a certain amount of force to something without consulting instruments? No, not really!

[man0005]

How do you tell what direction torque is (clockwise or anticlockwise)
e.g
While painting a tall building, a 70.0 kg painter stands 4.00 m from the end of a 6.00 m long plank that is supported by a rope at either end. Tthe plank has a mass of 20.0 kg. Determine the tension in each rope.

[man0005]

how would i do this one (attached)

i know it increases from B to P
but i dont get why it decreases for A and increases for B, from A to B

[onur369]

Is this motion or... ?

[Aurelian]

Is this motion or... ?

No it's structures and materials.

how would i do this one (attached)

i know it increases from B to P
but i dont get why it decreases for A and increases for B, from A to B

Net torque must be the same because the system is in rotational equilibrium. Thus, as it walks away from A, distance increases:

T = F*d

If T is constant, and d increases, then F must decrease.

The opposite is the case for the other beam.

[man0005]

ahh i get it now. thanks!
just one more thing, could you please elaborate on this:

The opposite is the case for the other beam.

why is this so?

[Aurelian]

ahh i get it now. thanks!
just one more thing, could you please elaborate on this:

The opposite is the case for the other beam.

why is this so?

So for that, T is constant once again, but in this case as you go closer to B, the distance between you and B decreases. Ie:

T = F*d

d is decreasing, T is constant, and therefore F must increase to compensate