I suck at Physics, help me :(

[tl]

 Ok.. well it's like less than a week to school and I need to do some holiday homework  :buck2:

And I have this question i can't seem to get the hang of:

'Two masses of 4.5kg and 2.5kg, respectively, are connected by a light rope that passes over a smooth pulley as shown in figure 1.20'

I've created a 'figure 1.20' myself and attached it below..

the questions are:

'a What is the magnitude of the acceleration of the two masses?
b What is the tension in the rope?
c What is the net force acting on the 4.5kg mass?'

So if anyone has a clue, please help me.

And try your best to explain to me (with monkey-understandable terms too please ^^)

EDIT: I have more Questions for you peeps to help me with 

'A Mass of 5.0kg is being hauled up a smooth plane inclined at 30 degrees to the horizontal with an acceleration of 1.5ms^-2 by means of a string parallel to the incline.

a What is the tension in the string?
b If the plane was rough and there was a force of friction of 20N between the mass and the incline, what is the force in the string?'

I'm just no good at questions like these. Perhaps it doesn't help not doing Unit 1/2 Physics.

If anyone could provide me with like a formula chart of some sort that'd help 

[jay1993]

Draw the system in a straight line. It will be easier to understand. 2.5kg pulling to the left and 4.5kg pulling to the right. The force of the 'pulling' can be calculated by multiplying both masses by 10m/s (Or 9.8m/s whichever you prefer), as this is the acceleration due to gravity. (F=ma)
so if 25 N is pulling to the left and 45 N pulling to the right, what is the Net Force? 45-25 = 20 N toward the right hand side.
so the FOrce is 20N to the right on the whole system.
F=ma.
20 = (2.5+4.5)*a.
20 = 7a.
a= 20/7
a= 2.9 ms^-2 

At least i think thats a)..

Maybe you can take it from there? ( i just dont know how to answer to the others..)

[tl]

Thanks Jay, yeah that helped heaps  :smitten:

p.s What the hell is tension...? Is it like an opposing force of some sort?

[jay1993]

the force in the string...hard to explain of the top of my head..

[coletrain]

continuing for jay1993's answers;
b) since the acceleration = 2.9ms^-2 in the direction of the 4.5kg mass, the net force on the 4.5kg mass is equal to mg- T
mg-T=ma
4.5 x 10 -T= 4.5 x 2.9
T= 31.95N (approx 32N)

btw tension is the force applied by a sting (simple definition)

[appianway]

Thanks Jay, yeah that helped heaps  :smitten:

p.s What the hell is tension...? Is it like an opposing force of some sort?

Pretty much. It's a restoring force. Actually, maybe restoring's the wrong terminology. I meant to say that it "pulls" back towards the centre of the rope.

[coletrain]

for past c) we've already figured out, only a matter on rewriting it
net force= mg-T
=45-32
13N down

[tl]

Thanks heaps guys, I fully understand it now. 

[jay1993]

here...a diagram showing where the tension is on the 4.5kg mass. its in the opposite direction fo the 2.5kg mass.. ie: tension in a single rope is = but in opposite direction.

[fady_22]

here...a diagram showing where the tension is on the 4.5kg mass. its in the opposite direction fo the 2.5kg mass.. ie: tension in a single rope is = but in opposite direction.

But only if the object is not accelerating, right?

[jay1993]

Im not 100% sure but when i think about this pulley system in my mind, i beleive that there is no tension unless there is a force. If there is a non-zero net force then the object will accelerate. Think about it. If you where holding the system in your hands and pulled the masses away from each other (one slightly harder than the other) then only will the rope go rigid (ie: become tense, and therfore have tension).

[appianway]

There can be tension even if it's not accelerating. The net force will give you the magnitude of the acceleration... however, the net force is essentially the sum of the tension and weight force. Therefore no acceleration occurs only if the net force is 0, so when the tension and weight forces are of equal magnitudes.

[fady_22]

Yeah, I know, but I was just saying that the tension in the rope= weight force (in opposite directions), unless the object is accelerating (i.e. net force=0)