Physics again, masses/pulleys, accel, tension

[MikeMike]

two blocks are connected by a lightweight, flexible cord which passes over a frictionless pulley

m1=??
m2=??

what is the acceleration of the two blocks, and what is the tension in the cord.

(the blocks are hanging vertical)

accell = what? i cant think of the equation for this.

does it have to do with the difference in masses?

and tension i have tried 4 of 5 times, with no luck.

 

[biostud]

damn, thought this would be something about physX

 

[SynthDude2001]

Originally posted by: biostud
damn, thought this would be something about physX

 

[Fenixgoon]

assuming m1 is on the table and m2 is hanging off the table, the force of tension in the cord will be mass2(g). since this is the only force acting on mass1, m2(g) = m1(a) and your answer will obviously be in m/s^2

 

[pray4mojo]

F=MA

/thread

 

[TankGuys]

Originally posted by: nourdmrolNMT1
two blocks are connected by a lightweight, flexible cord which passes over a frictionless pulley

m1=??
m2=??

what is the acceleration of the two blocks, and what is the tension in the cord.

(the blocks are hanging vertical)

accell = what? i cant think of the equation for this.

does it have to do with the difference in masses?

and tension i have tried 4 of 5 times, with no luck.

Yes, it does!

Think about that for a second, see what you come up with. If you still have problems, reply to the thread or PM me

 

[MikeMike]

Originally posted by: Fenixgoon
assuming m1 is on the table and m2 is hanging off the table, the force of tension in the cord will be mass2(g). since this is the only force acting on mass1, m2(g) = m1(a) and your answer will obviously be in m/s^2

both are hanging, no table.

 

[TankGuys]

Originally posted by: nourdmrolNMT1

Originally posted by: Fenixgoon
assuming m1 is on the table and m2 is hanging off the table, the force of tension in the cord will be mass2(g). since this is the only force acting on mass1, m2(g) = m1(a) and your answer will obviously be in m/s^2

both are hanging, no table.

Think of it this way:

Both are hanging off opposite sides of the string, right? So if they were the same mass, What would happen to the system? Would it move, and if so, how fast?

If they are different masses, what happens to the system? Does it move then? How fast?

 

[pray4mojo]

Draw a force diagram. It'll help.

 

[MikeMike]

it they were both the same mass, it would be 9.8m/s^2 .

it will move to the side with the larger mass,

m1(g) = m2(a)

 

[TankGuys]

Originally posted by: nourdmrolNMT1
it they were both the same mass, it would be 9.8m/s^2 .

Okay, that's right if they are being pulled by gravity in the same direction, but if they are on opposite sides of the pulley, they are pulled in opposite directions, right?

Originally posted by: nourdmrolNMT1

it will move to the side with the larger mass,

m1(g) = m2(a)

Now you're getting somewhere, you're on the right track. Let's look at it this way now: tell me what forces (and use + or - to denote up or down) are acting on M1.

EDIT: I'm sorry, I can't just come out and give you the answer, gotta make you work for it. It's the teacher blood in me

 

[MikeMike]

m1 would have a -g and a +a caused by the -a of m2 which has a larger mass....

theres a reason i didnt give any numbers, i dont want the answer.

 

[TankGuys]

Originally posted by: nourdmrolNMT1
m1 would have a -g and a +a caused by the -a of m2 which has a larger mass....

theres a reason i didnt give any numbers, i dont want the answer.

Good idea

So, M1 has these forces acting on it:

Gravity: (Mass 1) x (9.81) (units omitted)
Tension from block 2: __________________ (fill in the blank)


M2 has these forces acting on it:

Gravity: (Mass 2) x (9.81)
Tension from block 1: __________________(Fill in blank)




Get equations for those bits, and then we'll hit the next step

 

[MikeMike]

tension from block 2 is 42.14 N

block 1 is 27.44N

 

[TankGuys]

So, M1 has these forces acting on it:

Gravity: (Mass 1) x (9.81)= -42.14N
Tension from block 2: 27.44N


M2 has these forces acting on it:

Gravity: (Mass 2) x (9.81)= -27.44N
Tension from block 1: 42.14N


Okay. So, you've got some numbers here. What do you think you ought to do now?

 

[MikeMike]

find the sum to find the tension in the rope?

27.44 + 42.14?

or do i need to find the difference?

 

[TankGuys]

Originally posted by: nourdmrolNMT1
find the sum to find the tension in the rope?

27.44 + 42.14?

or do i need to find the difference?

In a way, you're making it more complicated than you need to

Just look at one of the masses. There are only 2 forces acting on it. Gravity, and Tension. One pulls down, one pulls up. Since you know the mass, and you know the all the forces, you can find the acceleration of the block, yes?

Example: I push a 1KG block with 10N to the right. You push it with 20N to the left. That means there is a net force or 10N to the left... so you have 1KG, with a net force of 10N... not too hard to find the acceleration from that right?

 

[MikeMike]

well i would divide 10N by 1kg. but im confused, i used to know how to do this, but forgot. i took AP phys two years ago, now im in College Physics and forget how to do the basics.

 

[DrPizza]

hint: Once you find the net force acting on the system, don't forget that both blocks are accelerating, i.e. in F=ma, the mass isn't only the mass of one of the blocks.

On the "simpler" one mass is on a frictionless table, frictionless pulley then other hanging mass, a lot of people make the mistake in that problem of figuring out the weight of the hanging object, and using that as the force causing acceleration of the table object. While it's the correct force, both objects are accelerating and both masses must be accounted for..

 

[MikeMike]

grr its my final question, i went ahead and did all of the other questions.

so confused i forget how to do this.

 

[Howard]

"Free body diagram"

 

[kevinthenerd]

I'm a mechanical engineering major, but I'm not going to do the problem for you.

Summation of forces = mass * acceleration

Force of gravity = 9.8 * mass

Do you know vectors?

 

[Hacp]

stupid junior! Listen in class!

 

[kevinthenerd]

You need to find the acceleration. They're both hanging down, right?

Summation of forces = force of block 1 minus force of block 2 (since they're acting in separate directions)
(and be sure to remember, then, that your acceleration will be in the direction of block 1's force if it's the positive one)

Total force = total mass * acceleration

therefore total acceleration = total force / total mass

total force = force 1 - force 2

total mass = mass 1 + mass 2

force of one block = its mass * gravity

(The units of force are kg*m/s^2. Gravity is in m/s^2. You'll never forget formulas if you learn the units and derivation behind them.)

force = mass1*g - mass2*g
(g = 9.8 m/s^2)

force = force
(mass1 * g) - (mass2 * g) = (mass 1 + mass 2) * acceleration

I don't want to do it for you. I might have said too much already.

 

[kevinthenerd]

Originally posted by: nourdmrolNMT1
it they were both the same mass, it would be 9.8m/s^2 .

it will move to the side with the larger mass,

m1(g) = m2(a)

the acceleration? No. If they were the same mass, they'd weigh the same, and they'd pull down equally. (Mass and force are two different things. The word "pound" has been lying to us our whole lives.) If there wasn't a mass on one side (and assuming no friction from the cable), it would fall at 9.8 m/s^2 as though nothing was attached to it at all.