# Difficulty with this Problem involving a Pully and Cart

#### i_love_space_and_eng

Homework Statement
A cart with a mass of 1.75 kg is pulled by a string over a pulley attached to a hanging
mass of 400 g.
a. What is the acceleration of the cart if the pulley is considered massless?
b. The pulley has a radius of 3.5 cm and a mass of 100 g. It can be considered a
solid disc. What is the actual acceleration of the cart?
Homework Equations
Attached
My attempt is also attached. Basically, I think I have (a) down. The only issue is I am getting the wrong answer, and I was hoping one of you could let me know if the answer key is wrong or the way I did it was wrong? The answer is 1.823m/s^2. Also, for the second part, I am really not sure where to go from here. If someone could get any advice that'd be awesome. (Also, the answer to (b) is 1.782m/S^2.

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#### haruspex

Homework Helper
Gold Member
2018 Award
Problem Statement: A cart with a mass of 1.75 kg is pulled by a string over a pulley attached to a hanging
mass of 400 g.
a. What is the acceleration of the cart if the pulley is considered massless?
b. The pulley has a radius of 3.5 cm and a mass of 100 g. It can be considered a
solid disc. What is the actual acceleration of the cart?
Relevant Equations: Attached

My attempt is also attached. Basically, I think I have (a) down. The only issue is I am getting the wrong answer, and I was hoping one of you could let me know if the answer key is wrong or the way I did it was wrong? The answer is 1.823m/s^2. Also, for the second part, I am really not sure where to go from here. If someone could get any advice that'd be awesome. (Also, the answer to (b) is 1.782m/S^2.
Also, please do not post algebraic working as an image. That is for diagrams and textbook extracts. Take the trouble to type in your work. That is generally much easier to read and allows those responding to quote specific lines for reference.

#### i_love_space_and_eng

Also, please do not post algebraic working as an image. That is for diagrams and textbook extracts. Take the trouble to type in your work. That is generally much easier to read and allows those responding to quote specific lines for reference.
Sorry, was told to post again with writing everything. I'll be sure to write out the work next time.

#### i_love_space_and_eng

Sorry, was told to post again with writing everything. I'll be sure to write out the work next time.
So, I tried setting it up as ma for cart = ma for the crate, but I go the wrong answer. Could you let me know where I am going wrong with that?

#### haruspex

Homework Helper
Gold Member
2018 Award
So, I tried setting it up as ma for cart = ma for the crate, but I go the wrong answer. Could you let me know where I am going wrong with that?
As I posted, draw separate FBDs for the hanging block and the cart. Assign an unknown T as the tension. Write out the ΣF=ma equation for each

#### i_love_space_and_eng

Alright,
Here i have my fbd1 and fbd2 attached. Any advice on where to continue from here?

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#### Chestermiller

Mentor
Alright,
Here i have my fbd1 and fbd2 attached. Any advice on where to continue from here?
$m_2a_2$ is not a force acting on the cart. How are the magnitudes of the accelerations of the cart and the crate related?

#### i_love_space_and_eng

$m_2a_2$ is not a force acting on the cart. How are the magnitudes of the accelerations of the cart and the crate related?
Are they related by the force of the string connecting them?

#### Chestermiller

Mentor
Are they related by the force of the string connecting them?
How are they related kinematically, given that the length of the string is constant?

#### i_love_space_and_eng

I would say the gravity pulls on the mass, and the mass pulls on the string, which pulls on the cart?

#### Chestermiller

Mentor
I would say the gravity pulls on the mass, and the mass pulls on the string, which pulls on the cart?
So geometrically how are the acceleration magnitudes related? (I don’t want to hear about gravity, masses, or forces)

#### haruspex

Homework Helper
Gold Member
2018 Award
I would say the gravity pulls on the mass, and the mass pulls on the string, which pulls on the cart?
That does not express the fact that the string stays the same length. What relationship between the two accelerations does express that?
And as Chet notes, your ∑F=ma for the cart is wrong. m2a2 is not an applied force.

#### i_love_space_and_eng

So geometrically how are the acceleration magnitudes related? (I don’t want to hear about gravity, masses, or forces)
Are the acceleration magnitudes the same? I don't know how not to mention gravity when that is what is pulling on the mass.

#### i_love_space_and_eng

That does not express the fact that the string stays the same length. What relationship between the two accelerations does express that?
And as Chet notes, your ∑F=ma for the cart is wrong. m2a2 is not an applied force.
Geometrically, they are at 90degree angle? Or atleast the string is.

#### i_love_space_and_eng

Are the acceleration magnitudes the same? I don't know how not to mention gravity when that is what is pulling on the mass.
They are at 90degree angles geometrically?

#### haruspex

Homework Helper
Gold Member
2018 Award
Geometrically, they are at 90degree angle? Or atleast the string is.
That is not a relationship between the magnitudes of the two accelerations and has nothing to do with the total length of string being constant.

#### Chestermiller

Mentor
Are the acceleration magnitudes the same? I don't know how not to mention gravity when that is what is pulling on the mass.
Yes, the acceleration magmoudes are the same? If the crate moves downward 1 cm, how mich does the cart move horizontally?

#### i_love_space_and_eng

That is not a relationship between the magnitudes of the two accelerations and has nothing to do with the total length of string being constant.
I actually think I found a and it makes sense... Now here is my work for (b).
Σt=Ia
T-mg=(6.125*10^-5kgm^2)a
T-3.92N= (6.125*10^-5kgm^2)a
Not sure how to find T though... Did I even set that up correctly? Any advice on where to go from here?

#### i_love_space_and_eng

Yes, the acceleration magmoudes are the same? If the crate moves downward 1 cm, how mich does the cart move horizontally?
I actually think I found a and it makes sense... Now here is my work for (b).
Σt=Ia
T-mg=(6.125*10^-5kgm^2)a
T-3.92N= (6.125*10^-5kgm^2)a
Not sure how to find T though... Did I even set that up correctly? Any advice on where to go from here?

#### haruspex

Homework Helper
Gold Member
2018 Award
I actually think I found a and it makes sense... Now here is my work for (b).
Σt=Ia
T-mg=(6.125*10^-5kgm^2)a
T-3.92N= (6.125*10^-5kgm^2)a
Not sure how to find T though... Did I even set that up correctly? Any advice on where to go from here?
As I wrote in the other thread, for part b you need three FBDs; one for the cart, one for the hanging mass and one for the pulley.
The tensions in the two sections of string are now different.
Again, the accelerations are related, but for the pulley it is an angular acceleration. To avoid confusion, please use a different label for that; α is customary.

#### i_love_space_and_eng

As I wrote in the other thread, for part b you need three FBDs; one for the cart, one for the hanging mass and one for the pulley.
The tensions in the two sections of string are now different.
Again, the accelerations are related, but for the pulley it is an angular acceleration. To avoid confusion, please use a different label for that; α is customary.
Great. Okay, so here I have drawn out the fbds for all three.
Is this correct?

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#### Chestermiller

Mentor
Great. Okay, so here I have drawn out the fbds for all three.
Is this correct?
Good. Now let's see the force and moment balances that go along with the relevant diagrams.

#### i_love_space_and_eng

Alright, I think I have that done. Please see below.

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#### jbriggs444

Homework Helper
You realize that you've been asked to type this stuff in, right? "Writing" means "typing". Taking pictures of handwritten text does not count.

#### Chestermiller

Mentor
Alright, I think I have that done. Please see below.
I want to see 3 separate equations: a force balance on the cart, a force balance on the crate, and a moment balance on the pulley. These equations should have net forces or moments set equal to ma or $I\alpha$ (I don't want to see any $\Sigma F's$).

"Difficulty with this Problem involving a Pully and Cart"

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