• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Can someone help me with this rotational dynamics problem.

I ended up getting a=.901m/s^2
 

SammyS

Staff Emeritus
Science Advisor
Homework Helper
Gold Member
11,095
879
I ended up getting a=.901m/s^2
[STRIKE]I didn't get that[/STRIKE].

I'll re-check what I got, but I have already gone over my answer a few times.

Added in Edit:

Your result here is good!
 
Last edited:
I'm sorry I meant .934 m/s^2 I was practicing this problem with different quantities and accidentally wrote the answer one of the others.
 

SammyS

Staff Emeritus
Science Advisor
Homework Helper
Gold Member
11,095
879
I'm sorry I meant .934 m/s^2 I was practicing this problem with different quantities and accidentally wrote the answer one of the others.
I didn't get that either.

When you got a = 0.552 m/s2 your moment of inertia was 4 times what it should have been, so when you had Fp = 6a, that was also 4 times the size it should have been.
 

ehild

Homework Helper
15,313
1,739
See attached picture. The acceleration of the plank is ap. F-2Fp=Map (1)

The motion of the cylinder is translation of the centre of mass and rotation about the CM.
The CM accelerates with acm. The friction between cylinder and plank accelerates the CM of the cylinders forward, and accelerates forward rotation; the rotational resistance Fg between ground and cylinder drives translation forwards, but hinders rotation. .
So Fp+Fg=macm. (2)

r(Fp-Fg)=Iβ (β is the angular acceleration)

The cylinders do not slip: βr=0.5 m.
The moment of inertia about the CM is I=0.5 mr2.
The torque equation becomes:

Fp-Fg=0.5 macm.(3)

Adding equations (2) and(3):

2Fp=1.5macm (4)


The plank does not slip on the cylinders, so its velocity is the same as the topmost point of the cylinders, which is twice the velocity of the CM:
acm=0.5 ap.

Now you have the equations (1) and (4)

F-2Fp=Map
2Fp=1.5map/2

Add them:

F=Map+0.75map.

ehild
 

Attachments

Last edited:

SammyS

Staff Emeritus
Science Advisor
Homework Helper
Gold Member
11,095
879
I ended up getting a=.901m/s^2
OK ...

I went over my calculations yet again.

The answer is a = 6.4/7.1 = 0.9014... .

However you came up with this for a previous post (post #51) of yours, this answer looks good.

I apologize for my repeatedly poor arithmetic.
 
Last edited:

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving

Hot Threads

Top