need help with a physics torque problem

Apr 7th, 2015
Price: $10 USD

Question description

I'm trying to do this problem from a physics book but I'm getting the wrong answer.

A thin, uniform, 15.0kg post, 1.75m long, is standing vertically. A string is attached to the post 1.25m up from the base of the post, passes over a massless, frictionless, pulley, and is attached to a 5 kg weight. What is the angular acceleration of the post about its base when this system is released from rest (i.e. when you remove the cable that's holding the post up in the image below)?

My solution is:

1. Calculate force from the weight (5 kg) * 9.8 = 49N

2. Calculate torque using the weight's force. Since it's perpendicular to the post, I'm using (force)(lever arm), which is (49N)(1.25m) = 61.25 N m

3. Calculate moment of inertia for the post about its base: 1/3ML^2 = 15.3125

4. Get angular acceleration:

(torque) = (moment of inertia)(angular acceleration)

(torque) / (moment of inertia) = (angular acceleration)

(61.25) / (15.3125) = 4 rad/s^2

.. but the book is saying the answer should be 2.65 rad/s^2. What am I missing here?


Tutor Answer

(Top Tutor) Daniel C.
School: UCLA

Studypool has helped 1,244,100 students

Review from our student for this Answer

Apr 7th, 2015
Ask your homework questions. Receive quality answers!

Type your question here (or upload an image)

1819 tutors are online

Brown University

1271 Tutors

California Institute of Technology

2131 Tutors

Carnegie Mellon University

982 Tutors

Columbia University

1256 Tutors

Dartmouth University

2113 Tutors

Emory University

2279 Tutors

Harvard University

599 Tutors

Massachusetts Institute of Technology

2319 Tutors

New York University

1645 Tutors

Notre Dam University

1911 Tutors

Oklahoma University

2122 Tutors

Pennsylvania State University

932 Tutors

Princeton University

1211 Tutors

Stanford University

983 Tutors

University of California

1282 Tutors

Oxford University

123 Tutors

Yale University

2325 Tutors