Turn on thread page Beta
    • Thread Starter
    Offline

    7
    ReputationRep:
    Hi,

    I don't really understand how to label forces on moments questions.
    I came across these two moments questions and found out that sometimes the forces act perpendicular to the rod and sometimes perpendicular to the floor/ground.

    For example:
    a) A uniform ladder AB, of length 6 metres and mass 22 kg, rests with its foot, A, on rough horizontal ground. The ladder rests against the top of a smooth vertical wall at the point C, where the length AC is 5 metres. The vertical plane containing the ladder is perpendicular to the wall, and the angle between the ladder and the ground is 60. A man, of mass 88 kg, is standing on the ladder. The man may be modelled as a particle at the point D, where the length of AD is 4 metres. The ladder is on the point of slipping. (According to the MS, resultant force acts perpendicular to the floor at A, and perpendicular to the rod at C.) So why is it different? why not perpendicular to the rod/floor either way?

    b) A uniform rod, PQ , of length 2a , rests with one end, P, on rough horizontal ground and a point T resting on a rough fixed prism of semicircular cross-section of radius a, as shown in the diagram. The rod is in a vertical plane which is parallel to the prism’s cross-section. The coefficient of friction at both P and T is m.
    (MS: Resultant force acts perpendicular to the ground at P while resultant force acts perpendicular to the rod at T.)

    It would be really nice of you if you could explain to me with easy words
    Thanks for helping!
    • Study Helper
    Offline

    15
    Study Helper
    (Original post by liemluji)
    It would be really nice of you if you could explain to me with easy words
    Thanks for helping!
    At the point of contact, at least one of the two bodies is a plane. The normal reaction is perpendicular to that plane.

    If there is no friction, then the normal force is the only one acting at the point of contact.

    If there is friction, you will get a force perpendicular to the normal one.

    Try those rules against the examples you've given.
 
 
 
Reply
Submit reply
Turn on thread page Beta
Updated: January 18, 2017

University open days

  • Heriot-Watt University
    School of Textiles and Design Undergraduate
    Fri, 16 Nov '18
  • University of Roehampton
    All departments Undergraduate
    Sat, 17 Nov '18
  • Edge Hill University
    Faculty of Health and Social Care Undergraduate
    Sat, 17 Nov '18
Poll
Black Friday: Yay or Nay?
Useful resources

Make your revision easier

Maths

Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

Equations

How to use LaTex

Writing equations the easy way

Equations

Best calculators for A level Maths

Tips on which model to get

Student revising

Study habits of A* students

Top tips from students who have already aced their exams

Study Planner

Create your own Study Planner

Never miss a deadline again

Polling station sign

Thinking about a maths degree?

Chat with other maths applicants

Can you help? Study help unanswered threads

Groups associated with this forum:

View associated groups

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.