FOLLOW US ON TWITTER
SHARE THIS PAGE ON FACEBOOK, TWITTER, WHATSAPP ... USING THE BUTTONS ON THE LEFT


YOUR PARTICIPATION FOR THE GROWTH OF PHYSICS REFERENCE BLOG

Monday, June 15, 2020

A rod PQ is attached at P to a vertical wall, as shown in Fig. 3.1. The length of the rod is 1.60 m. The weight W of the rod acts 0.64 m from P.


Question 42
A rod PQ is attached at P to a vertical wall, as shown in Fig. 3.1.



The length of the rod is 1.60 m. The weight W of the rod acts 0.64 m from P. The rod is kept horizontal and in equilibrium by a wire attached to Q and to the wall at R. The wire provides a force F on the rod of 44 N at 30° to the horizontal.
(a) Determine
(i) the vertical component of F,          [1]
(ii) the horizontal component of F.     [1]

(b) By taking moments about P, determine the weight W of the rod.           [2]

(c) Explain why the wall must exert a force on the rod at P.             [1]

(d) On Fig. 3.1, draw an arrow to represent the force acting on the rod at P. Label your arrow with the letter S.             [1]






Reference: Past Exam Paper – June 2015 Paper 22 Q3





Solution:
(a)
(i) Vertical component = 44 sin 30° = 22 N

(ii) Horizontal component = 44 cos 30° = 38(.1) N


(b)
{The vertical component of F acts at a distance of (0.64 + 0.96 =) 1.60m from the pivot.
Clockwise moment = Anti-clockwise moment}
W × 0.64 = 22 × 1.60
W = 55 N


(c) F has a horizontal component (not balanced by W)

{The horizontal component of F acts on the wall and this is not balanced by W. From Newton’s 3rd law, there should be an equal and opposite force exerted by the wall on the rod.}

OR F has 38 N acting horizontally
OR 38 N acts on wall
OR vertical component of F does not balance W
OR F and W do not make a closed triangle of forces

(d) Line from P in the direction towards the point on wire vertically above W and direction up


{This force, along with F and W should form a system that is in equilibrium. For equilibrium, the resultant force and the resultant moment should be zero. If all the 3 forces pass through the same point, the resultant moment would be zero.

Moment = Force × perpendicular distance from line of action of force to pivot.
In this case, the pivot is that point where all the 3 forces pass. As they pass on the point, the ‘distance …’ is zero, and thus, the moment at that point is zero.}

No comments:

Post a Comment

If it's a past exam question, do not include links to the paper. Only the reference.
Comments will only be published after moderation

Currently Viewing: Physics Reference | A rod PQ is attached at P to a vertical wall, as shown in Fig. 3.1. The length of the rod is 1.60 m. The weight W of the rod acts 0.64 m from P.