TURNING EFFECT OF FORCES MOMENTS Lesson Objectives l

TURNING EFFECT OF FORCES MOMENTS

Lesson Objectives l Identify what are the forces that will give rise to clockwise / anticlockwise moments l Identify the axes of rotation and the line of action l Explain what is clockwise / anticlockwise moments l Explain and apply the principle of moments to solve problems.

Inquiry approach activity 1 Distance moved VS Loads applied

MOMENTS What is moments? A force can cause many things to move or stop. When a force causes an object to turn, this turning effect is called moments. Example: A person sitting on a see-saw.

Types Of Moments There are 2 types of moments: l. Clockwise moment l. Anticlockwise moment

CALCULATING MOMENTS Moment = force perpendicular distance between force and pivot In symbols: Moments = F d Unit for moments: Newton-metre (Nm) d Weight => F

Example 1: A cat of weight 20 N stands on one end of a see-saw and the distance between the cat and the pivot is 3 m, find the moment. Solution: In this case the cat is causing a clockwise moment. Clockwise moment =Fxd = 20 x 3 = 60 Nm d = 3 m F = 20 N

Example 2: A duck stands on one end of a see-saw, 5 m away from the pivot. If the weight of the duck is 10 N, find the moment. Solution: The duck’s weight is causing an anticlockwise moment. Anticlockwise moment =Fxd = 10 x 5 = 50 Nm d = 5 m F = 10 N

Group Assignment: Identify at least 5 objects from the picture, indicate 1) the distance from the pivot 2) The direction of of force applied 3) Clockwise or anti-clockwise?

Examples which is the distance? Where is the force & pivot? d F F d d d F d

Principle of Moments For an object to be in equilibrium(stable/not moving), the total clockwise moment must be equal to the anticlockwise moment about the same pivot point. Clockwise moment = anti-clockwise moment weight pivot It is the fixed(non moving) point

SOLVING PROBLEMS RELATED TO PRINCIPLE OF MOMENTS Step 1: Identify what are the forces that will give rise to clockwise / anticlockwise moment Step 2: Find the clockwise / anticlockwise moment Step 3: Equate the clockwise and anticlockwise moments

WORKED EXAMPLE 6 m 10 N Find the value of d. d 30 N

Step 1: Identify what are the forces that will give rise to clockwise / anticlockwise moment

WORKED EXAMPLE 6 m Anticlockwise moment 10 N Find the value of d. d 30 N Clockwise moment

Step 2: Find the clockwise / anticlockwise moment

WORKED EXAMPLE 6 m Anticlockwise moment 10 N d 30 N Find the value of d. Solution: Clockwise moment = Force x distance between force and pivot = 30 x d = 30 d Nm Anticlockwise moment = Force x distance between force and pivot = 10 x 6 = 60 Nm Clockwise moment

Step 3: Equate the clockwise and anticlockwise moments

WORKED EXAMPLE 6 m Anticlockwise moment 10 N d 30 N Clockwise moment Find the value of d. Solution: Clockwise moment = Force x distance between force and pivot = 30 x d = 30 d Nm Anticlockwise moment = Force x distance between force and pivot = 10 x 6 = 60 Nm Using the principle of moments, Clockwise moment = Anti-clockwise moment 30 d = 60 d = 60 30 d = 2 m

Points to note: 1. The unit force must be in Newtons, the unit for distance must be in metres. 2. The distance must measured from the force to the pivot.

Turning effect of forces Moments Definitions of a torque, i. e. Force x perpendicular distance from the pivot Principle of moments for a body in equilibrium Describe and apply to examples Anti-clockwise moments = Clockwise Moments Next lesson! Centre of gravity (COG) Weight of body taken to be acting at a single point Qualitative decription of COG on object’s stability