TURNING EFFECT OF FORCES MOMENTS

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

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: Clockwise moment Anticlockwise moment

Types Of Moments There are 2 types of moments: Clockwise moment 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 20N stands on one end of a see-saw and the distance between the cat and the pivot is 3m, find the moment. Solution: In this case the cat is causing a clockwise moment. Clockwise moment = F x d = 20 x 3 = 60 Nm F = 20N d = 3m

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

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

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

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 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 10N 30N 6m d Find the value of d.  

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

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

Step 2: Find the clockwise / anticlockwise moment

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

Step 3: Equate the clockwise and anticlockwise moments

WORKED EXAMPLE Find the value of d. Solution: Anticlockwise moment Clockwise moment Find the value of d.   Solution: Clockwise moment = Force x distance between force and pivot = 30 x d = 30d 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 30d = 60 d = 60  30 d = 2 m

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

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