Moment of Force - Wikispaces.ppt

1、Turning Effect of Forces,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 moments: Clockwise moment Anticlockwise moment,Eve

2、ryday Examples,turning a door knob opening a door scissors turning a steering wheel crane at the construction sites lifting objects electrical fan,Everyday Examples,7.spanner & nut,Everyday Examples,7.8. Wheelbarrow,Activity,Stand up and support your bag with your wrist. Straighten your hand. Lift i

3、t up slowly until it is perpendicular to your body. Is it easy to do that? Hang the bag at your elbow and repeat the motion. How do you feel now? How do you think the distance between the pivot and the force affect the moments?,Activity,Stand up and support your bag with your wrist. Straighten your

4、hand. Lift it up slowly until it is perpendicular to your body. Ask your partner to remove some books from your bag. How do you feel as the books are removed? What does it tell you about the relationship between force and moments?,What does the turning effect of a force depend on?,1. force 2. perpen

5、dicular distance between force and pivot,Your turn!,Try out the question in the notes on how you should hang your clothes on a bamboo.,Turning Effect of a Force,Note: turning effect = moment = torque The moment of a force is its turning effect about a pivot,Turning Effect of a Force,Moments is the p

6、roduct of the force and the perpendicular distance from the pivot to the line of action of the force.,d,CALCULATING MOMENTS,Moment = force perpendicular distance between force and pivot,In symbols: Moments = F d,Weight = F,d,Unit for moments: Newton-metre (Nm),Example 1: A cat of weight 20N stands o

7、n 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,Worked Example 1,Example 2: A duck stands on one end of a see-saw, 5m away from the pivot. If the weig

8、ht of the duck is 10N, find the moment. Solution: The ducks weight is causing an anticlockwise moment. Anticlockwise moment= F x d = 10 x 5 = 50 Nm,F = 10N,d = 5m,Worked Example 2,Try out the questions in the notes to find the moments caused by the force,Your turn!,It is easy to balance two objects

9、of the same weight,Anticlockwise moment,Anti- clockwise moment,Anticlockwise moment,Clockwise moment,Is it possible to balance a objects of different weight?,weight,weight,Principle of Moments,For an object to be in equilibrium(stable/not moving), the total clockwise moment must be equal to the anti

10、clockwise moment about the same pivot.,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

11、and anticlockwise moments,Find the value of d.,Worked Example 3,Step 1: Identify what are the forces that will give rise to clockwise / anticlockwise moment,Find the value of d.,Anticlockwise moment,Clockwise moment,Worked Example 3,Step 2: Find the clockwise / anticlockwise moment,Clockwise moment

12、= Force x distance between force and pivot = 30 x d = 30d Nm,Anticlockwise moment,Clockwise moment,Anticlockwise moment= Force x distance between force and pivot = 10 x 6 = 60 Nm,Find the value of d. Solution:,Worked Example 3,Step 3: Equate the clockwise and anticlockwise moments,Find the value of

13、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,Anticlockwise moment,Clockwis

14、e moment,Worked Example 3,Points to note:,The unit for force must be in Newtons, the unit for distance must be in metres. The distance must measured perpendicularly from the force to the pivot.,The figure shows a uniform rod of length 4 m and weight 50N. It is pivoted at Q. A boy tries to lift up th

15、e rod vertically from the end P. What is the magnitude of the force when P is just lifted ?,Worked Example 4,The figure shows a uniform rod of length 4 m and weight 50N. It is pivoted at Q. A boy tries to lift up the rod vertically from the end P. What is the magnitude of the force when P is just li

16、fted ?,Solution,When the body is in equilibrium,F x 4m = 50N x 2m F = 25 N,Worked Example 4,The light ruler in the figure shown below is balanced. Calculate (a) the value of W (b) the normal reaction at the pivot P.,Worked Example 5,The light ruler in the figure shown below is balanced. Calculate (a) the value of W (b) the normal