1. 5.1.4 Force and moments You should be able to: (a) understand that the weight of a body may be taken as acting at a single point known as its centre of gravity. (b) understand a couple as a pair of equal parallel forces tending to produce rotation only. (c) define and use the moment of a force and the torque of a couple. (d) show an understanding that, when there is no resultant force and no resultant torque, a system is in equilibrium. (e) apply the principle of moments to solve problems involving forces acting in two dimensions. Monday, 19 October 2015

2. The centre of mass is the point where all of the mass of the object is concentrated. When an object is supported at its centre of mass it will remain in equilibrium. If the object is uniform, for example a meter stick, the center of mass will be at the exact geometric center; if the object is irregular in shape the center of mass will be closer to the heavier end. An easy way to determine the location of the center of mass of a rigid pole is to support the pole on one finger from each hand. Gently slide your fingers together. When your fingers meet, you will be at the centre of mass. Try it with a bat or a broom.

3. To find the center of mass of an planar object use a plumb line. Suspend the mass from each vertex and trace the plumb line's location. Since the center of mass will fall below the suspension point the center of mass will be at the intersection of all of the plumb lines.

4. Where is the centre of mass here ?

5. The Moment of a Force (also called torque) The moment of a force is a measure of its turning effect. The moment can be calculate using the following equation. Moment = Force ×Perpendicular distance of force from "pivot"

6. The two obvious changes the person could make in order to open the door more easily are he/she could 1.increase the distance "r“ 2.push at 90° to the door. If the angle between the line of action of the force and the door is 90°, we have Moment = Fr This is the maximum value of the turning effect.

7. What happens when the pairs of forces shown below act on the green block ?

8. Two equal and oppositely directed parallel but not collinear forces acting upon a body. The moment of the couple (or torque) is given by the product of one of the forces by the perpendicular distance between them.torque If the distance between the horses is d and the force each one exerts is f. The moment of the couple is given by: Moment = (f  ½ d) + (f  ½ d) = f  d