LEARNING OBJECTIVES
• Know and use the relationship between the moment of a force and its perpendicular distance from the pivot:
moment = force × perpendicular distance from the pivot
• Know that the weight of a body acts through its centre of gravity
• Use the principle of moments for a simple system of parallel forces acting in one plane
• Understand how the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam
Moments: The basics
Specification Point 1.30P
• Know and use the relationship between the moment of a force and its perpendicular distance from the pivot:
Moment = Force perpendicular distance from the pivot
• A moment is the turning effect of a force.
• The moment of a force is given by the equation:
• Moments have the unit’s newton centimeters (N cm) or newton meters (N m), depending on whether the distance is measured in meters or centimeters.
Diagram showing the moment of a force exerted by a spanner on a nut
Center of Gravity
Specification Point 1.31P
• Know that the weight of a body acts through its center of gravity.
• The center of gravity of an object (sometimes called the center of mass) is the point through which the weight of that object acts.
• For a symmetrical object of uniform density (such as a symmetrical cardboard shape) the center of gravity is located at the point of symmetry:
The center of mass of a regular shape can be found by symmetry
• When an object is suspended from a point, the object will always settle so that it’s center of gravity comes to rest below the pivoting point.
• This can be used to find the center of gravity of an irregular shape:
Diagram showing an experiment to find the center of gravity of an irregular shape
• The irregular shape is suspended from a pivot and allowed to settle.
• A plumb line (lead weight) is then held next to the pivot and used to draw a vertical from the pivot (the center of gravity must be somewhere on this line).
• The process is then repeated, suspending the shape from two different points.
• The center of gravity is located at the point where all three lines cross.
The Principle of Moments
Specification Point 1.32P
• Use the principle of moments for a simple system of parallel forces acting in one plane
•The principle of moments states that:
• For a system to be balanced, the sum of clockwise moments must be equal to the sum of anticlockwise moments.
Diagram showing the moments acting on a balanced beam
• In the above diagram:
• Force F2 is supplying a clockwise moment;
• Forces F1 and F3 are supplying anticlockwise moments.
• Hence:F2 x d2 = F1 x d1 + F3 x d3
Supporting a Beam
Specification Point 1.33P
• Understand how the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam
• A light beam is one that can be treated as though it has no mass.
• The supports, therefore, must supply upwards forces that balance the weight of any object placed on the beam.
Md. Jahangir Alam, Assistant Teacher (Senior section)
O and A level Physics Teacher,
Bangladesh International School and College, Dhaka/Abrar Jahin