Chapter 35 Moments, Levers & Centre of Gravity

Objectives To introduce the term turning effect of a force To investigate how the turning effect can be increased. To state the equation for the moment of a force and do examples To define what a lever is and look at everyday examples

5. To investigate the law of the lever 6 5. To investigate the law of the lever 6. To calculate unknown weights and distances using the law of the lever 7. To define the centre of gravity and look at examples 8. To accurately find the centre of gravity of a piece of cardboard 9. To investigate equilibrium 10. To examine how everyday objects are designed for stability 11. To discuss factors affecting stability

Turning effect of a force A fixed point or line through which something swings is called the fulcrum A door swings about its hinges. Hinges = fulcrum of the door A large force is need to close the door in because the man is holding the door beside the fulcrum. The baby only needs to use a small force to close the door

The turning effect of a force can be increased by: Increasing the size of the force Increasing the distance from the applied force to the fulcrum. Moment of A Force Moment of a force is a measure of the turning effect of the force. Equation for moment of a force: Moment of a force = Force x perpendicular distance from force to fulcrum

Levers A lever is any rigid body that is free to turn about a fixed point or axis called a fulcrum Examples of levers (pick examples from the diagram): Spanner _________________

Properties of levers: 1. All levers are rigid and do not bend easily 2. Levers turn about a fixed point called a fulcrum. 3. Levers allow us to move or turn things more easily.

Experiment to investigate the law of the lever

Law of the Lever When a lever is balanced by any number of forces, the sum of the clockwise moments acting on it is equal to the sum of the anti-clockwise moments.

Centre of Gravity The centre of gravity of an object is the point through which the weight of the object appears to act.

Equilibrium An object that is balanced is said to be “in equilibrium” The stability of an object is closely linked to the position of its centre of gravity.

Yellow card is in stable equilibrium Yellow card is in stable equilibrium. The fulcrum is above the centre of gravity. If the card moves its centre of gravity rises. Red card is in unstable equilibrium. The fulcrum is below the centre of gravity. If moved slightly its centre of gravity falls. The green card is in neutral equilibrium. The fulcrum is at the centre of gravity. If the card is moved it will just stay in its new position and its centre of gravity does not rise or fall.

Designing for stability

Factors which affect stability Two main factors which help the stability of an object. A low centre of gravity helps stability. 2. A wide base.

Exam questions: 2011 Paper > Question 7 > Part b