MOVEMENT AND CHANGE Newton’s third law of motion Acceleration due to gravity

Lesson Aims To appreciate the work of Newton To understand Newton’s third law of motion To know the value of acceleration due to gravity. To understand the concept of terminal velocity. To know that all objects accelerate in gravity at the same rate.

Forces in history

Greek view In order to keep something moving a force is required. We now know this idea is wrong, but it fitted the facts of the ancient Greek world!

Sir Isaac Newton

'I don't know what I may seem to the world, but, as to myself, I seem to have been only like a boy playing on the sea shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.'

Newton’s third law of motion

Newton noticed that forces were always in pairs and that the two forces were always equal in size but opposite in direction. He called the two forces action and reaction.

Newton’s third law of motion Newton’s Third Law of Motion is: The action force and reaction force are equal and opposite. Every action has an equal and opposite reaction.

Newton’s third law of motion In a tug of war, if the rope is not moving what can you say about the forces exerted by the two teams?

Newton’s third law of motion In a tug of war, if the rope is not moving what can you say about the forces exerted by the two teams? The forces involved must be equal and opposite!

Newton’s third law of motion Your elbow rests on the table. What is the pair of forces here?

Newton’s third law of motion Your elbow rests on the table. What is the pair of forces here? The force of the elbow on the table (downwards) The force of the table on the elbow (upwards) =

Newton’s third law of motion What are the forces when you start to walk or run?

Newton’s third law of motion What are the forces when you start to walk or run? The force of your feet on the Earth (moves the Earth slightly backwards) The force of Earth on your feet (moves you forward) = Yes! The Earth moves backwards slightly as you move forwards

The linear air track

Movement in a straight line Lorry moves forward at a constant velocity

Movement in a straight line Lorry moves forward at a constant velocity Propulsive force pushes the lorry forwards

Movement in a straight line Lorry moves forward at a constant velocity Propulsive force pushes the lorry forwards Equal and opposite resistive force opposes the forward motion

Movement in a straight line Lorry accelerates forward

Movement in a straight line Lorry accelerates forward Propulsive force pushes the lorry forwards

Movement in a straight line Lorry accelerates forward Propulsive force pushes the lorry forwards The resistive force is smaller than the propulsive force

Movement in a straight line Lorry slows down Propulsive force pushes the lorry forwards is smaller

Movement in a straight line Lorry slows down Propulsive force pushes the lorry forwards The resistive force is larger than the propulsive force

Terminal velocity

The force of gravity pulls down all objects here on Earth. If objects are allowed to fall, they accelerate downwards.

Terminal velocity

If there is no air resistance or friction then all objects accelerate at the same rate. This was first proved in a famous experiment by Galileo.

Terminal velocity Accurate measurements show that the acceleration due to gravity = 9.8 m/s 2 We usually take the value as being 10 m/s 2

The “Guinea and Feather” experiment

Draw a neat diagram of this experiment Explain what happened and what this experiments tells us.

Terminal Velocity Physics text book p122

Terminal Velocity Read the section on terminal velocity Explain in your own words what terminal velocity is. Make a full page, neat and accurate copy of the diagram off p122

Lesson Aims To appreciate the work of Newton To understand Newton’s third law of motion To know the value of acceleration due to gravity. To understand the concept of terminal velocity. To know that all objects accelerate in gravity at the same rate.