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Objectives • Define force as part of an interaction. (6.1) • State Newton’s third law of motion. (6.2) • Given an action force, identify the reaction force. (6.3) • Explain why the acceleration caused by an action force and by a reaction force do not have to be equal. (6.4) • Explain why an action force is not cancelled by the reaction force. (6.5) • Describe the horse-cart problem. (6.6) • Explain why you can’t touch without being touched. (6.7) 7.1 Forces and Interactions • A force is always a part of a mutual action that involves another force. – Forces always occur in pairs. 7.2 Newton’s Third Law • Describes the relationship between two forces. • Newton’s third law – whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the other object. – Once force is called the action force, the other, reaction force. Questions 1. What happens when a force exerts a force on another object? 2. Does a stick of dynamite contain force? 3. A car accelerates along the road. Strictly speaking, what is the force that moves the car? 7.3 Identifying Action and Reaction • A simple way of determining action-reaction forces: – First Identify the interacting objects. • Action: Object A exerts force on Object B • Reaction: Object B exerts force on Object A Question • We know the Earth pulls on the moon. Does the moon also pull on the Earth? If so, which pull is stronger? 7.4 Action and Reaction on Different Masses • The Earth and a falling boulder. We know the boulder falls to the Earth. But does the Earth also fall to the boulder? – You must draw on Newton’s second law to help answer this question. – Given a force exerted on a small mass produces a greater acceleration than the same force exerted on a larger mass. • Lift – Helicopter • Rotating blades push down on air (action) air pushed on blades (reaction). – Planes • Wings receive a greater force from air underneath the wing (action), wing provides a force back on the air (reaction). 7.5 Defining a System • Action and reaction forces do not cancel each other when either of the forces is external to the system being considered. 7.6 The Horse-Cart Problem • If the horse in the horsecart system pushes the ground with a greater force than it pulls on the cart, then there is an net force on the horse, and the horse-cart system accelerates. – It’s the force outside the horse-cart system that pushes the system. 7.7 Action Equals Reaction • For every interaction between things, there is always a pair of oppositely directed forces that are equal in strength.