3.4 Newton’s Third Law of Motion Newton’s First Law of Motion applies to situations when the net force acting on an object is zero. All the forces acting on the object are balanced. ( 𝑭 𝒏𝒆𝒕 = 0) Newton’s Second Law of Motion applies to situations when the net force acting on an object is not zero. An unbalanced is acting on the object. ( 𝑭 𝒏𝒆𝒕 = m 𝒂 ) Newton’s Third Law of Motion applies to situations when two objects interact with one another and exert forces on each other.

3.4 Newton’s Third Law of Motion If you reach out and push the table forward, you can actually feel the table push you backwards. The action force is you pushing on the table forward. Meanwhile, the reaction force is the table pushing you backwards. Newton’s Third Law of Motion states that for every action force, there is a simultaneous reaction force that is equal in magnitude and opposite in direction. ( 𝑭 𝒂𝒙𝒏 = - 𝑭 𝒓𝒙𝒏 ) or ( 𝑭 𝟏 𝒐𝒏 𝟐 = - 𝑭 𝟐 𝒐𝒏 𝟏 )

3.4 Newton’s Third Law of Motion Another way to illustrate Newton’s Third Law is to consider a person standing on a skateboard: As the person pushes off the skateboard in one direction, the skateboard will accelerate in the opposite direction. Action Force → Person pushing the skateboard backwards. FBD of skateboard. Reaction Force → Skateboard pushing the person forward. FBD of person.

3.4 Newton’s Third Law of Motion It is important to note that the action and reaction forces occur simultaneously, and always act on different objects. When applying Newton’s Third Law, the action and reaction forces are shown on separate FBDs. The forces do not cancel each other out, and they are not added together.

3.4 Newton’s Third Law of Motion Explain each event in terms of Newton’s Third Law: A rocket accelerates upwards off a launch pad. A swimmer moves through the water. A small balloon releases air and flies around the room. You are walking across the floor. SP # 1 p.139

3.4 Homework Practice # 1-4 p.140 Questions # 1, 3-5, 7, 9 p.141