This lesson will introduce you to the last of three laws of dynamics This lesson will introduce you to the last of three laws of dynamics These laws are the fundamental laws of Dynamics. With them, you can predict how an object will move when under the influence of forces. These laws are the fundamental laws of Dynamics. With them, you can predict how an object will move when under the influence of forces. SPH 3U Grade 11 U Physics

By the end of this lesson, you will be able to: What are we going to cover today? Explain why forces exist in pairs SPH 3U Grade 11 U Physics State Newton’s 3 rd Law of Motion Explain Newton’s 3 rd Law of Motion Homework Solve problems with the 3 rd Law

SPH 3U Grade 11 U Physics Newton discovered that when two objects interact, two forces will always be involved These two forces are called action and reaction forces Can you identify the action – reaction forces in this diagram? CofG FnFnFnFn FgFgFgFg No contact = no force FnFnFnFn FaFaFaFa

SPH 3U Grade 11 U Physics For every action force that object A exerts on object B, object B exerts a reaction force that is equal in magnitude, but opposite in direction on object A Newton’s 3 rd law of motion tells us that action-reaction forces are equal in magnitude opposite in direction

SPH 3U Grade 11 U Physics Ex 1.Draw a free-body diagram of a 2.5 kg book resting on a table Sol’n 2.5 kg Force of table on book F N = 25 N [U] Force of gravity on book F g = 25 N [D]

SPH 3U Grade 11 U Physics Ex 2.State the reaction forces to the book in ex 1. Sol’n 1.If the table pushes on the book with 25 N [U], then the book pushes on the table with 25 N [D] There are two reaction forces, because there are two Forces on the free-body diagrams. 2.If the Earth pulls on the book with a force of 25 N [D], then the book must pull on the Earth with 25 N[U] That’s Right. The book pulls on the earth with an equal force, just opposite in direction.

SPH 3U Grade 11 U Physics 1. Walking When you walk, you apply a force to the earth (the action force). - In this case down and backwards - By Newton’s 3 rd law the Earth will push back on you (the reaction force) up and forward. The forces are equal in magnitude, but opposite in direction.

2. Swimming When you swim, you apply a force to the water (the action force). By Newton’s 3 rd law the water will push back on you (the reaction force). The forces are equal in magnitude, but opposite in direction. SPH 3U Grade 11 U Physics

Suppose you push on an object attempting to move it. The reaction force is the object will push back on you. SPH 3U Grade 11 U Physics

Question: Question: If the force of the object pushing back on you is always equal & opposite, then how are you able to move it (sometimes) ? Answer: Because they act on different objects.

Question: But don’t action - reaction forces cancel each other out causing Fnet = 0? SPH 3U Grade 11 U Physics Answer: They do cancel each other out! But they don’t cancel out all the other forces… To understand this, lets Draw the FBD of each object

SPH 3U Grade 11 U Physics FaFaFaFa FfFfFfFf FNFNFNFN FgFgFgFg This is the action force, the applied force by the boy on the man. FgFgFgFg FaFaFaFa FNFNFNFN This is the reaction force, the force which the larger object pushes back on the kid FfFfFfFf We are looking at F a in both of these FBD’s Question: Why should we expect to see acceleration? Question: But how does that happen if all these forces are equal and opposite?

SPH 3U Grade 11 U Physics SMART Response Question 1. a.67. N b.6.7 N c..67 N d..067 N e.6.7 x N

SPH 3U Grade 11 U Physics Read pgs Questions