Demonstration Questions 1.How did the direction the ball moved compare to the direction the teacher moved? 2.The ball moved because the teacher applied a force to it by pushing it. Why did the teacher move? What pushed the teacher?

Chapter 4: Newton’s 3 rd Law of Motion Force Pairs

What is a force? Force: a push or a pull upon an object. They result from interactions between two objects.

What is Newton’s 3 rd Law of Motion? Newton’s 3 rd Law of Motion: for every action, there is an equal and opposite reaction.

What is a force pair? Force pair: two things that exert force on one another. – Forces are always equal. – Forces are always opposite directions.

Where can we see Newton’s 3 rd Law? Fish swimming in the water.

Newton’s 3 rd Law and Flying Birds

Newton’s 3 rd Law and Driving Wheels interacting with the road.

Check for Understanding While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton’s 3 rd Law of Motion. The firefly hit the bus and the bus hit the firefly. Which of the 2 forces is greater? – The force on the firefly. – The force on the bus. – Neither force is greater than the other.

Check for Understanding For years, space travel was believed to be impossible because there was nothing which rockets could push off of in order to provide the propulsion necessary to accelerate. This statement is not true because… – Space is void of air so the rockets have nothing to push off of. – Gravity is absent in space. – Space is void of air and so there is no air resistance in space. – Rockets do accelerate in space because of Newton’s 3 rd Law of Motion.

Check for Understanding Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton’s 3 rd Law, the bullet pushes backward upon the rifle. How fast is the acceleration of the recoiling rifle? – Greater than the acceleration of the bullet. – Smaller than the acceleration of the bullet. – The same size as the acceleration of the bullet.

Check for Understanding In the top picture below Mr. Weinandt is pulling upon a rope which is attached to a wall. In the bottom picture, Mr. Weinandt is pulling on a rope attached to an elephant. In each case, the force scale reads 500 N. In which case is Mr. Weinandt pulling with more force? – Scenario 1: Rope attached to the wall. – Scenario 2: Rope attached to the elephant. – He is pulling with the same force in both scenarios.

Identifying the parts of force pairs Forces come in pairs that are known as “action-reaction” forces pairs. – Action: object A exerts a force on object B. – Reaction: object B exerts a force on object A.

Identifying “Action-Reaction” Pairs Action: baseball pushes the glove to the left. Action Reaction: glove pushes baseball to the right.

Identifying “Action-Reaction” Pairs Action: Bowling balls pushes the pins to the right. Action Reaction: Pins push bowling ball to the left.

Identifying “Action-Reaction” Pairs Action: Enclosed air particles push balloon wall outwards. Reaction: Ball wall pushes the air particles inward.

Check for Understanding Consider the interaction depicted below between foot A, ball B, and foot C. The 3 objects interact simultaneously. Identify 2 pairs of action-reaction forces. Use the terms foot A, ball b, and foot C when writing your statements.

Answers 1.Foot A pushes ball C to the right, ball C pushes foot A to the left. 2.Foot B pushes ball C to the left, ball C pushes foot B to the right.

Check for Understanding Identify at least 4 pairs of action-reaction force pairs in the following diagram.

Answers 1.The elephants feet push down on the ground, the ground pushes up on the elephant’s feet. 2.The elephant pulls the rope to the right, the rope pulls the elephant to the left. 3.The man pulls the right part of the rope to the left, the rope pulls the man to the right. 4.The tractor pulls the rope to the left, the rope pulls the tractor to the right. 5.The tractor’s tires push down on the ground, the ground pushes up on the tires. 6.The man pulls the left part of the rope to the right, the rope pulls the man to the left.

Exit Slip On a separate sheet of paper to be turned in, explain an “action-reaction” force pair that you have experienced in your own life. Identify the two objects and then identify the action and reaction. You may NOT use one of the examples discussed in class today!

How does this demonstrate Newton’s 3 rd Law?