Newton’s Third Law of Motion For every Action, there is an equal and opposite Reaction.

Example 1: When you sit on a chair, the chair pushes back up against your weight with an equal force. Notice the action-reaction pair Action force Reaction force

Example 2: When you swim, you push water backwards and the water pushes you forwards. Example 3: When birds push their wings down in the air, the air pushes them and their wings up.

Example 4: Car wheels push backward on the road, and the road pushes the wheels forward.

Sample Problems 1. Sam’s mass is 20 kg. After he began to run, his acceleration reached 5 m/s when he suddenly hit a tree. Ouch! What force did the tree exert on Sam? Givens: m=20 kg. a=5 m/s F=ma F=20(5) Sam’s F=100 N The tree exerts an equal and opposite force to Sam Answer: 100 N

2. The system is only the orange(in the circle). If the apple pulls with x Newtons, is there any acceleration, or do the forces cancel? If you remember pages 78 and 79 in your book, you know that forces internal to a system cancel each other out, but forces external to that system cause it to accelerate. Answer: There is acceleration.

Concepts to Keep in Mind Action and Reaction forces come in pairs The forces are always equal… Sometimes different masses cause different accelerations though(F=ma) Internal vs. external systems- forces cancel out in an internal system, but an external force acting on the system will cause acceleration.

The Quiz!

Question One… it’s a toughie! A bug is flying at 1 m/s and truck is traveling in the opposite direction at 40 m/s. When the two collide, which is the force of the collision greater on? This is a trick question! We all know that action and reaction forces are EQUAL, and therefore, the the collision force is greater on neither the truck nor the bug.

Question Two When a.5 kg bullet is fired from an 8 kg rifle, the action-reaction pair is the rifle pushing the bullet forward and the bullet pushing the gun backward. If the force from the blast causes the rifle to accelerate to 125 m/s^2, what is the acceleration of the bullet ? GivensF=? Bullet: Rifle: m=.5 kg m= 8 kg a= ? a= 125 m/s^2 F=ma F=8(125) F=1000 N 1000 N=.5a 1000 N/.5=a Answer:a= 2000 m/s^2

Question 3… this one is extra hard! If the action is Mrs. B pushing on a wall, the reaction is… A) The floor pushing on the wall B) The floor pushing on Mrs. B C) The wall pushing on Mrs. B D) That’s crazy! A wall can’t push back! In an action-reaction pair, the reaction is usually just the vice- versa of the action. In the question, the action is a person pushing on a wall, so the reaction would be the wall pushing on the person. The correct answer is C.

Question 4 Identify all the action-reaction force pairs. The man pushing on the ground/the ground pushing on the man The knee hitting the ball/the ball hitting the knee The barbell pushing down on the man/ the man pushing up on the barbell The balloon pulling the barbell up/the barbell pulling the balloon down The air pushing out of the balloon/the balloon side pushing the air in

Congratulations! You are now ready for the portion of the final concerning Newton’s 3rd Law! For more review, see Chapter 6 in your book.