Newton s Third Law Action & Reaction Forces
Or You cannot touch without being touched
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 first object.
Identifying action reaction pairs Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
Action The hammer hits the nail Reaction The nail hits the hammer
Identifying action reaction pairs Step 1 Identify the objects Step 2 - Identify the action as a push or a pull Step 3 Rewrite the event using push or pull Step 4 - Switch the position of the objects relative to the word push or pull Step 5 Rewrite the event with the action word
Step 2 This action is a push Action The hammer hits the nail Step 1 Objects Step 3 Step 4 The hammer pushes the nail The nail pushes the hammer Step 5 Reaction The nail hits the hammer
Action & Reaction on Different Masses The rocket engine expels the exhaust The exhaust pushes the rocket engine
Action & Reaction on Different Masses The rocket engine expels the exhaust The exhaust pushes the rocket engine F rocket engine on exhaust = F exhaust on rocket engine
Action & Reaction on Different Masses F rocket engine on exhaust = F exhaust on rocket engine M rocket x a rocket = m exhaust x a exhaust
A Wicked Smahhhht Farmer A talking horse (???) told a farmer that if he pulls on a cart the cart will pull on him with the same force and they will not move. Since the cart will not move he should go back to the barn.
A Wicked Smahhhht Farmer The farmer said to the horse, You re right, if you pull on the cart it will pull on you with the same force. A true action reaction pair. I have an idea, push on the ground and see what happens.
A Wicked Smahhhht Farmer F earth on horse = F horse on earth M earth x a earth = m horse x a horse
An Example Problem Two oranges started a tug-of-war in a frictionless world. Chuck tilted the scale at 120 kg, while Fred tilted the scale at 80 kg. Chuck pulled on Fred with a force of 60 N. Chuck Fred A) What is the direction and magnitude of Chuck s acceleration? B. What is the direction and magnitude of the force Fred exerts? C. What is the direction and magnitude of Fred s acceleration?
An Example Problem Two oranges started a tug-of-war in a frictionless world. Chuck tilted the scale at 120 kg, while Fred tilted the scale at 80 kg. Chuck pulled on Fred with a force of 60 N. Chuck 60 N Fred 120 kg 80 kg A) What is the direction and magnitude of Chuck s acceleration? a = F net / m = 60N/120kg = 0.50m/s/s in the direction of Chuck s pull.
An Example Problem Two oranges started a tug-of-war in a frictionless world. Chuck tilted the scale at 120 kg, while Fred tilted the scale at 80 kg. Chuck pulled on Fred with a force of 60 N. Chuck 60 N 60 N Fred 120 kg 80 kg A. What is the direction and magnitude of the force Fred exerts?
An Example Problem Two oranges started a tug-of-war in a frictionless world. Chuck tilted the scale at 120 kg, while Fred tilted the scale at 80 kg. Chuck pulled on Fred with a force of 60 N. Chuck 60 N 60 N Fred 120 kg 80 kg A. What is the direction and magnitude of Fred s acceleration? a = F net / m = 60N/80kg = 0.75 m/s/s in the direction of Fred s pull.
A downward force of 4N is exerted on the book by
A downward force of 4N is exerted on the book by EARTH
If the book is at rest, an upward force of is exerted on the by the hand
If the book is at rest, an upward force of 4N is exerted on the BOOK by the hand
Is the upward force exerted by the hand the reaction to the downward force exerted on the book?
Is the upward force exerted by the hand the reaction to the downward force exerted on the book? NO
The reaction to the force exerted downward on the book is a force of magnitude exerted on by. Its direction is.
The reaction to the force exerted downward on the book is a force of magnitude 4N exerted on EARTH by the BOOK. Its direction is UP.
The reaction to the force exerted upward by the hand is a force of magnitude exerted on by. Its direction is.
The reaction to the force exerted upward by the hand is a force of magnitude 4N exerted on the HAND by the BOOK. Its direction is DOWN.
The fact that the 4N force exerted upward by the hand is equal in magnitude and opposite in direction to the 4N force exerted downward on the book is an example of Newton s law.
The fact that the 4N force exerted upward by the hand is equal in magnitude and opposite in direction to the 4N force exerted downward on the book is an example of Newton s 2nd law.
The fact that the 4N force exerted upward by the book is equal in magnitude and opposite in direction to the 4N force exerted downward by Earth is an example of Newton s law.
The fact that the 4N force exerted upward by the book is equal in magnitude and opposite in direction to the 4N force exerted downward by Earth is an example of Newton s 3rd law.
Increased to 5N Remains at 4N Does the book remain in equilibrium?
Increased to 5N Does the book remain in equilibrium? NO Remains at 4N
Increased to 5N Remains at 4N Is the force exerted on the book by the hand equal in magnitude an opposite in direction to the force exerted on the book by Earth?
Increased to 5N Remains at 4N Is the force exerted on the book by the hand equal in magnitude an opposite in direction to the force exerted on the book by Earth? NO F net = 1N UP
Increased to 5N Remains at 4N Is the force exerted on the book by the Earth equal in magnitude an opposite in direction to the force exerted on Earth by the book? YES
Increased to 5N Remains at 4N Is the force exerted on the book by the hand equal in magnitude an opposite in direction to the force exerted on the hand by the book?
Increased to 5N Remains at 4N Is the force exerted on the book by the hand equal in magnitude an opposite in direction to the force exerted on the hand by the book? YES
Finally, the hand is removed suddenly from under the book while the book is rising upward. How many forces then act on the book?
How many forces then act on the book? ONE Remains at 4N
Remains at 4N Is the book in equilibrium?
What balances the downward force exerted on the book by the earth? Remains at 4N F net = 0 F earth on book = F book on earth M earth x a earth = m book x a book