Newton’s 3 rd Law of Motion

1 st Law Humor

Cat’s 3 rd law of motion Pic supplied by Michael Turner – Encounter journal Pic supplied by Michael Turner – Encounter journal.

Newton’s third law of motion Newton noticed that forces were always in pairs and that the two forces were always equal in size but opposite in direction. He called the two forces action and reaction.

You’ve heard this one before for every action force there is an equal (in size) and opposite (in direction) reaction force. Forces always come in pairs — known as "action-reaction force pairs." Identifying and describing action-reaction force pairs is a simple matter of identifying the two things interacting and then making two statements describing who is pushing on whom and in which direction.

Newton’s third law of motion In a tug of war, if the rope is not moving what can you say about the forces exerted by the two teams? The forces involved must be equal and opposite!

Newton’s third law of motion Your elbow rests on the table. What is the pair of forces here? The force of the elbow on the table (downwards) The force of the table on the elbow (upwards) =

The 3 rd Law in Action Whenever one object exerts a force on another object, the other object exerts an equal but opposite force (in direction) on the first object. Note: each of the two forces in the pair acts on a different object. Hammer pushes on stake. Stake pushes on hammer. The hammer acts, the stake re-acts. The drawing shown is NOT a free-body diagram. Why not? (Candy-worthy.)

Action-Reaction Comparison Kent Budgett is pulling upon a rope which is attached to a wall. In the bottom picture, Kent is pulling upon a rope which is secured to an elephant. In each case, the force scale reads 500 Newtons. Kent is pulling... a. with more force when the rope is attached to the wall. b. with more force when the rope is attached to the elephant. c. with the same force in each case. Since they are action-reaction pairs, they must have the same magnitudes.

Action-Reaction Pairs Does a rocket need something to push on, such as the ground? No! The reaction force causes its movement.

Examples: When you push on a wall (action), the wall ___________ with ___________ (reaction). If you don’t believe me, try punching a wall! pushes back equal force

Walking…. To walk, a person must push _________ on the ground (action). The ground then pushes the person _______ (reaction). The action/reaction forces in this case are _________ forces. backward forward frictional

What is the reaction force to your weight? Is it the normal force? ____! To see why, recall that your weight is the gravitational pull of ________ on you (action). So, the reaction is the gravitational pull of ____ on ________. So, when you are accelerating towards the earth in free fall, the earth is actually ____________ towards you!! But, again, while the forces are the same, the accelerations are NOT the same…. the earth you NO the earth accelerating up a = m ma

Earth/moon gravitation action/reaction pair: Unlike us, the moon is massive enough to affect the earth’s motion!! (From The Cartoon Guide to Physics by Gonick and Huffman)

One final thought…. Why don’t action and reaction forces cancel each other out and thus produce NO accelerations, since they are “equal and opposite”? They act on ____________________! Lawn Sprinklers…. Action/Reaction causes sprinkler to spin in a circle. WHY? Bathroom scale demo: When I’m standing on a scale and… -pushing down on table, table pushes back ___ on me, so scale reading ______ - pulling up on table, table pushes _____ on me, so scale reading _________. updecreases down increases different objects

Action-Reaction Pairs

If action-reaction pairs are equal but opposite… why don’t they cancel? Because they’re acting on different objects.

The earth pulls on a person, so the person pulls on the earth with an equal and opposite force. If the person falls (accelerates) toward the earth, does this mean that the earth also accelerates toward the person? Yes No Let’s see how much. F = mg = (80kg)(9.8 m/s 2 ) = 784N a E = F/m E = 784N/(6 x ) = 4.7 x m/s 2 Sure—not very large or measureable.

In your book… The man said, “Giddiup,” to which the horse said… “That would be futile. You see, if I pull on the cart, the cart will pull back on me. By Newton’s 3 rd Law, the forces are equal and opposite, so they’ll cancel out. A zero net force won’t get us moving.” What’s wrong with how this horse is thinking, in terms of the 3 rd law?

The 3 rd Law is not “Equilibrium”

Consider the forces acting on each – the horse and the cart What forces are acting on the cart? Is the net force zero? What forces are acting on the horse? Is the net force zero on the horse?

Action-Reaction Pairs Why does the gun recoil? Because the bullet pushes back on it! (It’s not very big, but it does push back with the same force.)

Action-Reaction Pairs Describe how birds fly… in terms of the 3 rd law.

Action-Reaction Pairs Describe how we walk… in terms of the 3 rd law.

Action-Reaction Pairs Describe what’s happening above.

More Action-Reaction Pairs The yellow ball moves because the blue ball exerts a force on it. Why does the blue ball stop? Because the yellow ball exerts a force back on it. Would the blue ball have stopped if it had more mass than the yellow ball? No.

Action-Reaction Pairs If action-reaction pairs are equal but opposite… why don’t they cancel? Because they’re acting on different objects.

Third Law action-reaction: Remember, forces always come in "action- reaction force pairs." We need to identify the two things interacting and then make two statements describing who is pushing on whom and in which direction. The two things interacting: A and B A pushes on B to the right. B pushes back on A to the left.

Describe what’s happening below: Remember, forces always come in "action- reaction force pairs." We need to identify the two things interacting and then make two statements describing who is pushing on whom and in which direction. The two things interacting: the ball and the bat The bat pushes on the ball to the right. The ball pushes back on the bat to the left.

Your turn You need to identify the two things interacting and then make two statements describing who is pushing on whom with what force and in which direction. Two things interacting: the weight-lifter and the weights The weight-lifter pushes the weights up with 300N. The weights push downward on the lifter with 300N.

Action-Reaction Pairs Two things interacting: the bowling ball and the pin The bowling ball pushes the pin to the right. The pin pushes back on the ball to the left. T F Because the bowling ball is much heavier than the pin, and it is moving, it must be applying a larger force on the pin than the pin applies back upon it?

Action-Reaction Pairs Two things interacting: the air and the balloon wall The air pushes the balloon wall outward. The balloon wall pushes back on the enclosed air inward.

Action-Reaction Pairs Two things interacting: The baseball and the glove The baseball pushes the glove to the left with 50N of force. The glove pushes on the baseball to the right with 50N of force.

Action-Reaction Pairs Two things interacting: the hand and the box The hand pushes the box to the right with 10N of force. The box pushes on the hand to the left with 10N of force.

Action-Reaction Pairs Two things interacting: the child and the wagon The child pulls the wagon to the left. The wagon pulls the child to the right.

Action-Reaction Pairs Two things interacting: The cannon and the cannonball The cannon pushes the cannonball to the right. The cannonball pushes on the cannon to the left.

Action-Reaction Pairs Two things interacting: The man and the wall The man pushes on the wall to the right with 100N of force. The wall pushes on the man to the left with 100N of force.

Action-Reaction Pairs Two things interacting: Yoshi and the box Yoshi pushes on the box to the left with 300N of force. The box pushes on Yoshi to the right with 300N of force.

Action-Reaction Pairs Two things interacting: the boxer and the bag The boxer pushes the bag to the right. The bag pushes back on the boxer to the left. Is this still true if the bag is very small? Yes.

What if one object cannot react with the same force acted upon it? On the left, the man weighs 600N, and the floor is able to push up with 600N of reactive force. On the picture which will soon appear on the right, the man still weighs 600N, but the floor is able to react with only 500N. What happens?

Can a ghost walk through walls? The ability to walk through walls is often listed as a talent of ghosts. But Newton’s laws of physics suggest that if a ghost can walk (which requires action-reaction pairs) it should not then be able to pass through walls, since that ignores action-reaction pairs. Newton says a body at rest will remain at rest until it’s acted on by an external force and for every action there is an equal but opposite reaction.