Objectives: The student will Objectives: The student will be able to:be able to:

State Newton's 3rd law of motion and give examples that illustrate that law.

Apply Newton’s 3rd law of motion to various situations.

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What is Newton’s third law of motion?What is Newton’s third law of motion?

DemoDemo

Push on wall Push on wall Push on wall with cartPush on wall with cart

NewtonNewton’’s Third Law of Motions Third Law of Motion

Whenever one object exerts a force on a second object, Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite the second exerts an equal force in the opposite direction on the first.direction on the first.

Newton’s third law of motion

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.

Newton’s 3rd Law• For every action there is an equal and

opposite reaction.

Book toearth

Table tobook

Think about it . . .

What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.

Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on

your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe

hurts).

Newton’s Third Law

• A bug with a mass of 5 grams flies into the windshield of a moving 1000kg bus.

• Which will have the most force?

• The bug on the bus• The bus on the bug

Newton’s Third Law

• The force would be the same.

• Force (bug)= m x A

• Force (bus)= M x a

Think I look bad?You should see the other guy!

Action: earth pulls on you

Reaction: you pull on earth

Action and Reaction on Different Masses

Consider you and the earth

4-5 Newton’s Third Law of MotionHelpful notation: the first subscript is the object that the force is being exerted on; the second is the source.

This need not be done indefinitely, but is a good idea until you get used to dealing with these forces.

(4-2)

Concept Question

What makes a car go forward?

Concept Question answer

By Newton’s third law, the ground pushes on the tires in the opposite direction, accelerating the car forward.

Action: tire pushes on roadReaction: road pushes on tire

Concept Question

Which is stronger, the Earth’s pull on an orbiting space shuttle or the space shuttle’s pull on the earth?

Concept Question Answer

According to Newton’s Third

Law, the two forces are equal and opposite. Because of the huge difference in masses, however the space shuttle accelerates much more towards the Earth than the Earth accelerates toward the space shuttle.

a = F/m

Action - ReactionAction - Reaction

• You constantly use action-reaction force pairs as you move about.

• When you jump, you push down on the ground.

• The ground then pushes up on you. It is this upward force that pushes you into the air.

Action - ReactionAction - Reaction

• When the rocket fuel is ignited, a hot gas is produced.

• As the gas molecules collide with the inside engine walls, the walls exert a force that pushes them out of the bottom of the engine.

Action - ReactionAction - Reaction

• This downward push is the action force.

• The reaction force is the upward push on the rocket engine by the gas molecules.

• This is the thrust that propels the rocket upward.

Consider hitting a baseball with a bat. If we call the force applied to the ball by the bat the action force, identify the reaction

force.

(a) the force applied to the bat by the hands

(b) the force applied to the bat by the ball

(c) the force the ball carries with it in flight

(d) the centrifugal force in the swing

(b) the force applied to the bat by the ball

Other examples of NewtonOther examples of Newton’’s s Third LawThird Law

• The baseball forces The baseball forces the bat to the left (an the bat to the left (an action); the bat forces action); the bat forces the ball to the right the ball to the right (the reaction). (the reaction).

• Why does the ballWhy does the ball

accelerate if the accelerate if the

forces are equal?forces are equal?

Action - Reaction• If you hit a tennis ball with a racquet,

the force on the ball due to the racquet is the same as the force on the racquet due to the ball, except in the opposite direction.

• If you drop an apple, the Earth pulls on the apple just as hard as the apple pulls on the Earth.

• If you fire a rifle, the bullet pushes the rifle backwards just as hard as the rifle pushes the bullet forwards.

“For every action there’s an equal but opposite reaction.”

Earth / AppleHow could the forces on the tennis ball, apple, and bullet, be the same as on the racquet, Earth, and rifle? The 3rd Law says they must be, the effects are different because of the 2nd Law!

Earth

apple

3.92 N

3.92 N

0.40 kg

5.98 1024 kg

A 0.40 kg apple weighs 3.92 N (W = mg). The apple’s weight is Earth’s force on it. The apple pulls back just as hard. So, the same force acts on both bodies. Since their masses are different, so are their accelerations (2nd Law). The Earth’s mass is so big, it’s acceleration is negligible.

Earth / Apple (cont.)

a = mm a

Apple’s big acceleration

Apple’s little mass Earth’s little

acceleration

Earth’s big mass

The products are the same, since the forces are the same.

Newton’s 3rd Law

• Suppose you are taking a space walk near the space shuttle, and your safety line breaks. How would you get back to the shuttle?

Newton’s 3rd Law

• The thing to do would be to take one of the tools from your tool belt and throw it is hard as you can directly away from the shuttle. Then, with the help of Newton's second and third laws, you will accelerate back towards the shuttle. As you throw the tool, you push against it, causing it to accelerate. At the same time, by Newton's third law, the tool is pushing back against you in the opposite direction, which causes you to accelerate back towards the shuttle, as desired.

Demolition Derby

When two cars of different size collide, the forces on each are the SAME (but in opposite directions). However, the same force on a smaller car means a bigger acceleration!

Newton’s Third Law

• Newton’s Third Law of Motion– When one object exerts a force on a

second object, the second object exerts an equal but opposite force on the first.

Newton’s Third Law• Problem:

– How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force?

NO!!!

– Aren’t these “balanced forces” resulting in no acceleration?

Newton’s Third Law

– forces are equal and opposite but act on different objects

– they are not “balanced forces”– the movement of the horse depends on the

forces acting on the horse

• Explanation:

Example of 3rd Law

A horse harnessed to a cart exerts an equal and opposite force to the cart as it exerts a force against the ground.

SwimmingDue to the 3rd Law, when you swim you push the water (blue), and it pushes you back just as hard (red) in the forward direction. The water around your body also produces a drag force (green) on you, pushing you in the backward direction. If the green and red cancel out, you don’t accelerate (2nd Law) and maintain a constant velocity.

Note: The blue vector is a force on the water, not the on swimmer! Only the green and red vectors act on the swimmer.

Putting the three laws together

• http://science360.gov/obj/video/642db496-d506-432e-85b4-4e38f75d9142/newtons-three-laws-motion

Show sample problems and demo

Section ReviewQuestion 1

Explain Newton’s third law of motion.

Answer

The third law says that forces always act in equal but opposite pairs. For every action, there is an equal and opposite reaction.

Section ReviewQuestion 2

If they are “equal but opposite,” why don’t action-reaction pairs cancel?

AnswerAction-reaction pairs don’t cancel because they act on different objects, not on the same object. Equal and opposite forces acting on the same object would cancel.

Homework

• Hand out on Newton’s Third Law

• Conceptual Development Practice Page

• Questions on pages 97-98 #s 5, 6, 7, 15, 17, 18

• Computer Activity if want it for reinforcement

Closure

• Kahoot