Newton’s 3Newton’s 3rdrd Law of Law of MotionMotion

Newton’s 3Newton’s 3rdrd law of law of motionmotion

For every action, there is an equal and For every action, there is an equal and opposite reaction.opposite reaction.

Spring scales demoSpring scales demo

Force pairsForce pairs Forces always come in Forces always come in

pairs pairs equal and equal and opposite action / reaction opposite action / reaction pairs.pairs.

Force pairs are acting ON Force pairs are acting ON TWO DIFFERENT TWO DIFFERENT OBJECTSOBJECTS

Action: Bug hits windshieldAction: Bug hits windshield

Reaction: Windshield hits Reaction: Windshield hits bugbug

Balanced Forces vs. Force Balanced Forces vs. Force pairspairs

Normal Force

Weight

The table is pushing up on the tissue box.

The Earth is pulling down on the tissue box.

ACTIONACTIONBalanced ForcesBalanced Forces (Both forces acting on the

tissue box)

ACTIONACTION

REACTIONREACTION

REACTIONREACTION

The tissue box is pushing down on the table.

The tissue box is pulling up on the Earth.

How does a fish swim?How does a fish swim?

A fish uses its fins to push water backwards. But a A fish uses its fins to push water backwards. But a push on the water will only accelerate the water push on the water will only accelerate the water backwards. According to Newton’s 3backwards. According to Newton’s 3rdrd law, the water law, the water reacts by pushing the fish forwards. Thus the fish is reacts by pushing the fish forwards. Thus the fish is propelled through the water!propelled through the water!

Which of the following Which of the following forces is considered a force forces is considered a force pair?pair?

1 2

50%50%

1.1. ActionAction:: Bat pushes baseball to the right Bat pushes baseball to the right ReactionReaction:: Baseball pushes bat to the left Baseball pushes bat to the left

2.2. ActionAction: Bat pushes baseball to the right : Bat pushes baseball to the right ReactionReaction: Air resistance pushes baseball to : Air resistance pushes baseball to the leftthe left

Action / ReactionAction / Reaction

Which of the following forces is Which of the following forces is considered a force pair?considered a force pair?

Action – bat pushes baseball to the right

Reaction – baseball pushes bat to the left

Action – bat pushes baseball to the right

Reaction – air resistance pushes baseball to the left

Force pairs involve two objects!Force pairs involve two objects!

While driving down the road, an While driving down the road, an unfortunate bug strikes the windshield of unfortunate bug strikes the windshield of a bus. Quite obviously, this is a case of a bus. Quite obviously, this is a case of Newton's third law of motion. The bug hit Newton's third law of motion. The bug hit the windshield and the windshield hit the the windshield and the windshield hit the bug. Which of the two forces is greater: bug. Which of the two forces is greater: the force on the bug or the force on the the force on the bug or the force on the bus? bus?

1 2 3 4

25% 25%25%25%1.1. Force on the bugForce on the bug

2.2. Force on the busForce on the bus

3.3. They are equalThey are equal

4.4. Not enough Not enough informationinformation

Trick Question!Trick Question!

Each force is the same size. For every Each force is the same size. For every action, there is an equal ... (equal!). The action, there is an equal ... (equal!). The fact that the bug splatters only means fact that the bug splatters only means that with its smaller mass, it is less able that with its smaller mass, it is less able to withstand the larger acceleration to withstand the larger acceleration resulting from the interaction. resulting from the interaction.

CompareCompare

BUGBUG

FF = = mm··aa Bus WindshieldBus Windshield

FF = = mm··aa

CompareCompare

m = 0.01 kgm = 0.01 kg

a = 200 m/sa = 200 m/s22

m = 1000 kgm = 1000 kg

a = 0.002 m/sa = 0.002 m/s22

F = 2 NF = 2 N F = 2 N F = 2 NF = maF = ma

Forces are equalForces are equal

Newton’s 3 Laws Newton’s 3 Laws HockeyHockey

http://www.nbclearn.com/portal/site/learn/science-of-nhl-hockey

A gun recoils when it is fired. The recoil A gun recoils when it is fired. The recoil is the result of action-reaction force is the result of action-reaction force pairs. As the gases from the gunpowder pairs. As the gases from the gunpowder explosion expand, the explosion expand, the gungun pushes the pushes the bulletbullet forwards and the forwards and the bulletbullet pushes pushes the the gungun backwards. The acceleration of backwards. The acceleration of the recoiling gun is ... the recoiling gun is ...

1 2 3

33% 33%33%

1.1. greater than the acceleration greater than the acceleration of the bullet. of the bullet.

2.2. smaller than the acceleration smaller than the acceleration of the bullet.of the bullet.

3.3. the same size as the the same size as the acceleration of the bullet.acceleration of the bullet.

AnswerAnswer

The force on the gun The force on the gun equals the force on the equals the force on the bullet. Yet, acceleration bullet. Yet, acceleration depends on both force depends on both force and mass. The bullet and mass. The bullet has a greater has a greater acceleration due to the acceleration due to the fact that it has a smaller fact that it has a smaller mass. Remember: mass. Remember: acceleration and mass acceleration and mass are inversely are inversely proportional. proportional.