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Newton’s 3 rd Law of Motion. Newton’s 3 rd law of motion. For every action, there is an equal and opposite reaction. Spring scales demo. Force pairs. Forces always come in pairs  equal and opposite action / reaction pairs. Force pairs are acting ON TWO DIFFERENT OBJECTS

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newton s 3 rd law of motion1
Newton’s 3rd law of motion
  • For every action, there is an equal and opposite reaction.
force pairs
Force pairs
  • Forces always come in pairs  equal and opposite action / reaction pairs.
  • Force pairs are acting ON TWO DIFFERENT OBJECTS
  • Action: Bug hits windshield

Reaction: Windshield hits bug

balanced forces vs force pairs
Balanced Forces vs. Force pairs

ACTION

Normal Force

REACTION

Balanced Forces

(Both forces acting on the tissue box)

The tissue box is pushing down on the table.

The table is pushing up on the tissue box.

The tissue box is pulling up on the Earth.

The Earth is pulling down on the tissue box.

REACTION

ACTION

Weight

how does a fish swim
How does a fish swim?
  • A fish uses its fins to push water backwards. But a push on the water will only accelerate the water backwards. According to Newton’s 3rd law, the water reacts by pushing the fish forwards. Thus the fish is propelled through the water!
which of the following forces is considered a force pair
Which of the following forces is 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
action reaction
Action / Reaction
  • Which of the following forces is considered a force pair?

Force pairs involve two objects!

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

slide9

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

  • Force on the bug
  • Force on the bus
  • They are equal
  • Not enough information
trick question
Trick Question!

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

compare
BUG

F = m·a

Bus Windshield

F = m·a

Compare
compare1
Compare

Forces are equal

m = 1000 kg

a = 0.002 m/s2

m = 0.01 kg

a = 200 m/s2

F = ma

F = 2 N F = 2 N

newton s 3 laws hockey
Newton’s 3 Laws  Hockey
  • http://www.nbclearn.com/portal/site/learn/science-of-nhl-hockey
slide14

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

  • greater than the acceleration of the bullet.
  • smaller than the acceleration of the bullet.
  • the same size as the acceleration of the bullet.
answer
Answer

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