1. For every action there is an equal and opposite reaction.
Newton’s Third Law
For every action there is an equal and opposite reaction.

2. Newton’s 3rd Law
For every action there is an equal and opposite reaction.
Book to
earth
Table to
book

3. 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).

4. 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

5. 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!

6. Newton’s Third Law of Motion
For every action, there is an equal and opposite reaction.
The block’s weight pushes on the ground
Forces come in action-reaction pairs.
Make the connection that the force of the ground on the block is the normal force discussed earlier in Day 2.
Image source:
A stack of concrete blocks: 2006 Justanother, Wikipedia Commons
The ground pushes back on the block

7. Examples of Newton’s Third Law
Identify several action-reaction force pairs in the photograph.
Ask students to identify the action-reaction force pairs in the image.
Newton’s third law explains the recoil of projectile weapons such as cannons and guns. A fun tie in is a scene from Wall-E in which the robot uses the fire extinguisher as a propulsion system (the reaction force causes the robot to move). Another good third law/recoil example is a garden hose dancing around the yard.
Image source:
A cannon firing with flames and smoke: 2009 David Brandt, Wikimedia Commons
The cannon exerts a force on the cannon ball, and the cannon ball exerts an equal and opposite force on the cannon.

8. Action: tire pushes on road Reaction: road pushes on tire

9. Reaction: gases push on rocket
Action: rocket pushes on gases

10. Consider hitting a baseball with a bat
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

11. Concept Review Inertia force accelerate velocity mass
1. ______________ is an object’s resistance to changing its motion. 2. Applying an unbalanced ___________ to an object causes it to ______________. 3. Based on Newton’s first law, if no forces are acting on an object, its ____________ will not change. 4. From Newton’s second law, an object’s acceleration depends on the object’s ___________ and the strength of the ___________ _____ acting on it. 5. Newton’s third law: For every action there is ________________________________________.
force
accelerate
velocity
Review slide for concepts from all three lessons in the unit.
Next, conduct the associated activity, Sliding Textbooks.
Then administer the final quiz for the unit.
mass
unbalanced force
an equal and opposite reaction

12. What Laws are represented?

13. Review Newton’s First Law:
Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration (F = ma).
Newton’s Third Law:
For every action there is an equal and opposite reaction.

14. Newton's Laws
1stlaw: Homer is large and has much mass, therefore he has much inertia. Friction and gravity oppose his motion.
2nd law: Homer’s mass x 9.8 m/s/s equals his weight, which is a force.
3rd law: Homer pushes against the ground and it pushes back.