1. Unit 3: Force & Newton’s Part III (Chp 6)
(Chp 4-6) 3 Laws of Motion
Part III (Chp 6)
Newton’s 3rd Law (action/reaction)
Print: 1-7,10,12,14,16,18
For every force, there is an equal and opposite force.

2. A force is a push or a pull.
6.1 Forces and Interactions
A force is a push or a pull.
When you push on the wall, the wall pushes on you.

3. A hammer exerts a force on the nail and drives it into a board.
6.1 Forces and Interactions
A hammer exerts a force on the nail and drives it into a board.
There must also be a force exerted on the hammer to stop it.
There is ALWAYS
a pair of forces.
(never just one force)
In this case,
one acting on the nail and the
other acting on the hammer.

4. Newton’s 3rd Law: action/reaction forces
For every action force on an object, there is an equal and opposing reaction force on the OTHER object.
One force is called the action force.
The other force is the reaction force.
Neither force exists without the other.
They’re equal in strength
and opposite in direction.
Do NOT cancel out
(they’re on different objects)

5. Forces always occur in pairs.
6.2 Newton’s 3rd Law
Forces always occur in pairs.
You push against the floor, and the floor simultaneously pushes against you.
The tires of a car push against the road, and the road simultaneously pushes back on the tires.
When swimming, you push the water backward, and…
the water pushes you forward.

6. 6.2 Newton’s 3rd Law
In many cases, the interactions depend on friction as one of the forces in the pair.
On ice friction is minimal and you may not be able to exert an action force against the ice.
Without the action force there cannot be a reaction force, and there is no resulting forward motion.

7. There is a simple process for treating action and reaction forces:
6.3 Action and Reaction Pairs
There is a simple process for treating action and reaction forces:
First identify the interaction. Let’s say one object, A, interacts with another object, B.
State the action and reaction forces in the form:
Action:
A exerts a force on B.
Reaction:
B exerts an equal force on A.

8. If we call the action Earth exerting a force on the boulder,
6.3 Action and Reaction Pairs
Sometimes the pair of action and reaction forces in an interaction is not obvious.
In the case of a falling boulder, what are the action and reaction forces?
If we call the action Earth
exerting a force on the boulder,
then the reaction is…
the boulder exerting a force on Earth.

9. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

10. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

11. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

12. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

13. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

14. 6.3 Action and Reaction Pairs
If action is A exerts force on B, then reaction is B exerts equal force on A.

15. equal force, but opposite direction
6.3 Action and Reaction Pairs
think!
We know that Earth pulls on the moon.
Does the moon also pull on Earth?
Which pull is stronger?
Which distance is greater—between New York and L.A., or between L.A. and New York?
The distances either way are the same.
It’s the same with force pairs.
Both Earth and moon pull on each other with equal and opposite forces.
yes
equal force, but opposite direction

16. 6.4 Action and Reaction on Different Masses
Earth is pulled up by the boulder with just as much force as the boulder is pulled down by Earth.
The forces are equal in strength and opposite in direction.
Although the pair of forces between the boulder and Earth is the same, the masses are NOT equal.
same F, but...
more m, less a.
Because Earth has a huge mass, we don’t sense its super tiny acceleration.

17. F F = ma m a = 6.4 Action and Reaction on Different Masses
The force a cannon exerts on a cannonball is exactly equal and opposite to the force the cannonball exerts on the cannon, but…
the cannonball moves much faster than the cannon.
Newton’s 2nd Law shows that we must also consider the masses. F m
F = ma
a =

18. 6.4 Action and Reaction on Different Masses
The cannonball has more acceleration than the cannon because it has less mass. F m
F = ma
a =

19. The air accelerates backward from the force of the balloon on it.
6.4 Action and Reaction on Different Masses
Cannon recoil from the cannonball launch can help us understand rocket propulsion.
The air accelerates backward from the force of the balloon on it.
The balloon accelerates forward from the force of air on it.

20. 6.4 Action and Reaction on Different Masses
Cannon recoil from the cannonball launch can help us understand rocket propulsion.
A rocket accelerates as it continually recoils from the exhaust gases ejected from its engine.
Each molecule of exhaust gas acts like a tiny molecular cannonball shot downward from the rocket.

21. 6.4 Action and Reaction on Different Masses
Lift
Using Newton’s 3rd Law, we can understand how a helicopter gets its lifting force.
The whirling blades force air
particles downward (action).
The air forces the blades
upward (reaction).
This upward reaction force
is called lift.
When lift equals the weight,
the helicopter hovers.
When lift is greater, it climbs upward.

22. Every interaction requires at least… a single force. a pair of forces.
Quick Quiz!
Every interaction requires at least…
a single force.
a pair of forces.
an action force.
a reaction force.

23. opposite in direction and equal in magnitude.
Quick Quiz.
Whenever one object exerts a force on a second object, the second object exerts a force on the first object that is…
opposite in direction and equal in magnitude.
in the same direction and equal in magnitude.
opposite in direction and greater in magnitude.
in the same direction and weaker in magnitude.

24. Quick Quiz.
The force that directly propels a car along a road is that provided by the…
engine.
fuel.
tires.
road.

25. When you jump vertically upward, what does the Earth do? move downward
Quick Quiz.
When you jump vertically upward, what does the Earth do?
move downward
also move upward with you
remain stationary
move sideways a bit

26. To answer this question, we must consider the system involved.
6.5 Defining Systems
Since action and reaction forces are equal and opposite, why don’t they cancel to zero?
To answer this question, we must consider the system involved.
Print: 26-36

27. reaction force ON apple
6.5 Defining Systems
The system (orange) accelerates from force.
The action force ON orange (from apple) is not cancelled by the reaction force ON apple. The orange still accelerates.
force
ON orange
reaction force ON apple
system

28. system 6.5 Defining Systems Action and Reaction forces cancel b/c…
IN system
NOT
ON system

29. system force ON orange-apple system
6.5 Defining Systems
system
Action and Reaction forces cancel b/c…
IN system
NOT
ON system
apple pushes on floor, so floor pushes on apple
The orange-apple system accelerates.
force ON
orange-apple system

30. 6.5 Defining Systems
Inside a baseball, trillions of atomic/molecular forces hold the ball together but do not accelerate the ball.
They are part of action-reaction pairs within the ball, but they combine to zero.
If the action-reaction forces are internal to the system, then they cancel
and the system does
not accelerate.
An external force,
such as a swinging bat
is needed to
accelerate the ball.

31. think! 6.5 Defining Systems
Suppose a friend who hears about Newton’s 3rd Law says that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to your kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move! What do you say to your friend?
If you kick a football, it will accelerate b/c no other force has been applied to the ball.
(the ball applies a reaction force to your foot, not a cancelling force on the ball)

32. IN system ON system ON system ON ground
6.6 The Horse-Cart Problem
Will the horse’s pull on the cart be canceled by the opposite and equal pull by the cart on the horse, thus making acceleration impossible?
IN system
ON system
ON system
ON ground

33. IN system ON system ON system ON ground
6.6 The Horse-Cart Problem
3 pairs of forces: P/P F/F f/f
IN system: P forward – P backward (cancels)
ON system: F forward – f backward
IN system
ON system
ON system
ON ground

34. Fnet mcart+horse a = F = F – f = Fnet IN system ON system ON system
6.6 The Horse-Cart Problem
ON system: F forward – f backward
F = F – f = Fnet
Fnet
mcart+horse
a =
IN system
ON system
ON system
ON ground

35. 6.6 The Horse-Cart Problem
Turn to
p. 81
in your book
NOW!

36. If you hit the wall, it will hit you equally hard.
6.7 Action Equals Reaction
If you hit the wall, it will hit you equally hard.
You cannot touch without being touched.
How hard can you hit a piece of paper in the air?
You can only be apply a force that can be applied back to you.

37. A system undergoes acceleration only when acted on by a(n)… net force.
Quick Quiz!
A system undergoes acceleration only when acted on by a(n)…
net force.
pair of forces.
action and reaction forces.
internal interactions.

38. If a net force acts on a horse while it is pulling a wagon, the horse…
Quick Quiz.
If a net force acts on a horse while it is pulling a wagon, the horse…
accelerates.
is restrained.
is pulled backward by an equal and opposite net force.
cannot move.

39. Quick Quiz.
At a pizza shop, the cook throws the pizza dough in the air. The amount of force the cook exerts on the dough depends on the…
mass of the dough.
strength of the cook.
weight of the dough.
height of the cook.
The cook can only exert a force on the dough that is equal to what the dough can exert back on him.