1. Objectives: The student will 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.
Explain the tension in ropes and strings in terms of Newton’s third law.
Determine the value of the normal force by applying Newton’s 2nd law.

2. Warm Up
What is Newton’s third law of motion?

5. Demo
Push on wall
Push on wall with cart

6. Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first.

7. Newton’s third law of motion

8. 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.

9. Interaction Forces 4.3 Identifying Interaction Forces
Section
Interaction Forces
4.3
Identifying Interaction Forces
When you exert a force on your friend to push him forward, he exerts an equal and opposite force on you, which causes you to move backwards.
The forces FA on B and FB on A are an interaction pair.
An interaction pair is two forces that are in opposite directions and have equal magnitude.

10. Section
Interaction Forces
4.3
Drag Force and Terminal Velocity

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

12. 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).

13. 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

14. 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!

15. Action and Reaction on Different Masses
Consider you and the earth
Action: earth pulls on you
Reaction: you pull on earth

16. Newton’s Third Law of Motion
Helpful 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)

17. Concept Question
What makes a car go forward?

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

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

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

21. 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

22. Action - 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.

23. Action - 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.

24. Action - 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.

25. 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

26. Other examples of Newton’s Third Law
The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).
Why does the ball
accelerate if the
forces are equal?

27. “For every action there’s an equal but opposite reaction.”
Action - Reaction
“For every action there’s an equal but opposite 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.

28. Earth / Apple
How 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!
0.40 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.
apple
3.92 N
Earth
3.92 N
5.98  1024 kg

29. a = m Earth / Apple (cont.) a m Apple’s Earth’s little mass big mass
The products are the same, since the forces are the same.
a = m m a
Apple’s
little mass
Earth’s
big mass
Earth’s little acceleration
Apple’s big acceleration

30. 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?

31. 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.

32. 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!

33. 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.

34. 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?
Aren’t these “balanced forces” resulting in no acceleration?
NO!!!

35. Newton’s Third Law Explanation:
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

36. 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.

37. Putting the three laws together

38. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Draw free-body diagrams for the two systems: the ball and Earth and connect the interaction pair by a dashed line.

39. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Identify known and unknown variables.
Known:
mball = 0.18 kg
mEarth = 6.0×1024 kg
g = 9.80 m/s2
Unknown:
FEarth on ball = ?
aEarth = ?

40. Step 2: Solve for the Unknown
Section
Interaction Forces
4.3
Earth’s Acceleration
Step 2: Solve for the Unknown

41. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Use Newton’s second and third laws to find aEarth.

42. Section
Interaction Forces
4.3
Earth’s Acceleration
Substitute a = –g

43. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Substitute mball = 0.18 kg, g = 9.80 m/s2

44. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Use Newton’s second and third laws to solve for FEarth on ball and aEarth.

45. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Substitute FEarth on ball = –1.8 N

46. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Use Newton’s second and third laws to find aEarth.

47. Interaction Forces 4.3 Earth’s Acceleration
Section
Interaction Forces
4.3
Earth’s Acceleration
Substitute Fnet = 1.8 N, mEarth= 6.0×1024 kg

48. The normal force is important when calculating resistance.
Section
Interaction Forces
4.3
The Normal Force
The normal force is the perpendicular contact force exerted by a surface on another object.
The normal force is important when calculating resistance.

49. Interaction Forces 4.3 Forces of Ropes and Strings
Section
Interaction Forces
4.3
Forces of Ropes and Strings
Tension forces are at work in a tug-of-war.
If team A, on the left, is exerting a force of 500 N and the rope does not move, then team B, must also be pulling with 500 N.
Click the Back button to return to original slide.

50. Section Review Question 1 Answer Explain Newton’s third law of motion.
The third law says that forces always act in equal but opposite pairs. For every action, there is an equal and opposite reaction.

51. Section Review Question 2 Answer
If they are “equal but opposite,” why don’t action-reaction pairs cancel?
Answer
Action-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.

52. Homework Tentative test Wednesday Period 3 and Thursday Period 4B
Hand out on Newton’s Third Law

53. Closure
Kahoot

54. If P is the magnitude of the contact force between the blocks, draw the free-body diagrams for each block.

56. An Atwood’s machine is two masses connected by a strong light string that are hung over an ideal pulley (light and frictionless). The masses have identical velocity and acceleration magnitudes at every instant. If we define up on the left and down on the right as positive directions, then the masses have identical velocities and accelerations period. This simplifies the analysis a lot.

57. Atwood Machine
Would this move?
100 kg
100 kg

58. Atwood Machine FT FT Fg = w Fg = w Would this move? Which way? Forces?
200 kg
100 kg
Fg = w
Fg = w

59. Atwood Device m1 m2
m1g T
m2g
Assume m1 < m2 and that the clockwise direction is +.
If the rope & pulley have negligible mass, and if the pulley is frictionless, then T is the same throughout the rope.
If the rope doesn’t stretch, a is the same for both masses.

60. Atwood Analysis a = T m2 – m1 m1 + m2 m1g g m2g
Remember, clockwise has been defined as +. m1 m2
m1g T
m2g
2nd Law on m1: T - m1g = m1a
2nd Law on m2: m2g - T = m2 a
Add equations: m2g – m1g = m1a + m2 a (The T ’s cancel out.) Solve for a:
m2 – m m1 + m2
a = g

61. Atwood Machines
Example without numbers

62. An Atwood’s machine has m1 = 2 kg, m2 = 3 kg, hung from an ideal pulley. What is the acceleration of the masses? Calculate the tension in the string attached to each mass.