1. QQ:
Five people are playing tug-of-war. Greg and Jacob pull to the right with 45 N and 35 N, respectively. Brandon and Sharon pull to the left with 53 N and 38 N, respectively. With what force and in what direction does Victor pull if the game is tied?
A large model rocket engine can produce a thrust of 12.0 N upon ignition. The engine part has a total mass of kg when launched.
Draw a free-body diagram of the rocket .
What is the net force that is acting on the model rocket just after it leaves the ground?
What is the initial acceleration of the rocket?

2. Today’s Objective:
I can use Newton’s Third Law to explain forces and motion.

3. Isaac Newton and the Bike

4. Newton’s third law describes something else that happens when one object exerts a force on another object.
According to Newton’s third law of motion, forces always act in equal but opposite pairs.

5. 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.

6. Example of 3rd Law
Space shuttle’s rocket boosters propel the orbiter into space by exerting an equal and opposite force to exhaust gasses.

7. Forces always come in pairs.

8. Interaction Pairs:
Two forces acting in opposite directions AND with equal magnitudes
The greater the force of the oars in this direction
The greater force the boat moves in this direction

9. The forces exerted by two objects on each other are often called an action-reaction force pair.
Either force can be considered the action force or the reaction force.

10. Beware of these common misconceptions:
1. They do NOT cancel each other out. If they did, there would never be any movement.
Hyperlink example on horse

11. Forces that act on different objects don't cancel - after all, they affect the motion of different objects!
Fins push water backwards.
Size of force on water = size of force on fish
The push on water moves the fish.
The direction of force on water is in the opposite of the direction the fish moves
Action pairs make it possible for the fish to move.
Water pushes the fish forward.

12. Mathematically, if the forces cancel each other, the net force would be zero. Think…. f = 0
If the net forces = 0, think how this would affect objects using Newton’s 2nd Law: f = ma or rewritten as a = f/m
If f is 0, acceleration would always be 0…therefore nothing would move.

13. Action and reaction force pairs don’t cancel because they act on different objects.
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.

14. Object A accelerates (or not) because of the forces that push or pull on it. (Newton's 2nd Law) Forces that push or pull on some other object have no effect on object A's motion - even if object A exerts them.

15. Only the forces that act on an object can cancel
Only the forces that act on an object can cancel. Forces that act on different objects don't cancel - after all, they affect the motion of different objects!
Wheels grip the road and push the road backwards. At the same time, the road pushes the wheels forward. The sizes of the forces are equal but in opposite directions. This moves the car forward.

16. These forces cancel each other out.
Both of these forces are acting on the box.

17. The normal upwards force and the force of gravity cancel each other because they are both acting on the same object.

18. Another common misconception:
In the beginning it is often assumed that the more powerful entity in an interaction – the faster, bigger, or stronger one – will exerts a larger force. According to Newton’s third law, this is NOT true. The objects exert forces of identical magnitude.

19. The 3rd Law Expert --- Bill Nye

20. Solving Problems:
Determine the system.
Draw a system schema.

21. What is a System Schema?

22. Solving Problems: Determine the system. Draw a system schema.
Create free-body diagrams for each system.
Connect interaction pairs by dashed lines.
Use 2nd law to relate net force & acceleration
Use 3rd law to find magnitudes & direction

23. When a softball with a mass of 0
When a softball with a mass of 0.18 kg is dropped, its acceleration toward Earth is equal to the variable g. What is the force on Earth due to the ball, and what is Earth’s resulting acceleration? The mass of Earth is 6.0 x kg.

24. F earth on ball = ? aearth = ?
Determine the system: Ball and earth
Draw a system schema.
3) Create free-body diagrams for each system.
4) Connect interaction pairs by dashed lines.
Solve problem.
mball = kg m earth = 6.0 x kg
g = 9.8 m/s2
F earth on ball = ? aearth = ?
Fball on earth B E
Fearth on ball

25. Use Newton’s 2nd law to find F earth on ball which is known as weight.
SOLVE:
mball = kg m earth = 6.0 x kg g = 9.8 m/s2
F earth on ball = ? aearth = ?
Use Newton’s 2nd law to find F earth on ball which is known as weight.
F earth on ball = mball a
Acceleration due to gravity is -9.8 m/s2
F earth on ball = 0.18 kg (- 9.8 m/s2)
F earth on ball = -1.8 kg *m/s2
F earth on ball = -1.8 N

26. F earth on ball = -1.8 N (from 1st problem solved)
Use Newton’s third Law to find F ball on earth
mball = kg m earth = 6.0 x kg
F earth on ball = -1.8 N (from 1st problem solved)
F ball on earth = -F earth on ball
F ball on earth = -(-1.8 N)
F ball on earth = 1.8 N
F earth on ball =

27. Substitute: Fnet = m earth a earth 1.8 N = 6.0 x 10 24 kg * a earth
Use Newton’s Second law to find the aearth because of the net force caused by the ball hitting it.
Fnet = 1.8 N m earth = 6.0 x kg
Substitute: Fnet = m earth a earth
1.8 N = 6.0 x kg * a earth
6.0 x kg x kg
3 X N/kg = a earth
3 X m/s 2 = a earth
kg * m/s2 kg

28. You are walking along when you slip on some ice and fall
You are walking along when you slip on some ice and fall. For a moment you are in free fall. During this time, what force do you exert on Earth if your mass is 55.0 kg? Hint: find the force Earth exerts on you first.

29. Determine the system.
Draw a system schema.
Create free-body diagrams for each system.
Connect interaction pairs by dashed lines.
Solve problems: Your mass = 55 kg
Acceleration due to gravity = 9.8 m/s2
Choose formula: f = ma
Solve: Fearth on you = 539 N

30. Solve: Fearth on you = 539 N
The force you exert on Earth is the same magnitude.
Fyou on earth = 539 N

31. Newton’ s Three Laws of Motion

32. online problems

33. Two 100-N weights are attached to a spring scale as shown
Two 100-N weights are attached to a spring scale as shown. Does the scale read zero, 100 N, or 200 N -- or some other reading?

34. The scale reads the tension in the string
The scale reads the tension in the string. The tension in the string is 100 N. This is the force the string must exert up on either of the 100-N weights at either end of the string.

35. Nothing is moving, nothing is accelerating, so the net force on the spring is zero. Likewise, the net force on either of the 100-N weights is also zero. But that is another question. The spring scale does not measure the net force. The spring scale simply measures the tension, the magnitude of the force exerted by the string.

36. Concept Question 1
Why are we able to walk?

37. Concept Question Answer
We walk forward because when one foot pushes backward against the ground, the ground pushes forward on that foot.
Force exerted
on the person’s foot by the ground. Fpg
Force exerted on the ground
by person’s foot. Fgp
Fgp=-Fpg

38. Concept Question 2
What makes a car go forward?

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

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

41. 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

42. Problem 1
What force is needed to accelerate the 60 kg cart at 2m/s^2?

43. How to solve Problem 1 Force = mass times acceleration F = m * a
What force is needed to accelerate the 60kg cart at 2 m/s^2?
Force = mass times acceleration
F = m * a
F = 60kg * 2m/s^2
F = 120kgm/s^2
Kgm/s^2 = Newton
Newton = N
F =120 N

44. Problem 2
A force of 200 N accelerates a bike and rider at 2 m/s^2. What is the mass of the bike and rider?

45. How to solve Problem 2 F = ma therefore:m =F/a m = 200N/2m/s^2
A force of 200 N accelerate a bike and rider at 2m/s^2. What is the mass of the bike and rider?
F = ma therefore:m =F/a
m = 200N/2m/s^2
N= kgm/s^2 so when divide your answer will be kg left.
m = 100kg

46. Why can you exert greater force on the pedals of a bicycle if you pull up on the handlebars?
The handlebars then pull down on you, somewhat as if someone were pushing down on your shoulders. This lets you exert a greater downward force on the pedals.

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