1. Newton’s Laws
The concept of force is simply a push or pull. This idea is made more quantitative with units of newtons and pounds. Students sometimes consider the force due to gravity and weight to be two different forces. This lesson reinforces the fact that weight IS the force due to gravity. The force of gravity is related to but distinct from the mass of an object.

2. Explaining motion mass force
Isaac Newton was the first person to explain WHY objects move the way they do.
He based his laws of motion on two key concepts:
mass
force
Point out to students that in chapters 2 and 3 they learned how to describe motion (using physics vocabulary like displacement, velocity, and acceleration). In chapter 4 they will begin the work of learning how to explain WHY objects move they way they do. This can be likened to the difference in biology between the study of taxonomy and science of genetics.

3. What is mass? All matter has mass and takes up space.
A solid rock has mass. So do gases and liquids.
With your hand out the window of a moving car, you feel the mass in the air pushing against you.
These next two slides review the concept of mass, which was introduced in Chapter 2.

4. What is force?
The concept of force:
A force is a push or a pull.

5. What is force? The concept of force: A force is a push or a pull.
Forces can cause an object to change its motion.
Can you give some examples of forces?
Give the students a couple minutes to name some forces they have heard about.

6. Types of forces The concept of force: A force is a push or a pull.
Forces can cause an object to change its motion.
Can you give some examples of forces?
Examples of forces:
weight
friction
tension from rope
force from a spring
electric force
Students may also mention atomic forces, magnetic forces, gravity, air resistance, buoyancy, etc. People can also apply forces by pushing or pulling on objects.

7. Units of force Quantitative: Force is measured in newtons.
A useful reference: one newton of force is about equal to the weight of a stick of butter pressing down on your hand as you hold it.

8. Units of force
When students learn Newton’s second law, F = ma, it will provide a useful tool for remembering the definition of a Newton: 1 N = (1 kg) (1 m/s2).

9. Newton’s first law
The first law has two parts. This first is obvious from experience.

10. Newton’s first law
The first law has two parts. The second part is not so obvious.
the second part of Newton’s first law is a conceptual hurdle. Students believe that a force is required to keep an object in motion, and will often invent such forces where they do not exist. Free-body diagrams are excellent for revealing these errors.

11. First law examples Give an example of the first part of the first law.
Allow students time to suggest answers.

12. Objects at rest Give an example of the first part of the first law.
A book on the table, a chair, a seated person – all are at rest and stay at rest. The net force is zero in each case.

13. Objects in motion Give an example that illustrates the second part.
Allow students time to suggest answers. This is harder.

14. Reality
In real life all moving objects eventually slow down and stop UNLESS you continually apply a force to keep them going.

15. Is the first law wrong?
In real life all moving objects eventually slow down and stop UNLESS you continually apply a force to keep them going.
So, is the first law wrong? Was Newton hallucinating?

16. Is the first law wrong?
In real life all moving objects eventually slow down and stop UNLESS you continually apply a force to keep them going.
So, is the first law wrong? Was Newton hallucinating?
What is the explanation?

17. Newton’s first law
The answer has to do with the word “net.”

18. Newton’s first law
In everyday life all moving objects eventually slow down and stop because the net force is NOT zero.

19. What forces act on the ball?
In everyday life all moving objects eventually slow down and stop because the net force is NOT zero.
What forces act on the ball?

20. What forces act on the ball?
The force we are looking for is friction.

21. Friction The force we are looking for is friction.
Friction occurs whenever there is relative motion between matter.
Friction creates forces that always opposes this motion.

22. Zero net force
To create motion at constant speed in the real world, a constant force must be applied to make the net force zero.

23. Zero net force
To create motion at constant speed in the real world, a constant force must be applied to make the net force zero.
The first law is correct (of course) and objects do move at the same speed and in the same direction when the net force is truly zero.

24. Looking deeper
Consider – sitting in your chair, are you at rest right now?
It depends on your frame of reference.
If the students say “yes” ask them to think again.

25. Looking deeper
Consider – sitting in your chair, are you at rest right now?
Again, it depends on your frame of reference.

26. Understanding the first law
These two parts of the first law are really identical. They are both telling you this:
This slide summarizes the key idea of the lesson.

27. Understanding the first law
The first law goes both ways:
This slide summarizes the key idea of the lesson.

28. Understanding the first law
Motion only changes through the action of a net force.
If the net force is zero – there can be no changes in motion.
This slide summarizes the key idea of the lesson.

29. The law of inertia
Newton’s first law is also known as the law of inertia.
Inertia is the tendency of an object to resist changes in the speed or direction of its motion.
The inertia of an object is related to its mass.
The more massive an object is, the more inertia it has, and the more it will resist having its motion changed.
This slide summarizes the key idea of the lesson.

30. Newton’s second law Net force (N) Acceleration(m /s2) Mass (kg)
The acceleration of an object equals the net force divided by the mass.
The second law is introduced. The mathematics is also translated into an English sentence.

31. How does the second law show that this statement is true?
Newton’s second law
Net force (N)
Acceleration(m /s2)
Mass (kg)
Velocity must change if a net force acts on an object.
How does the second law show that this statement is true?
The second law is introduced.

32. The meaning of the second law
Net force (N)
Acceleration(m /s2)
Mass (kg)
Velocity must change if a net force acts on an object.
According to the second law, a net force on an object causes it to accelerate. If an object accelerates, its velocity must change.
The second law is introduced.

33. How does the second law show that this statement is true?
The meaning of the second law
Net force (N)
Acceleration(m /s2)
Mass (kg)
The net force is zero on an object with constant velocity.
How does the second law show that this statement is true?
The second law is introduced.

34. The meaning of the second law
Net force (N)
Acceleration(m /s2)
Mass (kg)
The net force is zero on an object with constant velocity.
If velocity stays constant, then the acceleration is zero—so the net force must also be zero.
The second law is introduced.

35. Acceleration and force are vectors.
Direction of force and acceleration
Acceleration and force are vectors.
If the student knows the direction of the net force on an object, he/she knows that the object must also have an acceleration IN THAT DIRECTION.

36. Units The second law can help you remember the definition of a newton.
Always use mass in kilograms and acceleration in m/s2 when applying Newton’s second law.

37. Applying Newton’s second law
If you know the force on an object, you can predict changes in its motion.
If you know the acceleration of an object, you can determine the net force on it.
Point out that knowing the forces on an object allows you to predict its motion. AND the reverse is also true: the motion of an object can help you figure out the forces acting on it. The next set of slides will show students how to go in both directions.

38. Finding motion from forces
If you know the force on an object, you can predict changes in its motion.
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for seconds. What is its final velocity?
If you know the force on an object, then you can predict changes in its motion.

39. Steps
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for seconds. What is its final velocity?
Use force and mass to find acceleration through the second law.
Use the acceleration to find the change in velocity or position.

40. Solution Impacts can cause very large accelerations for short times!
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for seconds. What is its final velocity?
Use force and mass to find acceleration through the second law.
Impacts can cause very large accelerations for short times!

41. Solution The ball reverses direction!
A 0.25 kg ball is traveling 40 m/s to the right when it is hit with a force of 3,000 N for seconds. What is its final velocity?
Use force and mass to find acceleration through the second law.
Use the acceleration to find the change in velocity or position.
Point out to the students that acceleration is the key variable that lets them link the force on the ball to its velocity.
The ball reverses direction!

42. Applying Newton’s second law
If you know the force on an object, you can predict changes in its motion.
If you know the acceleration of an object, you can determine the net force on it.

43. Forces come in pairs
In your everyday life you can observe that forces always occur in pairs.
Consider throwing a ball.
What force moves the ball?
These first slides try to tap into students’ experience with forces in their everyday lives.

44. Forces come in pairs
In your everyday life you can observe that forces always occur in pairs.
Consider throwing a ball.
Your hand exerts a force on the ball – that is the action force that causes the ball to accelerate.

45. Forces come in pairs
In your everyday life you can observe that forces always occur in pairs.
Consider throwing a ball.
Your hand exerts a force on the ball – that is the action force that causes the ball to accelerate.
The ball exerts an equal and opposite reaction force back against your hand.
Hand pushes on ball. Ball pushes back on hand.

46. Forces come in pairs
How do you detect the presence of the ball? Can you feel the reaction force?
If the ball was heavier, or covered with a prickly material, it would make the reaction force even more noticeable.
Describe a situation where you put a force on something, and a force acted back on you.
Can the students think of other examples in which they have put a force on an object and realized that a force acted back on them. The recoil of a gun or cannon is another possible example to mention. Or ask the students “what would happen to an astronaut on a space walk if he or she threw a hammer?”

47. Action and reaction forces
Forces always come in pairs.
If one object puts a force on a second object, the second object always puts an equal and opposite force back on the first object.
Reaction
This slide brings up the idea that there are two objects involved in a force interaction.

48. Action and reaction forces
Forces always come in pairs.
If one object puts a force on a second object, the second object always puts an equal and opposite force back on the first object.
Newton’s third law:
For every action (force) there is always an equal and opposite reaction (force).
Reaction
Point out that to understand the motion of a SINGLE object, the student doesn’t need the third law. We need the third law to understand how objects put forces on each other. We need the third law when we need to understand more than one object.

49. Action and reaction forces
Forces always come in pairs.
If one object puts a force on a second object, the second object always puts an equal and opposite force back on the first object.
Newton’s third law:
For every action (force) there is always an equal and opposite reaction (force). Newton’s third law is a law of interactions between objects.
Reaction
Point out that to understand the motion of a SINGLE object, the student doesn’t need the third law. We need the third law to understand how objects put forces on each other. We need the third law when we need to understand more than one object.

50. An example Action: racquet pushes on ball.
Reaction: ball pushes back on racquet.

51. There are always two objects
Action: racquet pushes on ball.
Reaction: ball pushes back on racquet.
Action-reaction forces always act on different objects:
One force acts on the racquet. Its partner force acts on the ball.

52. Free-body diagrams Action: racquet pushes on ball.
Reaction: ball pushes back on racquet.
Only one of these forces appears on an object’s free-body diagram: the force that acts ON the object.
Additional forces may act on each of these objects. Understanding and properly sketching these action/reaction forces is just PART of the process of creating the free-body diagram for the racket or the ball.

53. Force pairs don’t cancel out
Action-reaction pairs don’t cancel out because they always act on different objects:
One force acts on the racquet.
Its partner force acts on the ball.
Only forces that act on the same object can cancel each other out.
Racquet pushes on ball. Ball pushes back on racquet.