1. Section 2 Newton’s Laws of Motion
Question of the Day
If you are sitting still in your seat on the bus to see the Nutcracker and the bus is traveling 70 mi/h on the highway, is your body at rest or in motion?
Explain your answer using a diagram showing the forces (velocities) acting on you and the bus.

2. Section 2 Newton’s Laws of Motion
Objectives
Describe Newton’s first law of motion, and explain how it relates to objects at rest and in motion.
State Newton’s second law of motion, and explain the relationship between force, mass, and acceleration.
State Newton’s third law of motion, and give examples of force pairs (pushes and pulls).

3. Newton’s First Law of Motion
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion
(1)An object at rest remains at rest, and (2)an object in motion remains in motion at a constant speed and in a straight line unless acted on by an outside force.
Newton’s first law of motion describes motions of objects when they are sitting still and when they are moving in terms of forces.

4. Newton’s First Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
Part 1: Objects at Rest Objects at rest will stay at rest unless they are pushed or pulled by a force.
Part 2: Objects in Motion Objects will continue to move with the same velocity (speed and direction) unless they are stopped or slowed by something (outside forces).
The image on the next slide shows how you can have fun with Newton’s first law.

5. Newton’s First Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued

6. Newton’s First Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
Friction and Newton’s First Law Friction between an object and the surface it is moving over is an example of an outside force that stops motion.
Why does a rolling ball eventually come to a stop on Earth?
Inertia and Newton’s First Law Newton’s first law is sometimes called the law of inertia. Inertia is the tendency of all objects to resist any change in motion.

7. Newton’s First Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s First Law of Motion, continued
Mass and Inertia Mass is a measure of inertia. Small mass = small inertia, large mass = large inertia.
So, changing the motion of an object with small mass is easier than changing the motion of an object with large mass.
Consider the following: Which is easier to move: your pencil across your desk? Or your refrigerator across your kitchen? Why?

8. Newton’s Second Law of Motion
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion
The acceleration of an object depends on the mass of the object and the amount of force applied.
Newton’s second law describes the motion of an object when an outside force (pushes or pulls) acts on the object.

9. Newton’s Second Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion, continued
Part 1: Acceleration Depends on Mass The acceleration of an object decreases as its mass increases.
Its acceleration increases as its mass decreases.
Part 2: Acceleration Depends on Force An object’s acceleration increases as the force on the object increases. The acceleration of an object is always in the same direction as the force applied.

10. Section 2 Newton’s Laws of Motion

11. Newton’s Second Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s Second Law of Motion, continued
Expressing Newton’s Second Law Mathematically
The relationship of acceleration (a) to mass (m) and force (F) can be expressed mathematically with the following equation: m a = F , or 

12. Section 2 Newton’s Laws of Motion

13. Newton’s Third Law of Motion
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.
Newton’s third law of motion can be simply stated as follows: All forces act in pairs or action and reaction.

14. Newton’s Third Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion, continued
Force Pairs Do Not Act on the Same Object A force is always exerted by one object on another object. This rule is true for all forces, including action and reaction forces.
Paired forces do not act on the same object. If they did, the combined force would always be 0 N and nothing would ever move!

15. Newton’s Third Law of Motion, continued
Section 2 Newton’s Laws of Motion
Newton’s Third Law of Motion, continued
The Effect of a Reaction Can Be Difficult to See When an object falls, gravity pulls the object toward Earth and pulls Earth toward the object.
You don’t notice Earth being pulled upward because the mass of Earth is much larger than the mass of the object. Thus, the acceleration of Earth is much smaller than the acceleration of the object.