1. The tendency of objects to resist change in their state of motion is called inertia  Inertia is measured quantitatively by the object's mass.  Objects will undergo changes in motion only in the presence of a net (unbalanced) force Chapter 4: Force and the Laws of Motion Net force,: ALL of the forces acting on one, unique object. These forces are often summarized in a freebody diagram. If the forces cancel each other, then the net force acting on the body is equal to zero. When this happens, the object is said to be in a state of equilibrium: static equilibrium occurs when the object is at rest; dynamic equilibrium occurs when the object is moving at a constant velocity or speed 1

2. Static or Dynamic equilibrium (Newton’s First Law) Types of Force (most common) weight normal friction tensions applied forces A man stands on a scale inside a stationary elevator N mg Reading on scale 2

3. N mg An object at rest must have no net force on it. If it is sitting on a table, the object exerts a downward force mg on the surface of the table. The surface of the table exerts an upward force on the block, called the normal force. It is exactly as large as needed to balance the force from the object. ** Important: The normal force is always perpendicular to the surface (90 degree) m The reason is for Newton’s 3 rd Law. 3

4. F1F1 F2F2 Action/Reaction Forces Whenever one object exerts a force on a second object, the second exerts an equal force in the opposite direction on the first. Newton’s Third Law F1F1 F2F2 F 1 = Box pushes on table F 2 = Table pushes on box F 1 = Box pushes on table F 2 = Table pushes on box Then is how we draw the FBD – not the action/reaction forces 4

5. More Free Body Diagrams – Equilibrium 1.A 20kg box at rest on a 4.5m table. 2.A 20kg box at rest on the floor 3.A 20 kg box at rest on a 400lb truck bed. 4.A 20 kg box pushed at a constant speed with opposing friction force of 3.0N. 5.A 20kg box pulled across a table with opposing friction force of 4.0N. 5

6. 6 Newton's First Law – Reading material In this unit (Newton's Laws of Motion), the ways in which motion can be explained will be discussed. Isaac Newton (a 17th century scientist) put forth a variety of laws that explain why objects move (or don't move) as they do. These three laws have become known as Newton's three laws of motion. The focus of Lesson 1 is Newton's first law of motion - sometimes referred to as the law of inertia. Newton's first law of motion is often stated as An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.unless acted upon by an unbalanced force

7. 7 State of Motion Inertia is the tendency of an object to resist changes in its state of motion. But what is meant by the phrase state of motion? The state of motion of an object is defined by its velocity - the speed with a direction. Thus, inertia could be redefined as follows:velocity Inertia: tendency of an object to resist changes in its velocity. An object at rest has zero velocity - and (in the absence of an unbalanced force) will remain with a zero velocity. Such an object will not change its state of motion (i.e., velocity) unless acted upon by an unbalanced force. An object in motion with a velocity of 2 m/s, East will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East. Such an object will not change its state of motion (i.e., velocity) unless acted upon by an unbalanced force. Objects resist changes in their velocity. As learned in an earlier unit, an object that is not changing its velocity is said to have an acceleration of 0 m/s/s. Thus, we could provide an alternative means of defining inertia:in an earlier unit Inertia: tendency of an object to resist accelerations.

8. Homework: Part A: Mass, Inertia, Weight, and Newton's First Law of Motion 1. Which of the following statements are true of inertia? List all that apply. a.Inertia is a force. b.Inertia is a force which keeps stationary objects at rest and moving objects in motion at constant velocity. c.Inertia is a force which brings all objects to a rest position. d.All objects have inertia. e.A more massive object has more inertia than a less massive object. f.Fast-moving objects have more inertia than slow-moving objects. g.An object would not have any inertia in a gravity-free environment (if there is such a place). h.Inertia is the tendency of all objects to resist motion and ultimately stop. i.In a gravity-free environment (should there be one), a person with a lot of inertia would have the same ability to make a turn as a person with a small amount of inertia.

9. Homework 2. Which of the following statements are true of the quantity mass? List all that apply. a.The mass of an object is dependent upon the value of the acceleration of gravity. b.The standard metric unit of mass is the kilogram. c.Mass depends on how much stuff is present in an object. d.The mass of an object is variable and dependent upon its location. e.An object would have more mass on Mount Everest than the same object in the middle of Lake Michigan. f.People in Weight Watcher's are really concerned about their mass (they're mass watchers). g.The mass of an object can be measured in pounds. h.If all other variables are equal, then an object with a greater mass would have a more difficult time accelerating. i.If all other variables are equal, then it would require less exerted force to stop a less massive object than to stop a more massive object. j.The mass of an object is mathematically related to the weight of the object.

10. 3. Which of the following statements are true of the quantity weight? List all that apply. a.The weight of an object is dependent upon the value of the acceleration of gravity. b.Weight refers to a force experienced by an object. c.The weight of an object would be less on the Moon than on the Earth. d.A person could reduce their weight significantly by taking an airplane ride to the top of Mount Everest. e.Two objects of the same mass can weigh differently. f.To gain weight, one must put on more mass. g.The weight of an object can be measured in kilograms. h.The weight of an object is equal to the force of gravity acting upon the object. i.When a chemistry student places a beaker on a balance and determines it to be 84.3 grams, they have weighed the beaker.