1. November 4, 2015 I will be expected to know how Newton’s First Law of Motion affects the world around me.

2. Causes of Motion Until now, we have been learning how to describe motion Displacement Velocity Acceleration Now we will learn what causes an object to move Force

3. Forces When we think of the word force, we usually imagine a push or pull exerted on an object. For example, you exert a force on a ball when you throw or kick it. You exert a force on a chair when you sit on it. (The chair also exerts a force on you!) Force represents the interaction of an object with its environment

4. Forces Cause Changes in Velocity A force often changes an objects state of motion. For example, when you kick a football on a tee, the force of your foot causes the stationary ball to soar quickly into the air. Forces can also slow or stop an object or change its direction. In all of these cases, forces cause a change in velocity, or acceleration, of the object.

5. The Newton The metric unit of force is the Newton, named after Sir Isaac Newton. Newton’s work contributed much to our modern understanding of force and motion.

6. The Newton The Newton is defined as the amount of force that, when acting on a 1kg mass, produces an acceleration of 1 m/s 2. Therefore, 1N = 1kg*m/s 2. The weight of an object is expressed in Newtons with one pound being equal to about 4.448 N.

7. Types of Forces There are two types of forces that can cause changes in the motion of an object. Contact forces involve pushing or pulling an object. This force requires direct contact with the object moving. A wagon being pulled by a child would involve a contact force. Field forces do not require physical contact between two objects. Gravity, and the attraction or repulsion of electrically charged objects would involve field forces.

8. Force Diagrams If you give a toy car a light push, it does not move as fast as it does when you give it a harder push. The effect of a force depends on the magnitude (how large or small) of the force. The effect of the force also depends on the direction of the force. The car moves differently if pushed from the front instead of the rear.

9. Force is a Vector Because the effect of force depends on its magnitude and direction, force is a vector quantity. Force diagrams can be a useful tool to determine how forces on an object can be resolved and motion of the object can be determined. Because force is a vector, we can add and subtract them exactly as we did when we studied vectors last six weeks.

10. Free-Body Diagrams Free-body diagrams are a special type of force diagram. Free-body diagrams include only the forces that are causing motion in the object. Forces that are not affecting the motion of the object are left out.

11. Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay 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

12. Balanced Forces What is an unbalanced force? In pursuit of an answer, we will first consider a physics book at rest on a table top. There are two forces acting upon the book. One force - the Earth's gravitational pull - exerts a downward force. The other force - the push of the table on the book (the normal force) - pushes upward on the book.

13. Balanced Forces These forces are equal but opposite of each other so they cancel each other out. The net external force acting on the book is zero. The net external force is the vector sum of all the forces acting on an object Objects that have a net external force of zero are said to be in equilibrium.

14. Balanced Forces An object in equilibrium can still be moving, But it will be moving at a constant velocity ( no acceleration).

15. Unbalanced Forces An object with an unbalanced force acting on it will usually be undergoing some kind of acceleration (speeding up, slowing down, or changing direction). The forces acting on the object do not balance out to zero. There will be a net external force greater than zero.

16. Inertia Newton’s first law is sometimes called the law of inertia. Inertia is the tendency of an object to resist changes in its motion. The more mass an object has the more inertia it has. If it isn’t moving, it wants to stay motionless. If it is moving, it wants to continue moving at the same velocity and direction.

17. Galileo Developed Inertia Galileo, the premier scientist of the seventeenth century, developed the concept of inertia. Galileo reasoned that moving objects eventually stop because of a force called friction.

18. Galileo’s Experiments In experiments using a pair of inclined planes facing each other, Galileo observed that a ball will roll down one plane and up the opposite plane to approximately the same height. If smoother planes were used, the ball would roll up the opposite plane even closer to the original height. Galileo reasoned that any difference between initial and final heights was due to the presence of friction. Galileo postulated that if friction could be entirely eliminated, then the ball would reach exactly the same height.

19. Galileo and Motion Galileo's reasoning continued - if the opposite incline was set at a 0-degree angle, then the ball would roll forever in an effort to reach the original height.

20. Force Is Not Needed Newton's first law of motion declares that a force is not needed to keep an object in motion. Slide a book across a table and watch it slide to a rest position. The book in motion on the table top does not come to a rest position because of the absence of a force; rather it is the presence of a force - that force being the force of friction - which brings the book to a rest position.

21. Force Is Not Needed In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! A force is not required to keep a moving book in motion; in actuality, it is a force which brings the book to rest.in actuality, it is a force which brings the book to rest

22. Inertia Have you ever accelerated suddenly while holding a drink without a lid? If you have, you have experienced inertia first hand ( you also got wet!) The drink wants to stay motionless while the car accelerated forward. “Objects want to keep on doing what they are doing”

23. Inertia A car hitting a wall is another example of inertia. If you don’t have a seat belt, you want to continue moving at the same speed the car was moving before it hit the wall. Ouch!!