1. Dr. Jie ZouPHY 10711 Welcome to PHY 1071 Physics of Sound and Music

2. Dr. Jie ZouPHY 10712 2 Newton’s First Law of Motion zNewton’s First Law of Motion-Inertia zNet Force zSupport Force zThe Equilibrium Rule zInertia and the Moving Earth

3. Dr. Jie ZouPHY 10713 Newton’s 1st Law of Motion zThe law of inertia: yEvery object continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. yContinues: an object continues to do whatever it happens to be doing unless an (unbalanced) force exerted upon it. zInertia: yThe property of objects to resist changes in motion.

4. Dr. Jie ZouPHY 10714 Understanding Inertia zInertia: the property of things to resist changes in motion. yIn the absence of external forces, a body at rest tends to remain at rest; if moving, it tends to remain moving. yOnce a body is in motion, no force is needed to keep it moving in the absence of retarding force, such as friction. yInertia is a property of matter to behave this way. It is not some kind of force that keeps things moving. yA force is needed to change motion (e.g. changing the direction or speed of the motion), not to keep a body moving, so long as friction is negligible.

5. Dr. Jie ZouPHY 10715 Examples of Inertia zYou see inertia everywhere! zExample 1: standing in a moving bus Standing in a moving bus STOP The bus suddenly slows down. Do you lurch forward or backward? And why? How about when the bus suddenly speeds up? Suddenly slows down. Keeps moving

6. Dr. Jie ZouPHY 10716 Net Force zForce: yChanges in motion are produced by a force or combination of forces. yA force, in the simplest sense, is a push or a pull. zNet force: yWhen more than a single force acts on an object, we consider the net force; force is a vector quantity-has both magnitude and direction. Applied forces Net force 5 N 10 N 5 N 0 N 10 N5 N

7. Dr. Jie ZouPHY 10717 Support Force zIt is a normal force: force that acts at right angles to the surface. zIt is a very real force. zExamples of support force: yA book supported by the table. yA person supported by a weight scale. zUnits of force: the Newton (N), pound zSome common forces: Gravity (G), Normal force (N) G Book N N=G or N + (-G) = 0  F=0

8. Dr. Jie ZouPHY 10718 The Equilibrium Rule zMechanical equilibrium: yWhen the net force on something is zero, we say something is in mechanical equilibrium. yEquilibrium is a state of no change. zForms of mechanical equilibrium: yRest is only one form of equilibrium. yUniform motion (constant speed in a straight line). yRotational equilibrium (Ch.8). zThe equilibrium rule: yFor any object in equilibrium, the net force on it must be zero, i.e.  F=0 or there is no unbalanced force.

9. Dr. Jie ZouPHY 10719 Examples of Equilibrium zExample 1: T1T2 N+G staging 500 N? N G person N Tip: separate objects when analyzing forces. The equilibrium rule  F=0: 1.The person stands at the exact middle. Thus, T1=T2=500 N. 2.The person is in equilibrium. Thus, N=G person. 3.The staging is in equilibrium. Thus, T1+T2=G person +G staging. 4.The person and staging weigh 1000 N in total. 170 N? N

10. Dr. Jie ZouPHY 107110 The Moving Earth zCan a bird at rest at the top of a tree drop down and catch a worm on the ground if the Earth moves at 30 km/s to circle the sun in one year? z Can you catch a flipped coin in a high- speed airplane assuming the airplane flies smoothly?

11. Dr. Jie ZouPHY 107111 Homework zChapter 2, Page 39, Exercises: #19, 20, 24, and 27. zNote: The above problems are from the 10 th edition of the textbook by Hewitt.