1 / 19

PH 201

PH 201. Dr. Cecilia Vogel Lecture. REVIEW. Projectiles Dropping Upward throw Range. OUTLINE. Newtons Laws Force, mass, inertia action, reaction forces. Special Case: Uniform Circular Motion. x-axis. q. Direction angle changes at a constant rate.

Download Presentation

PH 201

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. PH 201 Dr. Cecilia Vogel Lecture

  2. REVIEW • Projectiles • Dropping • Upward throw • Range OUTLINE • Newtons Laws • Force, mass, inertia • action, reaction forces

  3. Special Case: Uniform Circular Motion x-axis q • Direction angle changes at a constant rate • w is the (constant) “angular velocity” • positive if CCW • So x and y change sinusoidally

  4. Period of Motion • Period, T, is the time it takes • time to go • If it goes all the way around once, the angle changes by (rad) • If it goes all the way around once, the distance traveled is

  5. Special Case: Uniform Circular Motion • An object moves in a circle of constant radius, r, with constant speed, v. • Is the object accelerating? • Consider ways of ID’ing acceleration: • physical intuition: force needed, jerk felt • math:

  6. Acceleration • What is the acceleration of the object at • Consider the average acceleration from just before to just after: • Generally: centripetal acceleration is

  7. Magnitude of Acceleration • Same r and larger v yields: • Same v and smaller r yields: • Same T and larger r yields:

  8. Example • What is the speed and acceleration of the Earth in orbit? • r = 93,000,000 mi = 1.5X1011 m • T = 1yr = 3.156X107s = 30,000 m/s = 0.0059 m/s2 huge circle –

  9. Newton’s 1st Law An object at rest remains at rest, object in motion remains in uniform motion (i.e. constant velocity, no change in speed nor direction) unless acted on by a net external force. demos

  10. Inertia Inertia is the word used to describe this tendency: It is NOT a resistance to motion It is a resistance to CHANGE in motion Inertia is a property of matter. Define mass: if an object has inertia, then it has mass.

  11. Centripetal Force – 1st Law • Recall: 1st Law • An object in motion will continue • no change Can only continue in circle if

  12. Newton’s Second Law • An net external force can change an object's velocity, can accelerate it. • The force required is proportional to the acceleration, and in the same direction. • The force required is also proportional to the object’s mass.

  13. Newton’s Second Law Improved • What is meant by “net” external force? • take all the external forces, and add them as vectors. • the resultant is the net force. • Notation: S means sum • Note: • there may be several forces; add them. • there is only one acceleration.

  14. Centripetal Force – 2nd Law • Recall: centripetal acceleration • An object moving in a circle • is accelerating • Recall: 2nd Law • in order for object to accelerate • The acceleration is toward center, so • centripetal force is

  15. Centripetal Force – Examples • What force holds each in circular motion? • A ball swung around on a string: • You, as you rotate around with Earth’s surface: • Amusement-park ride: you’re plastered to the wall and the floor drops out • Coin on a turntable

  16. Newton’s Third Law • “For every action, there is an equal and opposite reaction” • Better said: “For every FORCE, there is an equal and opposite reaction FORCE.” • If A pushes on B, • then B pushes back on A • at the same time.

  17. Newton’s 3rd Law • For every FORCE, there is an equal and opposite reaction FORCE. • If I try to push the door open, the door pushes back on me, • making it hard to open a door in a wheelchair. • If the Earth pulls down on me with a gravitational force, then I pull up on the Earth • with exactly the same size force!

  18. More on the 3rd Law • The object pushes back, even if it accelerates. • when you shoot a gun • you accelerate bullet forward • you can feel it push _________ on you • “recoil” • when you catch a ball • you accelerate it backward (to stop it) • you can feel it push __________ on you

  19. 3rd Law Reaction on What? • The “action” FORCE and the “reaction” FORCE don’t act ON the same thing. • If the action force is Earth’s gravity pulling onME, • then the reaction force is me pulling on the Earth. • If the action force is your behind pushing down on the chair, • then the reaction force is the chair pushing up on your behind (keeping you from falling)

More Related