1 / 25

4. Newton's Laws

4. Newton's Laws. The Wrong Question Newton’s 1 st & 2 nd laws Forces The Force of Gravity Using Newton’s 2 nd Law Newton’s 3 rd Law. Ans: Human body: contact force Wind & water: fluid pressure Gravity: action-at-a-distance. What forces govern the motion of the sailboard?.

jasper-browning
Télécharger la présentation

4. Newton's Laws

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. 4. Newton's Laws The Wrong Question Newton’s 1st & 2nd laws Forces The Force of Gravity Using Newton’s 2nd Law Newton’s 3rd Law

  2. Ans: • Human body: contact force • Wind & water: fluid pressure • Gravity: action-at-a-distance What forces govern the motion of the sailboard?

  3. 4.1. The Wrong Question Q: Why do things move? Aristotle (~350BC) : Because some forces are acting on them. Galileo (~1600) : Wrong question ( things move until stopped ). … it always rises to its starting height … If a ball is released here … The right question: Why do moving things change direction? Newton: Because some forces are acting on them. … so this ball should roll forever.

  4. 4.2. Newton’s 1st & 2nd laws Fnet 0  v  Net force determines motion. Newton’s 1st law of motion: A body at rest or in uniform motion remains so unless acted on by a nonzero net force. Fnet= 0  v =const

  5. GOT IT? 4.1. On a horizontal tabletop is a curved barrier that exerts a force on a ball, guiding its motion in a circular path. After the ball leaves the barrier, which of the dashed paths shown does it follow?

  6. Newton’s 2nd Law (quantity of motion) Momentum: Newton’s 2nd law of motion: The rate of change of momentum is equal to the net force. For a constant mass:

  7. Mass, Inertia, & Force Inertia: resistance to changes in motion. 1st law = law of inertia Operational definition of mass  Loaded truck has greater mass, more inertia, less acceleration. 1 N (Newton)  force required to give a mass of 1 Kg an acceleration of 1 m / s2. 1 N = 1 Kg m / s2. 1 Dyne = 1 g cm / s2.

  8. Example 4.1. AcceleratingCar • A 1200 kg car accelerates constantly in a straight line from rest to 20 m/s in 7.8 s. • What is the net force on it? • What is the net force on it if it rounds a bend 85 m in radius at v = 20 m/s?

  9. Inertial Reference Frames Inertial Reference Frames: reference frame in which Newton’s 1st law works. • Examples of non-inertial reference frames: • Accelerating planes. • Car rounding a curve. • Merry-go-round. • Earth (rotating). • Examples of inertial reference frames: • Newton: Distant “fixed” stars. • Einstein: General relativity.

  10. 4.3. Forces Fc= Fg • Examples of forces: • Pushes & pulls. • Car collided with truck & stopped. • Moon circles Earth. • Person sitting on chair. • Climber on rope. (action-at-a distance) (contact force) (tension force) T = Fg

  11. The Fundamental Forces • The fundamental forces: • Gravity: large scale phenomena • Electroweak force • Electromagnetic force: everyday phenomena • Weak (nuclear) force • Strong (nuclear) force 1025 1036 1 1038

  12. 4.4. The Force of Gravity m in f = m a is the inertia mass (same everywhere). Weight = force of gravity on mass: For a mass of 65 kg, weight on Earth = (65 kg) (9.8 m/s2 ) = 640 N. weight on Moon = (65 kg) (1.6 m/s2 ) = 100 N. weight in outer-space = (65 kg) (0 m/s2 ) = 0 N. Caution: In daily use, weight is often measured in units of mass. E.g., a person “weights” 65 kg. Strictly speaking, m in w = m g is the gravitational mass. Galileo experiment: mI = mG(coincidence) Einstein: mI = mG(exact: gravity is geometry)

  13. Weightlessness These astonauts feel “weightless” because they are in freefall. Their weight is about 93% of that on surface. same a

  14. 4.5. Using Newton’s 2nd Law • Tactics 4.1. Free-Body Diagram • Identify object of interest & forces on it. • Object  dot. • Draw forces on object as vectors at dot.

  15. Example 4.3. Elevator A 740 kg elevator accelerates upward at 1.1 m/s2. Find the tension force on the cable (of negligible mass).

  16. GOT IT? 4.4. • How’s T compared to w = m g if the elevator • moves upward starting at rest. • decelerates to stop while moving upward. • starts moving downward, accelerating from rest. • slows to stop while moving downward. • moves upward with constant speed. greater less less greater equal

  17. Conceptual Example 4.1. At the Equator When you stand on a scale, the scale reading shows the force it pushes up to support you. If you stand on a scale at Earth’s equator, is the reading greater or less than your weight? Ans: You’re in uniform circular motion so that the net force on you is centripetal. m a W = m g Fscale

  18. Parabolic Flight

  19. 4.6. Newton’s 3rd Law Newton’s 3rd law: Action = Reaction Forces of 3rd law pair act on different objects. Hence, they don’t cancel each other out. Horse-Cart dilemma

  20. Example 4.4. Pushing Books 2 books lie on frictionless horizontal surface. You push with force F on books of mass m1. which in turn on book of mass m2. What force does the 2nd book exert on the 1st ? Acceleration of books: Net force on 2nd book: Force exerted by 2nd book on the 1st

  21. GOT IT? 4.5 Is the net force on the larger block (a) greater than 2 N , (b) equal to 2N or (c) less than 2 N ?

  22. Normal force n : contact force acting normal to contact surface. Not a 3rd law pair Normal force Fnet 0 3rd law also applies to non-contact forces such as gravity.

  23. Measuring Force Force can be measured using Newton’s 3rd law. Ideal spring (Hooke’s law): k = spring constant fsp= 0 fsp= fwall < 0 Works only in inertial frame. fsp= fwall > 0

  24. Example 4.5. Helicopter Ride • Helicopter rises vertically. • A 35 kg bag sits in it on a spring scale of k = 3.4 kN/m. • By how much the spring compresses • when helicopter is at rest • when it’s accelerating upward at 1.9 m/s2. (a) (b)

  25. GOT IT? 4.6 • Example 4.5: • Helicopter rises vertically. • A 35 kg bag sits in it on a spring scale of k = 3.4 kN/m. • By how much the spring compresses • when helicopter is at rest • when it’s accelerating upward at 1.9 m/s2. • Continued from Example 4.5: • Would the answer to (a) change if the helicopter were moving upward with constant speed? • Would the answer to (b) change if the helicopter were moving downward but still accelerating upward? No No

More Related