1 / 13

PHSX 114, Wednesday, September 10, 2003

PHSX 114, Wednesday, September 10, 2003. Reading for today: Chapter 4 (4-5 -- 4-6) Reading for next lecture (Fri.): Chapter 4 (4-7 -- 4-9) Homework for today's lecture: Chapter 4, questions 12, 14; problems 5, 6, 17. Last time: Newton's first and second laws of motion.

gabi
Download Presentation

PHSX 114, Wednesday, September 10, 2003

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. PHSX 114, Wednesday, September 10, 2003 • Reading for today: Chapter 4 (4-5 -- 4-6) • Reading for next lecture (Fri.): Chapter 4 (4-7 -- 4-9) • Homework for today's lecture: Chapter 4, questions 12, 14; problems 5, 6, 17

  2. Last time: Newton's first and second laws of motion • First law: What happens when there is no net force? • Second law: What happens when there is a net force?

  3. Your turn An object has mass 2 kg and two forces act upon it. F1= 3 N in the +x direction, F2= 4 N in the +y direction. Find the magnitude and direction of the acceleration vector. Answer: 2.5 m/s2 , 53.1 degrees from the x direction ax=( F1x+ F2x)/m=(3 + 0)/2 = 1.5 m/s2 , ay=( F1y+ F2y)/m=(0 + 4)/2 = 2 m/s2

  4. A force is applied by another object in the environment • Two basic types: 1) contact, 2) action-at-a distance (fundamental forces are of second type) • Newton's 3rd law discusses the relation between the object feeling the force and the object applying the force

  5. Here's a stupid question that the first two laws don't address: • Why can't I push myself across the floor?

  6. Newton's Third Law of Motion • Whenever one object exerts a force on a second object, the second object exerts a force on the first object that is equal in magnitude and opposite in direction. • Action-reaction • Example: woman pulling sled

  7. Introducing two forces • Force of Earth's gravity on an object • Contact force from a surface

  8. Weight vs. mass • FG=mg • Weight is a vector (units of Newtons or pounds) • Mass is a scalar (units of kilograms) • An object with a mass of 1 kg has a weight of 9.8 N (downward) • Note: m here is "gravitational mass"

  9. Projectile motion and Newton's laws • ΣF=ma • For projectile motion, only force is FG=mg • ΣF=ma=mg • a=g; ax=0, ay=-9.8 m/s2

  10. Contact forces from a surface • Normal force, FN, is perpendicular to the surface • Frictional force, Ffr, is along the surface • For today, we ignore friction, focus on normal force

  11. Consider a block on a horizontal surface • Examples

  12. Your turn • A 20-kg block is sitting on the table. You push down on the block with a force of 30 N. What is the normal force exerted by the table on the block? • Answer: 226 N • FN= 30 N + (20 kg) (9.8 m/s2) = 30 N + 196 N = 226 N

  13. Object on a vertical rope • ΣF=ma= FT – FG • Elevator example • FG=mg doesn't change with elevator's acceleration • Tension in rope, FT, determines elevator's acceleration • If a=0 (elevator has constant speed), FT = FG • If a>0 (elevator accelerating upward), FT > FG • If a<0 (elevator accelerating downward), FT < FG

More Related