Physics 211 Lecture 4

Transcription

1 Physics 211 Lecture 4 Today's Concepts: Newton s Laws a) Acceleration is caused by forces b) Force changes momentum c) Forces always come in pairs d) Good reference frames Mechanics Lecture 4, Slide 1

2 Stuff you asked about: I would like to go over a little bit more on Newton's third law. i had MAJOR difficulty with the first question in the pre-lecture... can we please go over the reason for the answer in class. I would like to cover momentum versus force a little more. I would like to have some more examples of the 3rd Law as I always had trouble with the equal and opposite force part. I always felt that that implied they cancel out. I get this stuff. Understanding Newton's Laws in different circumstances was the hardest part. It will be helpful to have versatile problems. Everything is fine! Mechanics Lecture 4, Slide 2

3 Newton s 2 nd Law Acceleration is caused by force. A bigger mass makes this harder Mechanics Lecture 4, Slide 3

4 Example 4.1 (Units) Units of force are Newtons. 1N = 1kg m/sec 2 Example: A net force of 100N is placed on a 1000kg truck in the positive x direction. What is the acceleration of the truck? 100N Mechanics Lecture 4, Slide 4

5 Example 4.2 (Force is a vector) F x =ma x F y =ma y F z =ma z Example: A 1000 truck is driving down the road at 30m/sec. The wind and the road put a new force of 50N on the side of the truck for 3 seconds. What is the velocity of the truck after this period of time? Mechanics Lecture 4, Slide 5

6 Checkpoint The net force on a box is in the positive x direction. Which of the following statements best describes the motion of the box : A) Its velocity is parallel to the x axis B) Its acceleration parallel to the x axis C) Both its velocity and its acceleration are parallel to the x axis D) Neither its velocity or its acceleration need be parallel to the x axis Mechanics Lecture 4, Slide 6

7 Concerns you raised: v F a v 1 Mechanics Lecture 4, Slide 7

8 Checkpoint You are driving a car with constant speed around a horizontal circular track. The net force acting on your car A) Points radically inward toward the center of the circular track B) Points radically outward, away from the center of the circular track C) Points forward in the same direction your car is moving D) Points backward, opposite to the direction your car is moving E) Is zero. Mechanics Lecture 4, Slide 8

9 Aside: Centripetal acceleration and force 1) Objects moving in a circle always have a component of acceleration, called centripetal, which is toward the center of the circle.* 2) Centripetal acceleration must be caused by a force: Friction, gravity whatever force keeps it moving in a circle. This force is often called the centripetal force 3) There is no new kind of force here. 4) There is no such thing as centrifugal force. * They can have also have tangential acceleration if their speed is not constant Mechanics Lecture 4, Slide 9

10 Momentum & Force We will return to momentum later! Mechanics Lecture 4, Slide 10

11 Checkpoint You are driving a car with constant speed around a horizontal circular track. The momentum of your car A) Points radically inward toward the center of the circular track B) Points radically outward, away from the center of the circular track C) Points forward in the same direction your car is moving D) Points backward, opposite to the direction your car is moving E) Is zero. Mechanics Lecture 4, Slide 11

12 v a Mechanics Lecture 4, Slide 12

13 p mv F ma Mechanics Lecture 4, Slide 13

14 Students Momentum Concerns I found the momentum pretty confusing, i would love review that. The concept of momentum was very confusing to me. Concept of momentum is bit confusing.. p mv We will return to momentum in greater detail later in this course. We re just introducing it now. Mechanics Lecture 4, Slide 14

15 Newton s 1 st Law Mechanics Lecture 4, Slide 15

16 What force causes hurricanes to form in this circular pattern? None! They form because the earth s surface is a slightly a non-inertial frame of reference! Hurricane Earl Mechanics Lecture 4, Slide 16

17 Newton s 3 rd Law Forces come in pairs! Fire-cart Mechanics Lecture 4, Slide 17