Dynamics – Ch. 6 – part 1

•  Static Equilibrium – not moving

•  Dynamic Equilibrium – constant v

•  Mass and Weight and Gravity

•  Ring of forces activity

•  Acceleration in the elevator – activity

•  Corrections due in class on Thursday, Oct 11

Exam 1 Corrections – 3 parts •  Grade for the “Initial Effort”:

–  Satisfactory initial effort : 4pt –  Missing explanations or steps:3pt –  Major problem parts missing (or didn’t finish): 2pt –  Very little coherent effort:1pt –  No initial effort: 0pt

•  Note: it is a grade per problem. 10 points total •  This grade has nothing to do with whether that effort is correct:

–  students can earn a full 4 points on this part and be completely wrong. •  No comments are made on the problem. •  Instructor posts the printed solutions on BB or on the website •  Students then use the printed solutions to correct their work using a

different color ink.

Exam 1 Corrections – 3 parts – 10 points total •  Instructor re-collects the work and grades for "Correction

Quality":

–  Solution is now completely correct 3

–  Minor issues were not corrected 2

–  Major issues were not corrected 1

–  No correction effort 0

•  and for "Correction Needed": –  No correction was necessary 3 –  Minor corrections were needed 2 –  Important corrections were needed 1 –  Initial effort needed a complete rewrite 0

Fermi Problem #1

•  Explicitly state all your (hopefully reasonable!) assumptions. Show all your work, including units!

•  There is no “right” answer? Estimate everything within an order of magnitude.

•  Write down who you worked with, roughly how long you spent, and what numbers you looked up, if any.

•  Due Thursday 10/11.

How many golf balls would it take to circle the Earth at the equator?

Using Newton’s Second Law

The essence of Newtonian mechanics can be expressed in two steps. •  The forces on an object determine its acceleration a = Fnet/m, and •  The object’s trajectory can be determined by using the equations of kinematics.

General Strategy

General Strategy

Important Concepts

EXAMPLE 6.2 Towing a car up a hill

EXAMPLE 6.3 Speed of a towed car

Mass

Mass is a scalar quantity that describes an object’s inertia. Loosely speaking, it also describes the amount of matter in an object. Mass is an intrinsic property of an object. It tells us something about the object, regardless of where the object is, what it’s doing, or whatever forces may be acting on it.

Gravity

It was Newton who first recognized that gravity is an attractive, long-range force between any two objects. Somewhat more loosely, gravity is a force that acts on mass. When two objects with masses m1 and m2 are separated by distance r, each object pulls on the other with a force given by Newton’s law of gravity, as follows:

Gravity

We can write the gravitational force even more simply as

where the quantity g is defined to be

Applications

• Application: elevator

Homework Tips for Force Problems

•  Given: •  Find: •  Interaction Diagram •  Free-Body Diagram •  Newton’s second Law •  broken into components

•  Algebra first •  Appropriate units •  Box answers

In Class Activity and Problems

•  Problems – assume no friction •  Push a box/pull a box •  Box on a incline •  Modified Atwood Machine

•  Activities •  ring stand and spring scales (vectors and forces) •  Elevator with Force Plate

In-Class Problems & Activities

•  Ring stand with spring scales.

•  A 100 kg block on which the gravitational force is 980 N hangs on a rope. Find the tension in the rope if the block is stationary, then if it’s moving upward at a steady speed of 5 m/s, and finally if it’s accelerating upward at 5 m/s2.

•  A 75 kg skier starts down a 50-m-high, 10° slope on frictionless skis. What is her speed at the bottom?

•  Extra-credit activity – Elevator with Force Plate

Upcoming Assignments

•  Read Chapter 6.4 to the end by Thursday 10/11 •  Pre-class MasteringPhysics assignment due before

class on Thursday 10/11 •  MasteringPhysics online HW due, Thursday 10/11 by

11 pm

Questions

1. Newton’s first law can be applied to

A.  static equilibrium.

B.  inertial equilibrium.

C.  dynamic equilibrium.

D.  both A and B.

E.  both A and C.

2. A Martian lander is approaching the

surface. It is slowing its descent by firing

its rocket motor. Which is the correct free-

body diagram for the lander?

Questions

3. An elevator that has descended from

the 50th floor is coming to a halt at the

1st floor. As it does, your apparent

weight is

A.  less than your true weight.

B.  equal to your true weight.

C.  more than your true weight.

D.  zero.

Newton’s first law can be applied to

A.  static equilibrium. B.  inertial equilibrium. C.  dynamic equilibrium. D.  both A and B. E.  both A and C.

A Martian lander is approaching the surface. It is slowing its descent by firing its rocket motor. Which is the correct free-body diagram for the lander?

An elevator that has descended from the 50th floor is coming to a halt at the 1st floor. As it does, your apparent weight is

A.  less than your true weight. B.  equal to your true weight. C.  more than your true weight. D.  zero.