Physics 151 Week 6 Day 3

Topics More vector math (time permitting)

Magnitude and Direction from componentsAdding vectors by components

Newton’s 1st Law of Motion Newton’s 2nd Law of Motion Practice with Free-Body Diagrams & System

Schema

To the Airport

Write out the simplest set of directions (fewest number of steps) to go from the Road Runner Station downtown (1st and Central) to the Sunport airport

Adding Displacement Vectors

Slide 1-35

Road Rally You are driving in a road rally race in the desert where you are

judged on how close you arrive at the finish line to a specific time. At the last pit stop, you are given the following directions for the last leg of the race:

• Drive East at 50 MPH for 30 minutes and• Then go 30 degrees East of North for 45 minutes at 60 MPH

• What would be your average speed from the pit stop to the finish line?

• What would be the magnitude and direction of your average velocity during this time?

• Find the magnitude and direction of the average acceleration during the first hour of driving.

Galileo (1564-1642)

Developed the idea of experimental science

Re-examined natural motion of objects and how objects move

Made astronomical observations that challenged Earth-centered solar system model.

Newton (1642-1727)

Newton's work based on experiments of how objects interact.

His laws of motion and law of gravity described how all objects interact with each other.

Summary

Slide 4-39

Summary

Slide 4-40

• Free-body diagram is another name for force diagram

• Weight force is another name for gravitational force

• All objects exert gravitational forces on one another • These forces are too small to be noticed unless one

object is at least the size of the Sandia Mountains

Quick Points

Slide 4-30

Clicker Question 11. A “net force” is

A. the sum of the magnitudes of all the forces acting on an object.

B. the difference between two forces that are acting on an object.

C. the vector sum of all the forces acting on an object.D. the force with the largest magnitude acting on an object.

Slide 4-7

Answer1. A “net force” is

A. the sum of the magnitudes of all the forces acting on an object.

B. the difference between two forces that are acting on an object.

C. the vector sum of all the forces acting on an object.D. the force with the largest magnitude acting on an object.

Slide 4-8

Clicker Question 22. Which of the following is NOT one of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the environment.

B. Name and label each contact force acting on the object.C. Draw a picture of the situation.D. Identify “the system” and “the environment.”E. Name and label each long-range force acting on the

object.

Slide 4-9

Answer2. Which of the following is NOT on of the steps used to identify

the forces acting on an object?

A. Name and label each force the object exerts on the environment.

B. Name and label each contact force acting on the object.C. Draw a picture of the situation.D. Identify “the system” and “the environment.”E. Name and label each long-range force acting on the

object.

Slide 4-10

An elevator, lifted by a cable, is going up at a steady speed. • Identify the forces acting on the elevator. • Is T greater than, equal to, or less than w? Or is there not

enough information to tell?

• A => FT > Fg

• B => FT = Fg

• C => FT < Fg

Example Problem

Slide 4-30

Newton's First Law of Motion Demonstrations

DEMO - Smash the HAND

DEMO - Tablecloth

Example ProblemThe hover puck

Slide 4-26

Example Problem: One book, Two book

Slide 4-26

Example Problem: Refrigerator MagnetUse a free body diagram to determine the force that holds up a kitchen magnet.

What is the magnitude of this force?

Slide 4-26

Example ProblemA block is dragged uphill by a rope. Identify all forces acting on the block.

Slide 4-26

Example ProblemBlock A hangs from the ceiling by a rope. Another block B hangs from A. Identify the forces acting on A.

Slide 4-27

A ball, hanging from the ceiling by a string, is pulled back and released. Identify the forces acting on it just after its release.

Example Problem

Slide 4-28

Newton’s Second Law

Slide 4-29

acceleration is a change in speed or a change in direction of speed.

What happens when Fnet is not = 0?