Physics 151 Week 6 Day 3
Topics
 More vector math (time permitting)
Magnitude and Direction from components
Adding 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
1. What would be your average speed from the pit stop to the
finish line?
2. What would be the magnitude and direction of your average
velocity during this time?
3. 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
Quick Points
•
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
Slide 4-30
Clicker Question 1
1. 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
Answer
1. 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 2
2. 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
Answer
2. 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
Example Problem
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
Slide 4-30
Newton's First Law of Motion Demonstrations
DEMO - Smash the HAND
DEMO - Tablecloth
Example Problem
The hover puck
Slide 4-26
Example Problem: One book, Two book
Slide 4-26
Example Problem: Refrigerator Magnet
Use 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 Problem
A block is dragged uphill by a rope. Identify all
forces acting on the block.
Slide 4-26
Example Problem
Block A hangs from the ceiling by a rope. Another block B hangs
from A. Identify the forces acting on A.
Slide 4-27
Example Problem
A ball, hanging from the ceiling by a string, is pulled
back and released. Identify the forces acting on it just
after its release.
Slide 4-28
Newton’s Second Law
What happens when Fnet is not = 0?
acceleration is a change in speed or a change in direction of speed.
Slide 4-29