Transcript Powerpoint

Physics 151 Week 8 Day 1
Topics
 System Schema and Force Diagrams
 Newton’s First Law of Motion
 Gravitational Force and g
 Working with tilted coordinate systems
 Applying Newton’s 1st Law of Motion
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
Replace “no force”
with “no net force”
Slide 4-39
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
Newton's First Law of Motion
DEMO - Cart at rest
- Pushing the cart on track
DEMO - Hoverpuck at rest
- Hoverpuck in motion


How much force is need to make an object move
with constant velocity?
How does an object move with a constant net force?
What Causes Motion?
In the absence of any forces acting on it, an object will continue
moving forever. Motion needs no “cause.”
Slide 4-8
Newton's First Law of Motion
Every object continues in a state of rest or a state of motion with
a constant speed in a straight line unless acted on by an
unbalanced force.
or
Velocity = constant if and only if (IFF) Fnet = 0
Seat Belts: An Application of Newton’s First Law
Slide 4-16
Newton's First Law of Motion Demonstrations
DEMO - Smash the HAND
DEMO - Tablecloth
DEMO - Pull the String
Quick Points
• Free-body diagram is another name for a 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
•
•
Pay attention to the lines indicating motion.
If there are no lines indicating motion and there is nothing
in the wording that indicates the object is moving,
do not assume the object is moving
•
Assume Drag forces are negligible unless otherwise stated
or implied
For friction, consider what the system/object would do
without friction. Friction will respond to minimize the motion
between the surfaces in contact.
See if Fnet = 0 N without friction
•
•
Example Problem
An elevator, lifted by a cable, is going up at a steady speed.
• Identify the forces acting on the elevator.
• Is FT greater than, equal to, or less than Fg? Or is there not
enough information to tell?
A => FT > Fg
B => FT = Fg
C => FT < Fg
Slide 4-30
Gravitational Force and g
Make measurements to make a data table of Fg and mass
Graph this relationship
What is the significance of the slope?
Slide 4-26
Gravitational Force & Coordinate Systems
A. Fg, y = -mg
B. Fg, y = -mgsinq
C. Fg, y = mg cosq
D. Fg, y = -mg cosq
E. Can’t tell what Fg,y is
Slide 4-19
Example Problem: What holds it up?
Use a free body diagram to determine what type of force
holds up a refrigerator magnet.
What type of force holds up a refrigerator magnet?
A.
B.
C.
D.
E.
Magnetic Force
Gravitational Force
Normal Force
Friction Force
Some other force
What is the magnitude of this force?
Slide 4-26
Fnet = 0 Problems
Determine tension in each cable.
(Hint: There is more than one way to define the system.)
5 kg
Fnet = 0 Problems
Determine tension in each cable.
(Hint: There is more than one way to define the system.)
Fnet = 0 Problems
The object hung from the cable has a weight of 25 N.
●
Write the equation for the sum of the forces in the y-direction.
●
What is the tension in the cable?
●
●
What if the angles are smaller?
What if the angles are not symmetrical?
Fnet = 0 Problems
The cable at left exerts a -30 N force.
a. Write the equation for the sum of the forces in the
x-direction. What is the value of T2?
b. Write the equation for the sum of the forces in the
y-direction. What is the force of gravity acting on
the ball?
General Force Model
Newton 0th Law of Motion
Newton 1st Law of Motion
Newton’s 3rd Law of Motion
Diagrams:
System Schema
Force Diagrams
Other Equations:
Fg = mg