Transcript Powerpoint

Physics 151 Week 7 Day 2
Topics
 What is a Force?
 Newton’s 0th Law of Motion (Not in the book)
 Force Diagrams and
System Schemas (Not in the book)
 Newton’s 1st Law of Motion
 More vector math
Vectors and Vector Components: Example
Displacement with 3 motion intervals
Slide 2-34
Vectors and Vector Components: Example
Velocity in 2D and vector equations
Slide 2-34
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
What Is a Force?
A force...
... is a push or pull.
... is a vector.
... acts on an
object.
... requires an
agent.
... is a contact force or a
long-range force.
Slide 4-10
Newton’s Zeroeth Law of Motion
Objects are dumb - They have no memory of the past and
cannot predict the future. Objects only know what acts
directly on them in a given moment.
Slide 2-34
Types of forces
Contact Forces vs. Non-contact Forces
Defining normal Forces and Friction Forces
Contact forces arise when two objects interact due to
surfaces in contact. These forces can be broken into two
components parallel and perpendicular to the surface
• The component parallel to the surface is the friction force
• The component perpendicular to the surface is the
normal force. (normal is a mathematical term meaning
perpendicular.)
Normal Force 
n
Slide 4-22
Friction

fk
and

fs
Slide 4-23

Tension ForceT
Slide 4-21

Drag D and Thrust

Fthrust
Slide 4-24
Force Vectors
Slide 4-18
Free-Body Diagrams
You should always describe a force by identifying the type
of force, the force agent and the object being acted on. For
a force diagram label forces like this => Fg, Earth=>box
System Schema
Draw a system schema:
• Draw a diagram where you write down the name of each object in
the system and then draw a solid circle drawn around it.
• Draw two sided arrows like this
between the object circles
of objects that interact (This illustrates all interactions between the
objects in this diagram).
• Draw an additional dotted line around the block to indicate it is the
object of interest. This diagram is called a system schema.
A system schema illustrates all the relevant interactions between the
objects in a given physical situation
Each double-headed arrow represents an action-reaction pair of forces
A.K.A. a Newton’s 3rd Law pair
Identifying Forces:
Freebody (Force) Diagrams and System Schema
Michael
Laura
Slide 2-34
Example Problem: One book, Two book
Slide 4-26
Newton's First Law of Motion
DEMO - Pushing the cart on track
DEMO - Hoverpuck
=> How much force is need to make an
object move with constant velocity?
Example Problem
The hover puck
Slide 4-26
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
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
Summary
Slide 4-39