Transcript Powerpoint

Physics 151 Week 7 Day 1
Topics
 Newton’s 1st Law of Motion
 Newton’s 2nd Law of Motion
 Newton’s 3rd Law of Motion
 Practice with Free-Body Diagrams & System
Schema
 Momentum
 Impulse
 Impulse-momentum Theorem
 Conservation of Momentum
Newton's Laws of Motion
Newton 0th Law
Objects are dumb - They have no memory of the past and cannot predict the
future. Objects only know what is acting directly on them right now
Newton's 1st Law
An object that is at rest will remain at rest and an object that is moving will continue
to move in a straight line with constant speed, if and only if the sum of the forces
acting on that object is zero.
Newton's 2nd Law
acceleration of an object = sum of forces acting on that object / the mass of the
object
Newton's 3rd Law
Recall that a force is an interaction between two objects. If object A exerts a force
on object B then object B exerts a force on object that is in the opposite direction,
equal in magnitude, and of the same type.
Slide 4-19
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
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
DEMO - Pull the string
Impulse
The force of the foot on
the ball is an impulsive
force.
Slide 9-8
Graphical Interpretation of Impulse
J = Impulse = area under
the force curve  Favg t
Slide 9-9
Momentum
Momentum is the product of an object’s mass and its velocity:


p = mv
Slide 9-10
The Impulse-Momentum Theorem
Impulse causes a change in momentum:




J =pf - pi = ∆p
Slide 9-11
Newton's Third Law of Motion Demonstrations
DEMO - Pushing off the wall
DEMO - Force Probes
One probe pushing the other
DEMO - Force Probes
One probe pulling the other
DEMO - Force Probes & Collisions I
Moving & Stationary Cart
Heavier vs. Lighter Cart
Newton’s 3rd Law Demonstrations
A.
FA=>B > FB=>A
B.
FA=>B < FB=>A
C.
FA=>B = FB=>A
D.
Not enough information given to tell
Slide 4-19
The Law of Conservation of Momentum
In terms of the initial and final total momenta:


Pf = Pi
In terms of components:
Slide 9-18