Mr. Rogers AP Physics C Study Guide  --  Newton's Laws

Unit Plan Practice Test Study Guide

Mathematical models

Dynamics

S F = ma

Statics

S Fx = 0, S Fy = 0, S Fz = 0       for a = 0

Components of weight force Fw on a slope with angle b

Fwn = Fw cos b

Fwp = Fw sin b

Key Principles

Definitions of "g"
  • unit of acceleration = 9.8 m/s2
  • acceleration due to gravity (freefall only)
  • measure of gravity field strength
Conditions for Static Equilibrium
  • a = 0
  • v = constant (often zero)
  • the sum of the forces in each dimension = 0
Newton's Laws
  • 1st: establishes inertial frame of reference
  • 2nd: F = ma
  • 3rd: Forces always appear in pairs acting in opposite directions on 2 different objects.

 

Free body diagrams (FBD):
  • Shows only forces from the outside world that act on a body.
  • Shows only a single force from an action/reaction pair.
  • Never shows internal forces.
  • Acceleration or velocity may be placed beside the FBD but never on it.
 

Problem Solving Tips: Newton's Laws

Use of "g": Note that "g" is usually used as measure of gravity field strength in dynamics problems. It is rarely used as the acceleration due to gravity. bathroom scales reading = normal force, NOT the weight force. Likewise the normal force not the gravity force creates the sensation of weight.
Ropes:
  • Can pull but can never push
  • Ropes are modeled as massless
  • Tension in an ideal rope is the same throughout the rope

 

Action reaction pairs: forces always come in pairs that are equal in magnitude and opposite in direction. For a person standing on the floor, the action reaction pair for the Earth's gravity force acting on the person is the person's gravity force acting on the Earth. The normal force is not the action reaction pair of the gravity or weight force.

Example Problems

Dynamics Statics
Elevator Problem
Bob has a mass = 100 kg. He has been told that he can lose weight by descending in an elevator. He places a bathroom scale in the elevator, stands on it, and presses the down button causing him to  descend at an acceleration of 4 m/s2. What does the bathroom scale read on the way down?
Buzzard Problem
Two telephone poles are separated by a distance of 200 m. A perfectly horizontal wire with a mass of zero is stretched between them. A 2 kg buzzard lands on the mid point of the wire and causes it to deflect by 1 cm. Find the tension in the wire.
Harness Type Elevator Problem
Bob still has a mass = 100 kg. He decides to install an elevator (of sorts in his house). The "elevator consists of a rope running over a pulley attached to the ceiling. To operate the make shift elevator, Bob ties the one of rope around his waist and pulls downward on the other dangling rope. This action makes him accelerate upward at a rate of 3 m/s2. What is the tension in the rope?
Scale  Problems
What will each of the spring scales read? Assume that the pulleys are massless and frictionless and the ropes and scales massless.
 

 

Vocabulary

freebody diagrams Static conditions  
inertial frame of reference    
     
Mr

SAM Team--Southside High School's STEM and Computer Science extra-curricular club (Mr. Rogers Sponsor)

Mr. Rogers' Twitter Site

Mr. Rogers Teacher's Blog

Mr. Rogers T-shirts

Mr. Rogers Information for Teachers

Mr. Rogers Science Fair Information

Check out other web sites created by Mr. R:

Check out Articles by Mr. Rogers:

 
Insultingly Stupid Movie Physics is one of the most humorous, entertaining, and readable physics books available, yet is filled with all kinds of useful content and clear explanations for high school, 1st semester college physics students, and film buffs.

It explains all 3 of Newton's laws, the 1st and 2nd laws of thermodynamics, momentum, energy, gravity, circular motion and a host of other topics all through the lens of Hollywood movies using Star Trek and numerous other films.

If you want to learn how to think physics and have a lot of fun in the process, this is the book for you!

 

First the web site,

now the book!


Mr. Rogers Home | Common Sylabus | AP Comp Sci I | AP Comp Sci II | AP Physics Mech | AP Physics E&M | AP Statistics | Honors Physics|IB Design Tech | Southside

[ Intuitor Home | Physics | Movie Physics | Chess | Forchess | Hex | Intuitor Store |

Copyright 1996-2011 T. K. Rogers, all rights reserved. Forchess is a registered trademark of T. K. Rogers.
No part of this website may be reproduced in any form, electronic or otherwise, without express written approval.