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

Unit Plan Practice Test Study Guide

Mathematical models


S F = ma


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.
  • 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.



freebody diagrams Static conditions  
inertial frame of reference    

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