1-Kinematics (motion)

TextBook: Chapter 1 & 3.5 in in Mastering Physics (get online code for registration on about page of google classroom)

Basic Overview

Kinematics is the study of motion of objects based of observations of change in position, velocity and acceleration. It does not look at the cause of motion (forces or inertia).

Equations

Using Kinematics Equations to solve 1D motion problems

The video embedded here shows how to select the right equation and apply it.

  • When motion is uniformly accelerated, we can use the 3 kinematic equations above to determine the speed, position, time or acceleration of the object.
    • choose the equation based on what information you are not given.
    • common assumptions include:

-being at rest or stopped indicates v=0 (including drops and vertical vy at the top of a throw)

-we often start at a position of 0, but can modify that position to make our work easier.

-ay = gravitational field strength = 9.8 m/s downward in frictionless (no air resistance) free-fall

  • When motion is graphed it can be easier to use graphing skills to find the information instead of using the formulas. On other occasions you will be required to show understanding of these graphing processes when you may not be given enough information to solve using equations.
    • Slope and area can be used to solve or describe motion in different ways depending on the axis of a graph.
  • 2 Dimensional motion can be solved by splitting the motion into X and Y axes.

Projectiles

  • Assuming air resistance is negligible we can always assume the following for projectiles (airborne objects without motive power):
    • motion on the x axis is unaccelerated (v0x = vx and ax = 0)
    • motion on the y axis is accelerated by gravity (ay = -g on earth ay = -10 m/s2)
    • Horizontal launch projectiles have the following assumptions in addition to the general assumptions above:
      • The initial velocity (muzzle velocity) is ALL on the x axis. (v0y = 0)
      • The time in the air is only based on height and g
    • Angled launches on level ground have the following assumptions in addition to the general assumptions above:
      • The starting and ending positions on the y axis can both be zero
      • The starting and ending velocities on the y axis are equal, but opposite. (–voy = vy)
      • In the absence of air resistance, the max height occurs at 1/2 the time of the whole trajectory.
    • As long as the object is thrown with some upward component, the maximum height occurs when the velocity on the y axis = 0. This is not happening at the full time of the trajectory, so if should be solved separately from final positions, etc.

Misconceptions

The following are common misconceptions from this unit. It is likely that the AP test will use these as a basis for writing believable distractors in multiple choice questions or look to uncover these in your writing in the free response section. See if you can explain why they are not always true (some are true sometimes, while others are not true at all).

  • Misconception 1: Freely falling bodies can only move downward.
    • What does free-fall really mean? Can you come up with examples of free fall that are not moving downward?
  • Misconception 2: Gravity only acts on things when they are falling.
    • Name some times when this is not true.
  • Acceleration and velocity are always in the same direction.
    • What happens if they are opposite each other? What happens if they are 90º to each other?
  • The acceleration of a falling object depends upon its mass. (Heavier objects fall faster than light ones.)
    • What makes this true for many real-life situations? What conditions do we assume in Physics problems that makes it not true?
    • Can you show/explain why they fall at the same rate?
  • If velocity is zero, then acceleration must be zero too. Really?? Why not?

A couple more common mistakes to make sure you don't make:

  • Two objects side by side must have the same speed.
  • Velocity is a force.
  • Acceleration is the same as velocity.

My Handouts/Notes

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