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Motion in the plane

This part defines two things that every later part of the course uses: a way to state the Position of an object, and a way to predict its later position and Velocity. Velocity and Acceleration are time derivatives of position. So this part defines position first.

Part 1 of 22 chapters35–45 min8 numbered equations2 knowledge checks

  1. Position and Coordinate Systems

    Locate a moving point in the plane in Cartesian and polar coordinates, and read velocity and acceleration off position as time derivatives.

    20–30 min7 equations1 checkdraft

  2. Velocity Under Constant Acceleration

    Predict an object's velocity at any later time from its initial velocity and a constant acceleration.

    15 min1 equation1 checkdraft

Notation used on this page (5)
a\mathbf{a}Accelerationdraft

The rate at which velocity changes with time. In this course it is held constant over a scenario, so it is the same vector at every instant.

a=d vdt\explain{motion.acceleration}{\mathbf{a}} = \dfrac{d\,\explain{motion.velocity}{\mathbf{v}}}{d\explain{motion.time}{t}}

Units: meters per second squared

v0v_0Launch speeddraft

The object's speed at the start of the scenario being analyzed — its initial velocity in a straight-line problem, or its launch speed the instant a projectile leaves the ground.

Units: meters per second

r\mathbf{r}Positiondraft

The object's location at elapsed time t, taken as a vector from a fixed origin so a two-dimensional motion keeps its horizontal and vertical parts separate.

r=(x, y)\explain{motion.position}{\mathbf{r}} = (\explain{motion.position.horizontal-coordinate}{x},\ \explain{motion.position.vertical-coordinate}{y})
Symbols
xxhorizontal coordinate(meters)
yyvertical coordinate(meters)

Units: meters

ttTimedraft

Elapsed time since the object was launched or released, measured in seconds.

Units: seconds after launch

v(t)v(t)Velocitydraft

The rate at which position changes with time. It is a vector; in a straight-line problem only its size and sign matter, and that is what the scalar v(t) tracks.

v=d rdt ;v(t)=v0+a t\explain{motion.velocity}{\mathbf{v}} = \dfrac{d\,\explain{motion.position}{\mathbf{r}}}{d\explain{motion.time}{t}}\,;\quad \explain{motion.velocity}{v(t)} = \explain{motion.launch-speed}{v_0} + \explain{motion.velocity.constant-acceleration}{a}\,\explain{motion.time}{t}
Symbols
aaconstant acceleration(meters per second squared)

Units: meters per second