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# 6: Motion in Two and Three Dimensions

To give a complete description of kinematics, we must explore motion in two and three dimensions. After all, most objects in our universe do not move in straight lines; rather, they follow curved paths. From kicked footballs to the flight paths of birds to the orbital motions of celestial bodies and down to the flow of blood plasma in your veins, most motion follows curved trajectories. In this chapter we also explore two special types of motion in two dimensions: projectile motion and circular motion. Last, we conclude with a discussion of relative motion. In the chapter-opening picture, each jet has a relative motion with respect to any other jet in the group or to the people observing the air show on the ground.

• 6.1: Prelude to Motion in Two and Three Dimensions
Consider the Red Arrows, also known as the Royal Air Force Aerobatic team of the United Kingdom. Each jet follows a unique curved trajectory in three-dimensional airspace, as well as has a unique velocity and acceleration. Thus, to describe the motion of any of the jets accurately, we must assign to each jet a unique position vector in three dimensions as well as a unique velocity and acceleration vector.
• 6.2: Displacement and Velocity Vectors
The position function is graphed as a vector from the origin of a chosen coordinate system to describe the position of a particle as a function of time of a particle moving in two or three dimensions. The displacement vector gives the shortest distance between any two points on the trajectory of a particle in two or three dimensions. Instantaneous velocity is graphed as a vector that gives the speed and direction of a particle at a specific time on its trajectory in two or three dimensions.
• 6.3: Acceleration Vector
In two and three dimensions, the acceleration vector can have an arbitrary direction and does not necessarily point along a given component of the velocity. The instantaneous acceleration is produced by a change in velocity taken over a very short time period. Instantaneous acceleration is a vector in two or three dimension which can be found by taking the derivative of the velocity function with respect to time.
• 6.4: Projectile Motion
Projectile motion is the motion of an object subject only to the acceleration of gravity, where the acceleration is constant, as near the surface of Earth. To solve projectile motion problems, we analyze the motion of the projectile in the horizontal and vertical directions using the one-dimensional kinematic equations for x and y.
• 6.5: Relative Motion in One and Two Dimensions
When analyzing motion of an object, the reference frame in terms of position, velocity, and acceleration needs to be specified. Relative velocity is the velocity of an object as observed from a particular reference frame, and it varies with the choice of reference frame. If two reference frames are moving relative to each other at a constant velocity, then the accelerations of an object as observed in both reference frames are equal.
• 6.E: Motion in Two and Three Dimensions (Exercises)
• 6.S: Motion in Two and Three Dimensions (Summary)

Thumbnail: The Red Arrows is the aerobatics display team of Britain’s Royal Air Force. Based in Lincolnshire, England, they perform precision flying shows at high speeds, which requires accurate measurement of position, velocity, and acceleration in three dimensions. (credit: modification of work by Phil Long).