# Chapter 3: Motion in 2 or 3 Dimensions

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Chapter 3: Motion in 2 or 3 Dimensions

Position & Velocity Vectors

Position Vector To describe the motion of a particle in space, we first need to describe the position of the particle. Position vector of a particle is a vector that goes from the origin of coordinate system to the point. Position vector components are the Cartesian coordinates of the particle.

Position & Velocity Vectors
As the particle moves through space, the path is a curve. The change in position (the displacement) of a particle during time interval t is: Average velocity vector during this time interval is the displacement divided by the time interval:

Position & Velocity Vectors
Instantaneous velocity vector is the limit of the average velocity as the time interval approaches zero, and equals the instantaneous rate of change of position with time: Magnitude of the vector v at any instant is the speed v of the particle at that instant. Direction of v at any instant is the same as the direction in which particle moves at that instant.

Position & Velocity Vectors
As t0, P1 and P2 move closer and in this limit vector r becomes tangent to the curve. Direction of r in the limit is the same as direction of instantaneous velocity v. At every point along the path, instantaneous velocity vector v is the tangent to the path at that point.

Position & Velocity Vectors
Components of instantaneous velocity vector v : Magnitude of vector v by Pythagorean theorem:

Acceleration Vector

Acceleration Vector Acceleration of a particle moving in space describes how the velocity of particle changes. Average acceleration is a vector change in velocity divided by the time interval:

Acceleration Vector Instantaneous acceleration is the limit of the average acceleration as the time interval approaches zero, and equals the instantaneous rate of change of velocity with time:

Acceleration Vector In terms of unit vectors:

Acceleration Vector Components of acceleration:
Acceleration vector can be resolved into a component parallel to the path (and velocity), and a component perpendicular to the path.

Acceleration Vector Components of acceleration:
When acceleration vector is parallel to the path (and velocity), the magnitude of v increases, but its direction doesn’t change When acceleration vector is perpendicular to the path (and velocity), the direction of v changes, but magnitude is constant

Acceleration Vector Components of acceleration for a particle moving along a curved path: Constant speed Increasing speed Decreasing speed

Projectile Motion

Projectile Motion A projectile is any object that is given an initial velocity and then follows a path (trajectory) determined solely by gravity and air resistance. The motion of a projectile will take place in a plane (so, it is 2-D motion). For projectile motion we can analyze the x- and y-components of the motion separately. The horizontal motion (along the x-axis) will have zero acceleration and thus have constant velocity. The vertical motion (along the y-axis) will have constant downward acceleration of magnitude g = 9.80 m/s2. The initial velocity components, vox and voy, can be expressed in terms of the magnitude vo and direction ao of the initial velocity.

Projectile Motion

Projectile Motion We analyze projectile motion as a combination of horizontal motion with constant velocity and vertical motion with constant acceleration Initial velocity is represented by its magnitude and direction

Projectile Motion Trajectory of a body projected with initial velocity v0 h is maximum height of trajectory R is horizontal range

Projectile Motion

Projectile Motion Trajectory of a cow
A cow is launched from the top of a hill with an initial velocity vector that makes an angle of 45 degrees with the horizontal. The projectile lands at a point that is 10 m vertically below the launch point and 300 m horizontally away from the launch point. Determine the time the cow was in the air. Determine the initial speed of the cow.

Projectile Motion Initial speed of the cow

Projectile Motion Flight time of the cow

Motion in a Circle

Motion in a Circle When a particle moves along a curve, direction of its velocity vector changes. Particle must have component of acceleration  to the curved path even if the speed is constant. Motion in a circle is a special case of motion along a curved path. Uniform circular motion - when a particle moves with constant speed Non-uniform circular motion - if the speed of a particle varies.

Uniform Circular Motion
No component of acceleration parallel (tangent) to the path. Otherwise, speed would change. Non-zero component of acceleration is perpendicular to the path.

Uniform Circular Motion
A particle that is undergoing motion in such a manner that its direction is changing is experiencing a radial acceleration that has magnitude equal to the square of its velocity divided by the instantaneous radius of curvature of its motion. The direction of this radial, or centripetal, acceleration is toward the center of circular path of particle's motion.

Uniform Circular Motion
In uniform circular motion, the magnitude a of instantaneous acceleration is equal to the square of the speed v divided by the radius R of the circle. Its direction is  to v and inward along the radius. Centripetal  “seeking the center” (Greek)

Uniform Circular Motion
Period of the motion T  the time of one revolution, or one complete trip around the circle. In time T, particle travels the distance 2R of the circle, so its speed can be expressed as

Motion in a Circle: Example
Centripetal acceleration on a curved road A car has a “lateral acceleration” of 0.87g, which is (0.87)/(9.8m/s2)=8.5m/s2. This represents the maximum centripetal acceleration that can be attained without skidding out of the circular path. If the car is traveling at a constant speed 40m/s (~89mi/h, or 144km/h), what is the max radius of curve it can negotiate? IDENTIFY and SET UP Car travels along a curve, speed is constant  apply equation of circular motion to find the target variable R. EXECUTE We know arad and v, so we can find R:

Non-Uniform Circular Motion
An object that is undergoing non-uniform circular motion, or motion where the magnitude and the direction of the velocity is changing, will experience an acceleration that can be described by two components: A radial or centripetal acceleration equal to the square of speed divided by radius of curvature of motion directed toward the center of curvature of the motion, and Tangential component of acceleration that is equal to the rate of change of the particle's speed and is directed either parallel (in the case of speeding up) or anti-parallel (in the case of slowing down) to the particle's velocity.

Relative Velocity

Relative Velocity The velocity seen by particular observer is called relative to that observer, or relative velocity.

Relative Velocity in 1-D
Woman walks with a velocity of 1.0m/s along the aisle of a train that is moving with a velocity of 3.0m/s. What is the woman’s velocity? For passenger sitting in a train: 1.0m/s For bicyclist standing: 1.0m/s + 3.0m/s = 4.0m/s Frame of reference is a coordinate system + time scale

Relative Velocity in 1-D
Cyclist: frame of reference A Moving train: frame of reference B In 1-D motion, position of P relative to frame of reference A is given by distance XP/A Position of P relative to frame of reference B is given by distance XP/B Distance from origin A to origin B is given by XB/A

Relative Velocity in 1-D
Velocity VP/A of P relative to frame A is the derivative of XP/A with respect to time

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