🔋College Physics I – Introduction Unit 3 – Two–Dimensional Kinematics

Key Concepts and Definitions

• Two-dimensional kinematics studies the motion of objects in two dimensions (x and y) and includes concepts such as position, displacement, velocity, and acceleration
• Position refers to an object's location in space, typically described using Cartesian coordinates (x, y) or polar coordinates (r, θ)
• Displacement measures the change in an object's position and is a vector quantity with both magnitude and direction
• Velocity describes the rate of change of an object's position and is also a vector quantity
• Speed is the scalar magnitude of velocity and represents how fast an object is moving without considering direction
• Acceleration is the rate of change of velocity and can be caused by changes in speed, direction, or both
• Vectors are quantities that have both magnitude and direction, while scalars only have magnitude

Position, Displacement, and Distance

• Position is an object's location relative to a chosen reference point or origin and is typically expressed using Cartesian coordinates (x, y) or polar coordinates (r, θ)
  • Cartesian coordinates use perpendicular x and y axes to describe a point's location
  • Polar coordinates use the distance from the origin (r) and the angle from the positive x-axis (θ) to specify a point's location
• Displacement is the change in an object's position and is calculated by subtracting the initial position vector from the final position vector
  • Displacement is a vector quantity, meaning it has both magnitude and direction
  • The magnitude of displacement is always less than or equal to the distance traveled
• Distance is the total length of the path an object travels and is a scalar quantity
  • Distance is always positive and does not depend on the direction of motion
• The relationship between position, displacement, and distance can be summarized as follows:
  • Displacement = Final Position - Initial Position
  • Distance ≥ |Displacement|

Velocity and Speed

• Velocity is the rate of change of an object's position with respect to time and is a vector quantity
  • Velocity has both magnitude (speed) and direction
  • Average velocity is calculated by dividing the displacement by the time interval: $v_{avg} = \frac{\Delta x}{\Delta t}$
  • Instantaneous velocity is the velocity at a specific instant in time and is found by taking the limit of average velocity as the time interval approaches zero: $v_{inst} = \lim_{\Delta t \to 0} \frac{\Delta x}{\Delta t}$
• Speed is the scalar magnitude of velocity and represents how fast an object is moving without considering direction
  • Average speed is calculated by dividing the total distance traveled by the time interval: $s_{avg} = \frac{d_{total}}{\Delta t}$
  • Instantaneous speed is the magnitude of instantaneous velocity
• Velocity and speed are related but distinct concepts:
  • An object's speed can change without its velocity changing if the direction of motion remains constant
  • An object's velocity can change without its speed changing if the direction of motion changes while the magnitude remains constant

Acceleration

• Acceleration is the rate of change of velocity with respect to time and is a vector quantity
  • Acceleration has both magnitude and direction
  • Average acceleration is calculated by dividing the change in velocity by the time interval: $a_{avg} = \frac{\Delta v}{\Delta t}$
  • Instantaneous acceleration is the acceleration at a specific instant in time and is found by taking the limit of average acceleration as the time interval approaches zero: $a_{inst} = \lim_{\Delta t \to 0} \frac{\Delta v}{\Delta t}$
• Acceleration can be caused by changes in speed, direction, or both
  • An object moving in a straight line with increasing speed has positive acceleration in the direction of motion
  • An object moving in a straight line with decreasing speed has negative acceleration (deceleration) in the direction of motion
  • An object moving along a curved path experiences centripetal acceleration, which is always directed toward the center of the curve
• The relationship between position, velocity, and acceleration can be described using kinematic equations:
  • $v = v_0 + at$
  • $x = x_0 + v_0t + \frac{1}{2}at^2$
  • $v^2 = v_0^2 + 2a(x - x_0)$

Vectors and Scalars in 2D Motion

• Vectors are quantities that have both magnitude and direction, while scalars only have magnitude
  • Position, displacement, velocity, and acceleration are all vector quantities in 2D motion
  • Distance and speed are scalar quantities
• Vector addition and subtraction are used to combine or separate vectors in 2D motion
  • The resultant vector is found by adding the components of the individual vectors: $\vec{R} = \vec{A} + \vec{B}$
  • Vector subtraction is performed by adding the negative of the vector being subtracted: $\vec{A} - \vec{B} = \vec{A} + (-\vec{B})$
• Vector resolution is the process of breaking a vector into its components along the x and y axes
  • The x-component of a vector is found using: $A_x = A \cos \theta$
  • The y-component of a vector is found using: $A_y = A \sin \theta$
• Scalar multiplication is used to scale a vector by a constant factor
  • Multiplying a vector by a positive scalar changes its magnitude but not its direction
  • Multiplying a vector by a negative scalar changes its magnitude and reverses its direction

Projectile Motion

• Projectile motion is a type of two-dimensional motion in which an object is launched with an initial velocity and follows a parabolic path under the influence of gravity
  • Projectile motion consists of two independent components: horizontal motion (constant velocity) and vertical motion (constant acceleration due to gravity)
  • The time of flight for a projectile launched horizontally from a height h is given by: $t = \sqrt{\frac{2h}{g}}$
  • The range of a projectile launched at an angle θ with initial velocity v₀ is given by: $R = \frac{v_0^2 \sin 2\theta}{g}$
• The horizontal and vertical components of a projectile's motion can be analyzed separately
  • Horizontal motion: $x = v_{0x}t$
  • Vertical motion: $y = y_0 + v_{0y}t - \frac{1}{2}gt^2$
• The maximum height reached by a projectile launched at an angle θ with initial velocity v₀ is given by: $h_{max} = \frac{v_{0y}^2}{2g}$
• The time to reach the maximum height is given by: $t_{max} = \frac{v_{0y}}{g}$

Relative Motion

• Relative motion describes the motion of an object as observed from different reference frames
  • A reference frame is a coordinate system used to describe the position and motion of objects
  • An inertial reference frame is one in which Newton's laws of motion hold true (no acceleration)
• The relative velocity between two objects is the velocity of one object as observed from the reference frame of the other object
  • The relative velocity formula is given by: $\vec{v}_{AB} = \vec{v}_A - \vec{v}_B$
  • If two objects are moving in the same direction, their relative velocity is the difference between their individual velocities
  • If two objects are moving in opposite directions, their relative velocity is the sum of their individual velocities
• The relative position and displacement between two objects can also be determined using vector subtraction
  • The relative position formula is given by: $\vec{r}_{AB} = \vec{r}_A - \vec{r}_B$
  • The relative displacement formula is given by: $\Delta \vec{r}_{AB} = \Delta \vec{r}_A - \Delta \vec{r}_B$

Problem-Solving Strategies

• Identify the given information and the quantity to be determined
  • List the known variables and their values
  • Identify the unknown variable to be solved for
• Sketch a diagram of the problem situation
  • Include relevant information such as initial position, velocity, acceleration, and any obstacles or boundaries
  • Establish a coordinate system and label important points and vectors
• Determine the appropriate equations or principles to use
  • Select kinematic equations based on the given information and the quantity to be determined
  • Consider whether the motion is one-dimensional or two-dimensional and if any special cases apply (e.g., projectile motion, relative motion)
• Solve the equations for the unknown quantity
  • Substitute known values into the selected equations
  • Perform algebraic manipulations to isolate the unknown variable
  • Calculate the numerical value of the unknown quantity
• Check the solution for reasonableness
  • Verify that the calculated value has the correct units and a reasonable magnitude
  • Consider whether the answer makes sense in the context of the problem situation
  • If possible, compare the solution to known values or limiting cases