Conceptual Physics Notes on Chapter 2 Motion in One Direction

Conceptual Physics Chapter 2 HOW FAST Motion in one plane / direction Motion is relative! –relative to the Earth, Sun, Galaxy, Universe All moving objects have speed Speed is a measure of how fast something is moving or rate in which distance is covered

Conceptual Physics Chapter 2 HOW FAST Speed is defined as the distance covered per unit of time. Speed is defined as the distance covered per unit of time. Two types of speed: Two types of speed: Average & Instantaneous Average & Instantaneous –Average speed is the total distance covered divided by total time interval –Instantaneous speed is the speed you are going at any given time

Conceptual Physics Chapter 2 HOW FAST Velocity Velocity is speed in a given direction –Speed & Velocity are different in Physics

Conceptual Physics Chapter 2 HOW FAST Equation Equation v = d / t v = d / t v = speed or velocity v = speed or velocity d = distance d = distance t = time t = timeUnits m/s km/s mm/s m/s km/s mm/s meters meters seconds seconds

Conceptual Physics Chapter 2 HOW FAST EXAMPLE: EXAMPLE: If a cheetah can run a constant speed of 25 m/s, how far will it run in 10 seconds? v = 25 m/s d = ? t = 10 s v = d / tv = d / tv = d / tv = d / t (25 m/s) = d / (10 s) (25 m/s )( 10 s ) = d d = 250 m

Conceptual Physics Chapter 2 HOW FAST Constant velocity vs Changing velocity Constant velocity is constant speed and direction Constant velocity is constant speed and direction Changing velocity is changing EITHER speed or direction Changing velocity is changing EITHER speed or direction

Conceptual Physics Chapter 2 HOW FAST EXAMPLE: The speedometer in a moving southward car reads 60 km/hr. It passes another car moving northward at 60 km/hr. Do both cars have the same speed? Do they have the same velocity?

Conceptual Physics Chapter 2 HOW FAST Acceleration is the rate Acceleration is the rate in which velocity changes with time. in which velocity changes with time. a = Δ v / t a = Δ v / t Δ means a change in Units Δ v = m/s Δ v = m/s t = seconds t = seconds a = m/s/s or m/s 2 a = m/s/s or m/s 2 Acceleration

Conceptual Physics Chapter 2 HOW FAST EXAMPLE: Pg 16 Question #2

Conceptual Physics Chapter 2 HOW FAR Free Fall : How Far? Gravity causes objects to fall and accelerate toward the Earth. Such objects are said to be in FREE FALL. Gravity causes objects to fall and accelerate toward the Earth. Such objects are said to be in FREE FALL. Gravity is 9.8 m/s 2 Gravity is 9.8 m/s 2 We will use 10 m/s 2 We will use 10 m/s 2

Conceptual Physics Chapter 2 HOW FAR Recall objects let go start at rest or ZERO velocity a = Δv / t  Δv = at if the acceleration is toward the earth, a = g : g is gravity which is equal to a = g : g is gravity which is equal to 10 m/s 2 10 m/s 2 Δv = gt Δv = gt

Conceptual Physics Chapter 2 HOW FAR EXAMPLE: What would the speed be of a rock be after 4.5 sec after it is dropped from rest? What would the speed be of a rock be after 4.5 sec after it is dropped from rest? After 8 sec.? After 8 sec.? After 15 sec.? After 15 sec.?SOLUTION: v = gt v = gt v = (10 m/s/s)(4.5 s) = 45 m/s v = (10 m/s/s)(4.5 s) = 45 m/s v = (10 m/s/s)(8 s) = 80 m/s v = (10 m/s/s)(8 s) = 80 m/s v = (10 m/s/s)(15 s) =150 m/s v = (10 m/s/s)(15 s) =150 m/s

Conceptual Physics Chapter 2 HOW FAR We are trying to find a distance a object falls and the average of that fall. Remember we drop a(n) object from rest! So we take the start velocity and the end velocity and divide by two (2) d = vt but v = gt from above d = (g/2 t)t or d = (g/2 t)t or d = ½ gt 2

Conceptual Physics Chapter 2 HOW FAR Example: A apple drops from a tree and hits the ground in one second. A apple drops from a tree and hits the ground in one second. What was the speed at impact? What was the speed at impact? What was the distance the apple fell? What was the distance the apple fell? What if the apple fell for 3 seconds, how far above the ground was the apple? What if the apple fell for 3 seconds, how far above the ground was the apple?

Conceptual Physics Chapter 2 HOW FAR Air Resistance In the absence of air, ALL objects fall at the same rate here on Earth g = 10 m/s 2 g = 10 m/s 2