Describing and Measuring Motion, Velocity & Acceleration Basics of Physics (21:14)

Motion An object is in motion when its distance from another object is changing. A reference point is a place or object used for comparison to determine if something is in motion. An object is in motion if it changes position relative to a reference point.

Motion and Reference Points When a passenger in train A looks over at a passenger in train B it looks as though both are not moving even though they both are: What can we infer about their speed? What visual evidence is available, from either train, to let them know that they are in fact moving? What is this evidence called? Both have the same speed Trees Reference Point

Calculating Speed The distance an object travels in a certain amount of time, Speed. Speed = Distance/Time If a car travels 90 km in 1 hour, the speed of the car would be: 2. If an ant crawls 2 cm in 1 second, the speed of the ant would be: S = d/t 90Km/hr S = d/t 2cm/sec

Types of Speed Constant Speed – If the speed of an object does not change over a period of time. Speed = Distance/Time If a horse gallops 21 m in 3 seconds, what is the horses speed? Formula: Speed = distance/time Work: = 21m/3sec Answer: = 7m/s

Ave. Speed = Total Dist./Total Time Types of Speed Average Speed – most objects do not move at constant speed for long periods of time but change their speed throughout the course of time. Ave. Speed = Total Dist./Total Time A cyclist in a race travels 32 km during the first two hours of his race, and 13km during the next hour. Formula: Ave Speed = Tot. Dist/Tot. Time Work: = (32 km + 13 km)/ (2hr + 1hr) Answer: = 15km/hr

Constant or Average Speed? From Start to A: From A to B: From B to C: From C to D: Constant Speed – speed stays the same over time, 10m/s Average Speed – speed changes over time. Constant Speed – speed stays the same over time, 30m/s Average Speed – speed changes over time.

Velocity When you know both the speed and direction of an object’s motion you know the velocity of the object. A weather forecaster may give the speed of an approaching storm as 25km/hr but should you be concerned if you live in Ovid? you don’t know the storm’s velocity until they give you the direction that the storm is moving too. This would be helpful to know if you had to plan for the impending storm! The storm is currently over Penn Yan and traveling 25km/hr, East – yes, you should be concerned!

Calculating Velocity A eastbound train has traveled 150 miles in 2 hours, what is its velocity? Formula: Work: Answer: Velocity = distance/time + direction V = 150 mi/2hr V = 75mi/hr Velocity = 75mi/hr East

Acceleration = final velocity – initial velocity/time The rate at which velocity changes = acceleration. Can be: (speed or direction) An increasing speed - acceleration An airplane accelerates in preparation for takeoff. Decreasing speed – deceleration A car slowing down for a stop sign. Changing direction – acceleration Earth accelerates as it revolves around the sun Acceleration = final velocity – initial velocity/time = Vf – Vi/t = m/s/s or m/s2

Calculating Acceleration As an airplane travels down a runway it goes from 0m/s to 40m/s in 5sec. What is the airplane’s acceleration? Formula: =Vf – Vi/t Work: =(40-0m/s)/5sec Answer: = 8 m/s/s

Calculating Acceleration A car is traveling down the road at 90km/hr and after 7 sec the car was found to be traveling at 20km/hr. What was the car’s acceleration? Formula: =Vf – Vi/t Work: =(20-90km/hr)/7sec Answer: = -10km/hr/sec What type of acceleration was this car having? Negative acceleration or decelerating

Calculating Acceleration What type of acceleration is taking place from: A to B: C to D: What is the acceleration of the car from: Start to A: Acceleration Deceleration A to B: Formula: Vf – Vi/t Formula: Vf – Vi/t Work: 10-10/150 Work: 30-10/100 Answer: 0m/s2 Answer: 0.2 m/s2

Formulas for tonight’s HW: Calculating Acceleration (#4) Vf = (a x t) + Vi Vi = Vf – (a x t)