Describing and Measuring Motion, Velocity & Acceleration

1. Describing and MeasuringMotion, Velocity & Acceleration Basics of Physics (21:14)

2. 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.

3. 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

4. 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

5. 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

6. 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

7. 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.

8. 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!

9. 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

10. Acceleration • 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

11. 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

12. 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

13. 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

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