Chapter 2 Linear Motion Aristotle on Motion (350 BC) Aristotle attempted to understand motion by classifying motion as either (a) natural motion or (b) violent motion. Large object tend to 'strive harder'. The Earth remains at rest. Geocentric Model - Earth Centered Universe Copernicus (1500's) "The Earth and planets orbit the Sun. He reasoned this from his astronomical observations. Galileo (1600's) Scientist who supported Copernicus Dropped objects with different weights from the Leaning Tower of Pisa Found that all objects fall at the same rate if you can account for air resistance Galileos Incline Planes Friction - a force that resists motion e.g. air resistance and sliding on rough surfaces Inertia - the resistance of an object to change in its state of motion Demo: Ball and incline plane Would it be correct to say that inertia causes a moving object to continue in motion? In a strict sense, no. See this question on page 22. * Description of Motion instantaneous speed - the speed that something has at any one instance The average speed for a trip might be 53 miles/hour. However, during this trip your instantaneous speed might have been 0 miles/hour at a stoplight or 70 miles/hour on the open road. 1. What is the average speed of a cheetah that sprints 100 meters in 4 seconds? 2. How about if it sprints 50 m in 2 s? See questions on page 24. * What are the units of speed? miles/hour.mph meters/second.m/s kilometer/hour.km/h furlongs/fortnight? If a car moves with an average speed of 60km/h for an hour, it will travel a distance of 60 km. (a) How far would if travel if it moved at this rate for 4 hours? (b) For 10 h? (c) Would if be possible for the car to have an average speed of 60km/h and never exceed a reading of 60km/h? See this question on page 24. * Velocity = {speed with a direction} Examples: 70 mph is a speed. 70 mph North is a velocity. Velocity Acceleration - rate of change in velocity due to change in speed or direction Acceleration Example: 9.8 meters/second 2 downward "She moves at a constant speed in a constant direction. Say the same sentence in fewer words. Answer: She moves at constant velocity. See questions on page 25. * The speedometer of a car moving to the east reads 100km/h. It passes another car that moves to the west at 100km/h. Do both cars have the same speed? Do they have the same velocity? During a certain time, the speedometer of car reads a constant 60km/h. Does this indicate a constant speed? Constant velocity? Example Multiple-Choice Questions 1. The two measurements necessary for calculating average speed are (a) acceleration and time. (b) velocity and time. (c) distance and time. (d) distance and acceleration. 2. What is the average speed of a horse that gallops a distance of 10 kilometers in a time of 30 minutes? (a) 10km/h (b) 20km/h (c) 30km/h (d) more than 30km/h 3. What is the acceleration of a car that maintains a constant velocity of 100km/h for 10 seconds? (a) 0 km/h s (b) 10 km/h s (c) 10 m/s 2 (d) 1000 km/h s Free fall is a state of falling free from air resistance and other forces except gravity. Free Fall (a) you drop an object from rest at t=0. (b) velocity acquired = acceleration time v = g t (c) For Free Fall... 6. Dan drops a stone down a well and sees it hit the water 4 seconds later. Neglecting air resistance, how deep is this well? (a) 9.8 m(b) 19.6 m (c) 78.4 m (d) m(e) 39.2 m 7. Dan drops a stone down a well and sees it hit the water 4 seconds later. Neglecting air resistance, what is the speed of impact of the stone? (a) 9.8 m/s(b) 19.6 m/s (c) 4.9 m/s (d) 78.4 m/s(e) 39.2 m/s End of Chapter 2