Chapter 4 Linear Motion

You can describe the motion of an object by its position, speed, direction, and acceleration.

I. Motion Is Relative A. Everything moves. Even things that appear to be at rest move. 1. Motion is described by motion relative to something else. a. Relative to the sun, the center of the galaxy, etc. b. We will discus motion (things in our environment) relative to the surface of the Earth.

II. Speed A. Speed is measure of how fast something is moving (rate at which distance is covered) 1. Rate term used to describe something divided by time. 2. Speed = Units of distance / units of time (distance covered per unit of time)

3. Common Units a. miles/hour (mi/h) b. kilometers/hour (km/h) c. meters/second (m/s) Used in physics

Continental drift: 2 mm/year (1mm = one millimeter = 10-3 meters) Walking speed: 3 miles/hour = 4.4 feet/second = 1.3 m/s (1m = 1 meter, s = seconds) Driving speed: 60 miles/hr = 100 km/hr = 88 ft/s = 26 m/s Orbital speed (near Earth orbit) = 8 km/s = 17,000 miles/hr Earth's speed around the sun = 30 km/s Speed of light = 300,000 km/s = 186,000 miles/s.

B. Instantaneous Speed the speed at any instant (What you see on a car s speedometer)

C. Average Speed total distance covered/time interval 1. Does not indicate variations in speed over time. 2. still describes rate at which distance traveled Average speed = Total distance covered Time interval v d t d t 1 1 d t 0 0

Speed think! If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 meters every second. At this rate, how far will it travel in 10 seconds? In 1 minute?

Speed think! If a cheetah can maintain a constant speed of 25 m/s, it will cover 25 meters every second. At this rate, how far will it travel in 10 seconds? In 1 minute? Answer: In 10 s the cheetah will cover 250 m, and in 1 min (or 60 s) it will cover 1500 m.

Speed think! The speedometer in every car also has an odometer that records the distance traveled. If the odometer reads zero at the beginning of a trip and 35 km a half hour later, what is the average speed?

D. Velocity 1. Velocity and speed are often used interchangeably, but in physics are different. a. Velocity is speed in a given direction. b. Speed is how fast object moves (direction does not matter)

E. Constant Velocity must have constant speed and direction 1. Object moves in straight line 2. Object s path does not curve

F. Changing Velocity 1. Velocity will change if either speed or direction changes. 2. Constant speed and constant velocity are not the same.

Velocity think! The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h. Do both cars have the same speed? Do they have the same velocity?

Velocity think! The speedometer of a car moving northward reads 60 km/h. It passes another car that travels southward at 60 km/h. Do both cars have the same speed? Do they have the same velocity? Answer: Both cars have the same speed, but they have opposite velocities because they are moving in opposite directions.

III. Acceleration (4.4) A. acceleration is the rate at which the velocity is changing 1. applies to increases as well as decreases in velocity. 2. decrease in velocity often called deceleration or negative acceleration Acceleration = Change of velocity Time interval a v t v t 1 1 v t 0 0

B. Acceleration applies to changes in direction as well as speed 1. When motion is in straight line the term speed and velocity are often used interchangeably.

1. When motion is in straight line the term speed and velocity are often used interchangeably. Acceleration along a straight line = Change of speed Time interval

2. Units for acceleration a bit more complicated Acceleration = 10m / 1s m s Change of speed 10 Time interval 2 s

Acceleration Accelerate in the direction of velocity speed up

4.4 Acceleration Accelerate in the direction of velocity speed up Accelerate against velocity slow down

4.4 Acceleration Accelerate in the direction of velocity speed up Accelerate against velocity slow down Accelerate at an angle to velocity change direction

Acceleration think! Suppose a car moving in a straight line steadily increases its speed each second, first from 35 to 40 km/h, then from 40 to 45 km/h, then from 45 to 50 km/h. What is its acceleration? Answer: The speed increases by 5 km/h during each 1-s interval in a straight line. The acceleration is therefore 5 km/h s during each interval.

Acceleration think! In 5 seconds a car moving in a straight line increases its speed from 50 km/h to 65 km/h, while a truck goes from rest to 15 km/h in a straight line. Which undergoes greater acceleration? What is the acceleration of each vehicle? Answer: The car and truck both increase their speed by 15 km/h during the same time interval, so their acceleration is the same.

IV. Free Fall: How Fast A. The force of Gravity causes object to accelerate downward 1. If we disregard air resistance (air friction) then free falling objects only affected by gravity. Called free fall 2. Use letter (g) to represent gravity 3. gravity varies slightly around the Earth. Average value is about 10 m/s 2 4. More accurately, g is 9.8 m/s 2

B. The instantaneous speed of an object falling from rest is equal to the acceleration multiplied by the amount of time it falls. Instantaneous speed = acceleration x elapsed time (v symbolizes both speed and velocity) v gt

1. speed decreases at the same rate with an object moving upwards as it increases when moving downward 2. An object thrown upward will reach a velocity of zero when it gets to its highest point

V. Free Fall: How Far A. Relationship between distance traveled, acceleration, and velocity 1 v gt d gt 2 2 Rearrange and solve for t t 2d g

Free Fall: How Fast think! During the span of the second time interval in Table 4.2, the object begins at 10 m/s and ends at 20 m/s. What is the average speed of the object during this 1- second interval? What is its acceleration?

Free Fall: How Fast think! What would the speedometer reading on the falling rock be 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped?

Free Fall: How Fast think! What would the speedometer reading on the falling rock be 4.5 seconds after it drops from rest? How about 8 seconds after it is dropped? Answer: The speedometer readings would be 45 m/s and 80 m/s, respectively.

VI. Graphs of Motion A. Equations and tables not the only way to describe relationships such as velocity and acceleration. 1. Linear relationship- e.g. speed and time a. Forms straight line curve. b. Has constant slope (direct proportion)

2. Parabolic relationship e.g. distance versus time a. Not straight line. Curved line b. Tangent at any point gives slope at that point (slope of this curve is instantaneous speed. Remember that slope is rise/run or change in y over change in x. Distance/time = speed

VII. Air Resistance and Falling Objects A. Air resistance noticeably alters the motion of things (like feathers, paper, etc.) B. Less effect on more dense (compact) objects C. Air resistance is small enough to be neglected in most cases.

VIII. How Fast, How Far, How Quickly. How Fast Changes A. speed and velocity-used to describe how fast something free falls from rest. equation to use: v gt v v 0 gt

B. To specify how far the object has fallen we are talking about distance. equation to use: d 1 gt 2 2

What is this graph telling us? Change in velocity over time = acceleration

C. Acceleration how quickly does speed or velocity change 1. Very complex concept 2. rate of a rate

Constant Positive Velocity

Constant Negative Velocity

Positive Velocity and Positive Acceleration

Positive Velocity and Negative Acceleration

Motion Diagram

Motion Diagram Standing still Moving constant speed Speeding up Slowing down

Particle Model Replace object with single point Simplified version of motion diagram

Particle Model Standing still Moving constant speed Speeding up Slowing down

Assessment Questions 1. Jake walks east through a passenger car on a train that moves 10 m/s in the same direction. Jake s speed relative to the car is 2 m/s. Jake s speed relative to an observer at rest outside the train is a. 2 m/s. b. 5 m/s. c. 8 m/s. d. 12 m/s.

Assessment Questions 2. A gazelle travels 2 km in a half hour. The gazelle s average speed is a. 1/2 km/h. b. 1 km/h. c. 2 km/h. d. 4 km/h.

Assessment Questions 2. A gazelle travels 2 km in a half hour. The gazelle s average speed is a. 1/2 km/h. b. 1 km/h. c. 2 km/h. d. 4 km/h. Answer: D

Assessment Questions 3. Constant speed in a constant direction is a. constant velocity. b. constant acceleration. c. instantaneous speed. d. average velocity.

Assessment Questions 3. Constant speed in a constant direction is a. constant velocity. b. constant acceleration. c. instantaneous speed. d. average velocity. Answer: A

Assessment Questions 4. A vehicle undergoes acceleration when it a. gains speed. b. decreases speed. c. changes direction. d. all of the above

Assessment Questions 4. A vehicle undergoes acceleration when it a. gains speed. b. decreases speed. c. changes direction. d. all of the above Answer: D

Assessment Questions 5. If a falling object gains 10 m/s each second it falls, its acceleration can be expressed as a. 10 m/s/s. b. 10 m/s 2. c. v = gt. d. both A and B.

Assessment Questions 5. If a falling object gains 10 m/s each second it falls, its acceleration can be expressed as a. 10 m/s/s. b. 10 m/s 2. c. v = gt. d. both A and B. Answer: D

Assessment Questions 6. A rock falls 180 m from a cliff into the ocean. How long is it in free fall? a. 6 s b. 10 s c. 18 s d. 180 s

Assessment Questions 6. A rock falls 180 m from a cliff into the ocean. How long is it in free fall? a. 6 s b. 10 s c. 18 s d. 180 s Answer: A

Assessment Questions 7. The slope of a speed-versus-time graph represents a. distance traveled. b. velocity. c. acceleration. d. air resistance.

Assessment Questions 7. The slope of a speed-versus-time graph represents a. distance traveled. b. velocity. c. acceleration. d. air resistance. Answer: C

Assessment Questions 8. In a vacuum tube, a feather is seen to fall as fast as a coin. This is because a. gravity doesn t act in a vacuum. b. air resistance doesn t act in a vacuum. c. greater air resistance acts on the coin. d. gravity is greater in a vacuum.

Assessment Questions 9. Speed and acceleration are actually a. one and the same concept, but expressed differently. b. rates of one another. c. entirely different concepts. d. expressions of distance traveled.