motion an introduction. thoughts about motion: a short history
TRANSCRIPT
MOTION
An Introduction
Thoughts about Motion:
A Short History
Aristotle
• Greek philosopher • (384-322 BCE)
Assumptions:
1.Natural laws could be understood by logical reasoning
2.Heavy objects fall faster than light objects
3.Moving objects must have forces exerted on them to keep them moving
Galileo Galilei
• Italian (1564-1642)
Assumptions:
1. Natural laws could be understood by experimentation
2. Objects of different weights fall at the same rate (except air resistance)
Leaning Tower of Pisa
Assumptions (continued)
3. Moving things, once moving, continue in motion without the application of forces
(ignoring friction)
Sir Issac Newton
• English (1642 -1727)
Sir Issac Newton
Assumption:
Newton’s First Law : Inertia
Every object remains at rest or in motion (unless acted upon by an outside force)
Motion : Speed
1. = how fast an object is moving2. speed = distance / time
Units = mi/hr, km/s, m/s, ft/sec, cm/s, in/s
Motion : Speed
• Average speed =
total distance covered time interval
Speed examples
1. It took me 12.8 hours to drive to Vegas. (914 miles)
What was my average speed?
Speed Example 1
Av speed = total distance covered time interval
Speed Example 1
= 914 mi = 71.4 mi / hr 12.8 hr
Speed example 2
2. If I drive at an average speed of 79 mph ( mi / hr) ,
how many miles can I cover in 4.5 hours?
Speed example 2
total distance = (av. speed) (time)
= (79 mi ) (4.5 hr) = (hr)
355.5 miles
Speed example 3
3. How long will it take to drive to Chicago (1000 miles) if your average speed
is 63 mi/hr?
Speed example 3
Time = distance = av. speed
Speed example 3
(1000 mi) =(63 mi/hr)
Speed example 3
15.9 hrs.
SPEED IS RELATIVE
• Everything is moving• Earth is rotating (spinning)• Earth is orbiting around the sun• Galaxy is expanding
SPEED IS RELATIVE
• Motion is measured relative to something1. e.g. Train relative to track 2. Space shuttle relative to
Earth 3. Other examples?
SPEED IS RELATIVE
• How fast is the Earth is moving?
SPEED IS RELATIVE
30 km/secRelative to the sun
• So… you are moving 30 km/sec• The desk is moving 30 km/sec
VELOCITY
• = speed plus a DIRECTION
of motion
v = distance time
VELOCITY Problem
• Two cars are driving in opposite directions.
• Car 1 is going 60 mi/hr.• Car 2 is also going 60 mi/hr.• Do both cars have the same
speed?
VELOCITY Problem 1
• Yes
• Do both cars have the same velocity? Why or why not?
VELOCITY Problem
• No.
• Because they are not traveling in the same direction.
• They have the same speeds, but opposite velocities.
Acceleration
Acceleration
• = (change in velocity)(change in time)
• = ∆v/ ∆ t
• = v2 – v 1
t2 – t 1
Acceleration
• NOTE : Deceleration = negative acceleration• So, stepping on the brake =
− acceleration
Acceleration Problem 1
• A 1965 T-bird with a 390 cubic inch engine can go from rest to 60 mi/hr in 8 seconds.
• What is its acceleration in m/sec2?
Acceleration Problem 1
• What formula to use?
• How about the acceleration formula?
Acceleration Problem 1
• Acceleration = ∆v/ ∆ t
• = v2 – v 1
t2 – t 1
• Right, but we need m/sec, not mi / hr, what to do?
Acceleration Problem 1
SO, we need to convert….
(6x101 mi) (1km) (103 m) (1 hr) (hr) (6.2x 10-1 mi) (1 km) (3.6x103 s)
= 6 x 104 m = 2.6 x 102 m / s
2.23x 102s
Acceleration Problem 1
• But, we need to know m/ s2
• (2.6 x 102 m/s) = 32.5 m/s2 (8 s)