Chapter 4 Newton’s Laws and Motion
I.
History of Explaining Motion
A.
Aristotle’s
Ideas
Not all
Correct
Aristotle’s Motion (384-322 B.C.)
1) Aristotle was a Greek “philosopher”
2)
Science by observation, then thinking, not experimentation
His ideas might match some of your initial conceptions of physics
3)
Force = push or pull on an object
a) Must have a force to have motion
b)
Velocity is directly proportional to strength of force
i. Heavy object falls faster than light object
ii. Strength of the force = heaviness of the object
c)
Velocity is inversely proportional to the resistance of the medium (air
water)
4)
Direct and Inverse proportions: how two variables are related
A
a) Directly Proportional: If A increases, B increases
d  vt
B
d v
If velocity increases, distance increases: if t = 2 s find d for v1 = 10 m/s and v2 = 11 m/s
b)
Inversely Proportional: If A increases, B decreases
d
d  vt  v 
t
1
v
t
A
1
B
If time increases, velocity decreases: if d = 30 s find v for t1 = 2 m/s and t2 = 11 m/s
5)
Velocity and Acceleration were the same thing to Aristotle
6)
How does a ball keep going once it
leaves your hand?
a) Motion requires a constant force
b) Air filling a vacuum behind the
ball pushes it forward
a) Reasonable answer, but not
experimentally true
B.
Galileo’s Motion (1564-1642)
1) Aristotle’s explanations were thought of as absolute truth
2) Galileo performed experiments testing these explanations
a) g is the same for all objects
b) Observations favored Sun-centered Solar System
3) Proposed an object in motion stays in motion without a constant force
4) Mathematical Description of Motion
a) Acceleration was different from Velocity
b)
C.
D.
1
d  v0 t  a t 2
2
Newton’s Motion (1642-1727)
1) Theory of motion that matched experiments
a) Worked for all objects: planets or dropped balls
b) No difference between “heavenly” and “earthly” objects
c) Led to predictions not yet observed (Planet Neptune)
2) Three Laws of Motion (rest of the chapter)
3) Universal Gravitation
4) Invented Calculus
Science builds on the work of those before you
II.
Newton’s First and Second Laws
A.
Newton’s First Law of Motion
1) An object’s velocity changes only if acted on by a force
a) An object at rest stays at rest unless acted on by a force
b) An object in motion maintains its velocity unless acted on by a force
2)
Aristotle: thrown ball or ice skater must have constant force
Newton: only needs a force to stop once it is going
3)
What eventually stops a moving object?
a) Friction: force opposing motion due to physical interaction of
surfaces
b) Examples: Air Resistance, Road/Tire interaction
B. Newton’s Second Law of Motion
1) Acceleration is directly proportional to force and inversely proportional to
mass.
1
F
a
2)
3)
4)
5)
m
aF
a
m
Acceleration, not velocity, is the property affected by a force
Force = interaction of one object with another causing acceleration
Inertia = property of objects to resist changes in motion
a) Mass = measure of inertia; how much object resists change in motion
b) Units = Kilogram = kg
Units of Force are called Newtons (N)
a
F
m
kg  m
F  ma  (kg)( m / s ) 
N
2
s
2
C.
Adding Forces
1) Force is a vector quantity: magnitude and direction
2) Often multiple forces act on an object at the same time
a) Total Force = Net Force = sum of the individual forces
b) Friction is a common force we must deal with in finding Net Force
c)
D.
m = 5 kg
a?
The 1st law is a special case of the 2nd law when Force = 0
F 0
a   0
m m
a
v
t
v  a t  (0)t  0
III. Mass and Weight
A.
a
F
m
Comparing Masses
1) Use 2nd Law to define mass
F  ma
m
F2 F1
  m2  m1
a2 a1
F
a
2) Compare masses by comparing
Acceleration caused by equal forces
B.
Weight
1) Use g for comparison of masses
2) Weight = gravitational force acting on an object
F  ma  W  mg  units  N
W  mg  (50kg)(9.8 m/s 2 )  490N
3)
4)
m = 50 kg. What is the weight?
1 lb. = 4.45 N
W  mg  (1kg)(9.8m /s 2 )  9.8N
5)
How many lbs in a kg?
 1 lb 
(9.8N)
  2.2 lbs
 4.45N 
5)
C.
Mass and Weight
1) Mass doesn’t change; it only depends on F and a
2) Weight depends on g, which changes depending on where you are
3) How much does a 110 lb person weigh on the moon? (gm = 1.6 m/s2)
 1kg 
(110 lbs) 
  50kg
 2.2 lbs 
W  mg  (50kg)(1.6 m/s 2 )  80N
Why is gE independent of mass?
1)
2)
 1 lb 
(80N)
  18 lbs
4.45N


F W mg
a 

g
m m
m
Force and acceleration are not the same
a) Heavy object has larger weight
(force) than a light object
a) Heavy and light objects
accelerate at g