Chapter 4 Newton’s Laws and Motion

I. History of Explaining Motion

A. 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)

Aristotle’s IdeasNot allCorrect

4) Direct and Inverse proportions: how two variables are related

a) Directly Proportional: If A increases, B increases

b) Inversely Proportional: If A increases, B decreases

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

BA

BA

1

tvd vd

t

dvtvd

tv

1

If velocity increases, distance increases: if t = 2 s find d for v1 = 10 m/s and v2 = 11 m/s

If time increases, velocity decreases: if d = 30 s find v for t1 = 2 m/s and t2 = 11 m/s

B. Galileo’s Motion (1564-1642)1) Aristotle’s explanations were thought of as absolute truth2) Galileo performed experiments testing these explanations

a) g is the same for all objectsb) Observations favored Sun-centered Solar System

3) Proposed an object in motion stays in motion without a constant force4) Mathematical Description of Motion

a) Acceleration was different from Velocity

b)

C. Newton’s Motion (1642-1727)1) Theory of motion that matched experiments

a) Worked for all objects: planets or dropped ballsb) No difference between “heavenly” and “earthly” objectsc) Led to predictions not yet observed (Planet Neptune)

2) Three Laws of Motion (rest of the chapter)3) Universal Gravitation4) Invented Calculus

D. Science builds on the work of those before you

20 2

1tatvd

II. Newton’s First and Second LawsA. 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.

2) Acceleration, not velocity, is the property affected by a force

3) Force = interaction of one object with another causing acceleration

4) 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

5) Units of Force are called Newtons (N)

m

Fa Fa

ma

1

Ns

mkgsmkgamF

22 )/)((

m

Fa

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) m = 5 kg a ?

D. The 1st law is a special case of the 2nd law when Force = 0

00

mm

Fa

t

va

0)0( ttav

III. Mass and WeightA. Comparing Masses

1) Use 2nd Law to define mass

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

3) m = 50 kg. What is the weight?

4) 1 lb. = 4.45 N

5) How many lbs in a kg?

m

Fa

a

FmamF

NunitsmgWamF

121

1

2

2 mma

F

a

F

490N)m/s(50kg)(9.8mgW 2

lbs .224.45N

lb 1(9.8N)

N8.9)/s(1kg)(9.8mmgW 2

5) 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)

C. Why is gE independent of mass?

1)

2) 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

gm

mg

m

W

m

Fa

lbs 184.45N

lb 1(80N)

80N)m/s(50kg)(1.6mgW

50kglbs 2.2

1kglbs) 110(

2