Title: Lecture 3 Newton
1Lecture 3Newtons Three Laws of Motion
2Origins of Modern Astronomy
- Sir Isaac Newton (1642-1727)
- Formulated the laws of motion and gravitation
that govern all bodies in the universe.
3Newtons First Law of Motion
- The Law of Inertia
- Every object continues in a state of rest or of
uniform speed in a straight line unless acted on
by a nonzero force. - Inertia The property of objects to resist
changes in motion.
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6The Earth in Motion
- Nicolaus Copernicus (1473-1543)
- Proposed that the Earth revolved around the Sun
from observations of the motion of planets. - Because the concept of inertia was unknown at his
time, the idea of a moving Earth was difficult to
comprehend.
7If the Earth moves at 30 km/s, how can the bird
drop down and catch the worm?
8Thanks to inertia, you can flip a coin in an
airplane without having it fly into your face at
500 mph.
9Newtons Second Law of Motion
- The acceleration produced by a net force on an
object - is directly proportional to the net force,
- is in the same direction as the net force,
- and is inversely proportional to the mass of the
object.
10Newtons Second Law of Motion
Acceleration net force / mass a F / m
11Newtons Second Law of Motion
- Net Force mass x acceleration
F ma Units 1 N 1 kgm/s2
12Newtons Second Law of Motion
F ma
F
F
F
m
a
a
m
a
m
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14You apply the same amount of force on two
separate carts one cart with mass of 1 kg and
another with a mass of 2 kg. Which of the
following is correct?
- The acceleration of the 2 kg cart will be ½ as
much as that of the 1 kg cart. - The acceleration of the 2 kg cart will be 2 times
greater than that of the 1 kg cart. - The acceleration will be the same for both carts.
- The acceleration of the 2 kg cart will be ¼ as
much as that of the 1 kg cart.
15A jumbo jet cruises at a constant velocity of
1000 km/h when the thrusting force of its engines
are a constant 100,000 N. What is the force of
air resistance on the jet?
- 0 N
- 100,000 N
- 1,000 N
- There is not enough information to answer this
question.
16How much force, or thrust, must a 20,000-kg jet
plane develop to achieve an acceleration of 2
m/s2?
- 10,000 N
- 10,000 m/s2
- 20,000 N
- 20,000 m/s2
- 40,000 N
- 40,000 m/s2
17Homework Assignment 2
- A few rules to remember
- At rest or constant velocity no change in
motion - No change in motion no acceleration No Net
Force (SF 0)
18A constant g on Earth
- Galileo was the first to measure the acceleration
of objects in free fall, but could not explain
why they all fall equally. - Greater the mass stronger gravitational pull.
19A constant g on Earth
20A constant g on Earth
- g (10m/s2) is independent of an objects mass.
21In a vacuum, a coin and a feather fall equally,
side by side. Would it be correct to say that
equal forces of gravity act on both the coin and
the feather in a vacuum?
- Yes
- No
22A constant g on Earth
23Weight
- Calculating Weight using Newtons Second Law
- F ma
- Weight mg
- g acceleration due to gravity on Earth
24Falling Objects and Air Resistance
- On Earth, air-resistance must be considered for
falling objects. - As falling speed increases so does the opposing
force of air-resistance. - Net force (SF) Weight Air-resistance
25Falling Objects and Air Resistance
- Acceleration of falling object calculated using
Newtons 2nd Law
a SF / m a (mg - R )/ m R force due to Air
Resistance mg weight
26Terminal Velocity
- Terminal velocity reached when the force of
air-resistance the falling objects weight. - No net force (SF 0) no acceleration no
change in velocity
a SF / m 0 a (mg - R )/ m 0
27Terminal Velocity
- Varies from 150 to 200 km/h for a human skydiver.
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29A bowling ball and a feather are dropped from the
same height at the same time. Which reaches
terminal velocity first?
- Bowling Ball
- Feather
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31A bowling ball and a feather are dropped from the
same height at the same time. Which has the
greater terminal velocity?
- Bowling Ball
- Feather
32Terminal Velocity
- Greater force of air resistance (R) needed to
cancel out the weight (mg) of heavier objects in
free fall - Greater R requires a greater velocity which
requires acceleration for a longer period of time.
33Effect of air-resistance on falling objects
Initially velocity is 0 Air-resistance is 0
Velocity has increased Air-resistance increases
Velocity continues to increase Air-resistance
increases
R 80 N
R 40 N
R 0 N
Weight 100 N
Weight 100 N
Weight 100 N
Net Force 100 N Initial Acceleration is 10m/s2
Net Force 60 N Acceleration is less due to
smaller net force
Net Force 20 N Acceleration has decreased more
34Effect of air-resistance on falling objects
Velocity no longer changes (Terminal
Velocity) Air-resistance is 100 N
R 100 N
No net force no acceleration no change in
velocity!
Weight 100 N
Net Force 0 N Acceleration 0 m/s2
35A bowling ball and a feather are dropped from the
same height at the same time. Which would strike
the ground first if it were on the Moon?
- Bowling Ball
- Feather
- Both at the same time
36Forces and Interactions
- A force is not a thing in itself but makes up an
interaction between one thing and another. - Force Pair two forces that are equal in
magnitude and opposite in direction. - Constitutes a single interaction.
37Forces and Interactions
- You can only exert as much force on an object as
it can exert back on you.
38Forces and Interactions
39Newtons Third Law of Motion
- Whenever one object exerts a force on a second
object, the second object exerts an equal and
opposite force on the first. - Action force and reaction force
40To every action there is always an opposed equal
reaction
41Action and Reaction
- Earth is pulled up by the boulder with just as
much force as the boulder is pulled down by
Earth. - Forces are equal in magnitude but what about the
acceleration of the two objects?
42A speeding bus and an insect experience a head-on
collision. The force of the bus on the insect
splatters it on the windshield. Is the
corresponding force of the insect on the bus
greater, less, or the same?
- Greater
- Less
- The same
43What about the resulting acceleration that the
bug experiences?
- Greater than the acceleration of the bus.
- Less than the acceleration of the bus.
- The accelerations are the same.
44Equal and opposite forces does not always mean
equal and opposite accelerations.
45Action and Reaction
a 20 m/s2
a 500 m/s2
F 10,000 N
F 10,000 N
Cannonball 20 kg Cannon 500 kg
46Chuck Norris delivers a roundhouse kick with a
force of 8,000 N to an opponent. Assuming that
the laws of physics apply to Chuck Norris, how
much force is exerted back on his foot?
- Less than 8,000 N
- More than 8,000 N
- 8,000 N
- Im too intimidated by Chuck Norris to answer
this question.
47Assume that Chuck Norris has a mass of 100 kg and
his opponent has a mass of 80 kg. The force
exerted on each was 8,000 N in the previous
question. What is the acceleration of his
opponent during impact?
- 8,000 m/s2
- 20 m/s2
- 100 m/s2
- 80 m/s2
- 640,000 m/s2
48Defining a System
- If action and reaction forces on an object are
equal and opposite, then how can an object
accelerate? - An acceleration of a system is only possible if a
force external to the system is involved.
49Flight
- Lift an upward reaction force that allows for
flight. - When the force of lift exceeds an objects weight
it will accelerate upward. - A helicopters whirling blades are shaped to
force air downward and the air forces the blades
up.
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51A birds wing pushes down on the air and the air
pushes back on the wing.