Download 12.2 Newton`s First and Second Laws of Motion

12.2 Newton’s First and Second Laws of Motion
12.2 Key Concepts
• How does Newton’s first law relate change in
motion to a zero net force?
• How does Newton’s second law relate force,
mass, and acceleration?
• How are weight and mass related?
12.2 Newton’s First and Second Laws of Motion
Aristotle _______________
incorrectly
thought that a force was
required to __________
an
keep
object moving
Almost 2000 years later
Galileo
______________
work helped
to correct this misconception
Galileo Galilei
(1564 – 1642)
12.2 Newton’s First and Second Laws of Motion
Aristotle, Galileo, and Newton
Galileo
By rolling balls down wood ramps, Galileo studied
how gravity produces _________________
constant
acceleration.
• He concluded that moving objects NOT subjected to
friction
______________
or any other force would continue
to move ___________________.
indefinitely
12.2 Newton’s First and Second Laws of Motion
Aristotle, Galileo, and Newton
Newton
Newton built on the work of
scientists such as Galileo.
• He published his results in book
Principia
called _________________
• In this book, Newton defined
mass
force
_________
and _____________
and
introduced his laws of motion.
12.2 Newton’s First and Second Laws of Motion
Newton’s First Law of Motion
According to Newton’s first law of motion, the state of
change
not
motion of an object does ______
______________
as long as the net force acting on the object is
zero
________.
remains
• So an object at rest ______________
at rest
continues
• A moving object ________________
moving
inertia
• This law is also called the law of ______________
the tendency of an object to resist a change in
motion
12.2 Newton’s First and Second Laws of Motion
Newton’s First Law of Motion
push
A strong kick or ___________
will overcome
static friction and make the ball move.
Without friction,
the ball would
continue to move
push
friction
Unbalanced forces
change motion
12.2 Newton’s First and Second Laws of Motion
Newton’s First Law of Motion
This crash sequence illustrates inertia. The
forward
test dummy continues its ______________
motion as the car slows and stops.
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
According to Newton’s second law of motion, the
net
acceleration of an object is equal to the ________
_________
acting on it divided by the object’s
force
mass
___________.
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
So the acceleration of an object is
directly
______________
proportional to the net force
acting on it.
harder
Example: The _____________
you throw a ball,
the more it accelerates
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
The acceleration of an object also depends
______________________upon
its mass - measure
inversely
of the ____________
of an object.
inertia
Example: Which will have more acceleration when
you throw it…a baseball or a bowling ball?
A baseball – less mass
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
The acceleration of an object is always in the
_____________
direction as the net force.
same
When a net force acts in the direction
opposite to the object’s motion, the force
deceleration
produces a _____________________
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
An automobile with a mass of 1000 kilograms
accelerates when the traffic light turns green. If
the net force on the car is 4000 newtons, what is
the car’s acceleration?
4000 N
1000 kg
= 4 N/kg
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
Recall that one newton is the force that causes a _______
1 kg
1 m/s
mass to accelerate at a rate of _________
each second
1N=
So 4 N = 4
kg
kg·m
s2
kg
kg·m
s2
=
= m/s2
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
1. A boy pushes forward a cart of groceries with a
total mass of 40.0 kg. What is the acceleration of
the cart if the net force on the cart is 60.0 N?
60. 0 N
40.0 kg
= 1.5 N/kg = 1.5 m/s2
12.2 Newton’s First and Second Laws of Motion
Newton’s Second Law of Motion
2. A 25-N force accelerates a boy in a wheelchair
at 0.5 m/s2. What is the mass of the boy and the
wheelchair? (Hint: Solve Newton’s second law for
mass.)
(M) 0.5 m/s2 = _25 N_ (M)
M
(M) 0.5 m/s2 = 25 N
0.5 m/s2
0.5 m/s2 =
M = 50 kg
12.2 Newton’s First and Second Laws of Motion
Weight and Mass
Mass and weight are related but are NOT the
same.
amount of
• Mass is the measure of the ____________
material an object contains.
acting
• Weight is the force of gravity _____________
on the mass of an object.
12.2 Newton’s First and Second Laws of Motion
Weight and Mass
If an astronaut has a mass of 112 kilograms,
what is his weight on Earth where the
acceleration due to gravity is 9.8 m/s2?
Weight = 112 kg x 9.8 m/s2
= 1098 kg·m/s2
= 1098 N
12.2 Newton’s First and Second Laws of Motion
Weight and Mass
On the moon, the acceleration due to gravity
1/6
is only about ___________
that on Earth.
Astronaut on Earth
Mass = 88.0 kg, Weight = 863 N
Astronaut on Moon
Mass = 88.0 kg, Weight = 141 N
12.2 Newton’s First and Second Laws of Motion
Assessment Questions
1. What is inertia?
a.
b.
c.
d.
the force of gravity acting on an object
forces of friction slowing an object’s motion
the mass of an object
the tendency of an object to resist change in its motion
12.2 Newton’s First and Second Laws of Motion
Assessment Questions
2. A 3600-N force causes a car to accelerate at a
rate of 4 m/s2. What is the mass of the car?
a.
b.
c.
d.
600 kg
900 kg
14,400 kg
1200 kg
12.2 Newton’s First and Second Laws of Motion
Assessment Questions
3. How would your mass and weight change if you
were on the moon’s surface?
a. They wouldn’t change.
b. Your mass would remain constant, and your weight
would increase.
c. Your mass and weight would decrease.
d. Your mass would remain constant, and your weight
would decrease.