Newtonian Physics
Laws of Motion
Force, Mass and Inertia
O FORCE
O Force is needed to change motion.
O There can be no change in an object’s motion
O
O
O
O
without the presence of a Force.
Forces can increase or decrease the motion of
an object
Forces can cause a change in direction
O Mass
Mass creates Inertia
Objects with greater mass have greater inertia, or
resistance to a change in their motion.
Units of Force
O The NEWTON is the Metric unit used to
express force
O 1 Newton is needed to cause 1 Kg to
speed up by 1 m/sec (Units for math
problems need to be meters and Kg)
O The Newton is smaller than a pound of
force
O 4.448 N to 1 Pound
Net Force
O Forces act together in nature
O Adding the forces that are acting on
an object is very different than just
adding the numbers
O Forces act in certain directions, and
the Net Force is the result
Balanced and Unbalanced
Forces
Newton’s First Law
O Considered the Law of Inertia
O An object in motion will remain in
motion, an object at rest will remain at
rest, unless acted upon by an
unbalanced force.
O Balanced forces result in no
movement/motion
O Inertia: An object’s resistance to a
change in its motion
1. What is the force that will stop
an object that is rolling across the
floor?
2. Describe how to find the Net
Force acting on an object.
3. What are the units for force?
Friction:
O Friction is a force between objects
that are sliding past one another.
O Friction creates the following:
O Heat
O Wear and tear on an object
O Decrease in speed
Newton’s Second Law
O Newton’s first law states force is needed to change an
object’s motion.
O Newton’s Second Law addresses and explains that
change in motion.
O The change in motion is called Acceleration
O Acceleration: The rate of change in an object’s speed.
O Units for Acceleration are m/sec2
Solve these problems
O If a car increases its
speed from rest to
40 miles per hour,
and it takes 5
seconds to do so,
what is its
acceleration?
O If a car changes its
speed from 60 miles
per hour to 20 miles
per hour in 4
seconds, what is its
rate of acceleration?
Try another!!
O A car accelerates at a rate of 5
m/sec/sec for 6 seconds. It
started at a velocity of 10 m/sec,
what is its final velocity?
Force, Mass and
Acceleration
O Newton’s Second law also
describes the relationship
between Acceleration and the
Net Force on that object or
Acceleration and the Mass of
that object.
Relationships
Acceleration is directly/positively
related to Net Force
Acceleration is inversely related
to Mass
O What is the difference
between Acceleration and
Velocity?
Solve the following
O If a 2000kg car has an acceleration
of 15m/sec/sec, what was the
force required to do so?
O A 20 N force was applied to a 100
kg box, what acceleration resulted?
Applying Newton’s Second Law
To use Newton’s second law properly, keep the
following important ideas in mind. They are a
good guideline for how to apply the second law
to physics problems.
1. The net force is what causes acceleration.
2. If there is no acceleration, the net force must
be zero.
3. If there is acceleration, there must also be a
net force.
4. The force unit of Newtons is based on
kilograms, meters, and seconds.
O What are the 2 different
mathematical formulas that are
going to be used when working with
Newton’s Second Law?
O What does Newton’s Second Law
address/explain.
Acceleration and Gravity
Acceleration Due to Gravity
O The acceleration of
objects that are
dropped, is a
constant rate.
O That rate is the
acceleration due to
gravity, (g)
O The value of (g) is 9.8
m/sec/sec
Acceleration Due to Gravity
OLargest Vacuum
Chamber
O If the acceleration of objects in
free fall is the same no matter
what, then why does a feather
take longer to fall than a
baseball?
How Do We Calculate Free
Fall
O The speed is equal to the acceleration
of gravity multiplied by the time that it
took for the object to fall
Solve the
Following:
O What is the speed of a rock,
dropped from a bridge, that
falls for 5 seconds?
Upward Launches
O When you drop an object it will
accelerate at 9.8 m/sec/sec due to
the downward force of gravity.
What do you think happens if you throw
an object up into the air?
The object will decelerate on its way
up, stop for an instant and then
accelerate as it falls to the ground
It will decelerate at a rate of 9.8
m/sec/sec until it reaches the
top
+ Sign means moving upward
- Sign means moving downward
O An object is thrown up into the air and when
it lands its speed was 29.4 m/sec.
1. How long was the object in the air?
2. Set up a chart of the object’s speed from
start to finish.
Draw a picture to help if necessary
Distance With Acceleration
When an object has constant speed, the
distance is easy to calculate: d = vt
The problem is when an object is
accelerating, the speed is changing and
the distance becomes difficult to predict.
The formula to use is:
d=
𝟏
𝟐
𝒂𝒕𝟐
Free Fall Distance
When an object is in free fall, you use
(g) as your acceleration
d=
𝟏
𝟐
𝟐
g𝒕
O \
O Before we go on, solve the following
problem!
A rock falls of a cliff and splashes into
a river 5 seconds later, how far did it
fall?
Air Resistance
O Earlier in this unit we said that forces
do not act alone
O Gravity is NOT the only force acting on
an object that is in “Free Fall”
O Air exerts an an additional force on
the object falling
Air Resistance and Terminal Velocity
O The size and shape of an
object affect the force of
air resistance
O A feather has its weight
spread out over a larger
surface area
O Shape of the object is
also a factor in the ease
of falling objects
O Objects will stop
accelerating at a certain
point and proceed to fall
at a constant speed.
O Terminal velocity is a ratio
of an object’s weight to its
air resistance.
O When the net force
becomes zero, the object
has reached terminal
velocity
O A falling object reaches terminal
velocity when the force of gravity is
balanced by the force of air
resistance.
O Explain this fact as it relates to
Newton’s first and second laws.