Download Newton`s 2 Law

Newton’s
nd
2
Law
Chapter 11 – Section 2
Pages 316 – 322 in textbook
Newton’s
nd
2
Law of Motion
DESCRIBES….
The
net force on an object is equal to the
product of its acceleration and its mass.
One Equation….
The relationship between the quantities of force,
mass, and acceleration can be written in one
equation:
Force = Mass x Acceleration
F = ma
Compare and Contrast
Which shopping cart will be easier to push/move?
How do the shopping carts relate to Newton’s 2nd Law?
Answer: The empty shopping cart has less mass than
the full shopping cart, therefore it will need a smaller
force to accelerate it.
Units: F = ma
Mass units example: kg
Acceleration units example: m/sec/sec
Force units: kg * m/sec/sec ???? What a mess!
Force is measured in units called the Newton
(or N for short!)
1N = 1 kg x 1 m / sec / sec
This equation says that one Newton equals the force
required to accelerate one kilogram of mass at 1 meter
per second per second!
Sample Problem #1
A bicycle has a mass of 40.0 kilograms. Suppose
the bike accelerates at 1.0 m/s/s. Find the net
force being exerted on the bicycle.
Step 1:
F = ma
Step 2:
F = 40.0 kg x 1.0 m/s/s
Step 3:
F = 40.0 N
Sample Problem #2
A speedboat is pulling a
52,000 gram skier. The force
causes her to accelerate at 2.0
m/s/s. Calculate the force.
F = ma
F = 52.0 kg x 2.0 m/s/s
F = 104.0 kg m/s/s OR
F = 104.0 N
F
M
A
Sample Problem #3
An object with a mass of 50 kg is
pushed with a force of 150
Newtons. What is the objects
acceleration?
A = F/M
A = 150 kg m/s/s / 50 kg
A = 3 m/s/s or 3 m/s2
F
M
A
Sample Problem #4
What is the mass of an object that
accelerates at a rate of 5 m/s2 after
a force of 100 N is applied to it?
M = F/A
M = 100 kg * m/s/s / 5 m/s/s
M = 20 kg
F
M
A
GRAVITY!
The force that pulls object’s to the center of the
earth is known as gravity.
Gravity attracts objects towards each other.
Gravitational force decreases as:
Distance increase between objects
Masses of objects decrease.
Law of Universal Gravitation
Gravity is everywhere in the universe!
The Law of Universal Gravitation states that
the force of gravity acts between all objects
in the universe!
Acceleration due to GRAVITY
Acceleration on the Earth due to gravity is
9.8 meters/second/second
This means:
For every second an object falls, it’s velocity
increases by 9.8 m/s!
Let’s figure it out…….
A parachutist jumps from a
plane. What is her acceleration
due to gravity? Complete the
below chart.
Time
1.0 second
2.0 seconds
3.0 seconds
4.0 seconds
Velocity
9.8 meters/second
19.6 meters/second
29.4
39.2
meters/second
meters/second
Weight
The weight of an object is the size of the
gravitational force exerted on an object.
Since weight is a force, you can rewrite
Newton’s second law of motion as W=ma.
Since acceleration due to gravity = 9.8 m/s2,
you can rewrite the formula for weight as:
W = m * 9.8 m/s2
Weight
How much would a 50 kg person weigh on the
surface of the earth?
Force = Mass * Acceleration (due to gravity)
Force (Weight) = 50 kg * 9.8 m/s2
Force (Weight) = 490 kg * m/s2
Force (Weight) = 490 Newtons
Who Wins? Acorn or Leaf?
Why does an oak leaf flutter
straight to the ground yet an acorn
will fall straight down?
Answer – AIR RESISTANCE
Air Resistance
Air resistance is a form of friction that acts to
slow down any object moving in the air.
Air resistance increases as:
1.
2.
The Surface area of the object increases
The velocity of the object increases
When the air resistance balances the force of
gravity, the net force is zero, also called terminal
velocity.
In a vacuum, where there is no air, all objects
fall with the same acceleration!