Download 12.2 Newton`s 1st and 2nd Laws of Motion

Name: _______________________________ Class: ____
12.2 Newton’s 1st & 2nd Laws of Motion
Chapter 12
Forces and Motion
What are we going to learn?
Newton’s Laws of Motion
(Imagine a toy car with an unanchored load
– what will change its motion?)
What is a Scientific Law?
A statement of what will happen in a given set
of circumstances.
FYI: Newton developed the Law of Universal
Gravitation in addition to the Laws of Motion
Newton’s First Law of Motion
An object at rest will remain at rest unless
acted upon by a nonzero net force
An object moving at a constant velocity will
continue moving at a constant velocity unless
acted upon by a nonzero net force
Inertia
The tendency of an object (moving or not) to
resist a change in its motion
Newton’s 1st Law is also known as the Law of
Inertia
Ex: You move forward in your seat when your
car stops suddenly
Relate inertia to safety bars on a roller coaster
_______________________________________
Inertia
Relate inertia to safety bars on a roller coaster
They keep you in your seat when changes in the
coaster’s motion moves you out of your seat
Mass and Inertia
The greater the mass of an object, the greater
its inertia...and the greater the force required to
change its motion
Use this information to explain why you feel
tossed around whenever a roller coaster goes
over a hill or through a loop...
Use this information to explain why you feel
tossed around whenever a roller coaster goes
over a hill or through a loop
Because of inertia, you resist changes in
motion. When the coaster accelerates over a
hill, inertia keeps you moving at a constant
velocity. You feel tossed
around.
Example
What keeps a ball rolling down a hill?
Example
What keeps a ball rolling down a hill?
Gravity and Inertia
What force is acting on the ball, resisting
motion?
What force is acting on the ball, resisting
motion?
Friction between the ball and the ground
If you put a ball on the floor and roll it across
the floor so it hits the wall, why did the ball
stop moving in the direction it was rolled?
If you put a ball on the floor and roll it across
the floor so it hits the wall, why did the ball
stop moving in the direction it was rolled?
It rolled into the wall, so it could not keep
going in the direction it was rolled
Now I get it!
Now I know that Newton’s first law of motion
states that _____________________________
Now I get it!
Now I know that Newton’s first law of motion
states that objects only change motion when a
nonzero net force acts on them
Newton’s Second Law of Motion
An object’s acceleration depends on its mass
and on the net force acting on it
Ex: Is a full or empty shopping cart easier to
push?
Ex: Who can cause a greater acceleration on
the cart, a small child or grown adult?
Newton’s Second Law of Motion
Ex: Is a full or empty shopping cart easier to
push? Empty cart – less mass
Ex: Who can cause a greater acceleration on
the cart, a small child or grown adult? Grown
adult – more force
Formula
The acceleration of an object is equal to the
net force acting on it divided by the object’s
mass
Acceleration = Net force
Mass
or
Net force = Mass x Acceleration
Mass x Acceleration = Net force x Mass
Mass
which gives
Mass x Acceleration = Net force
when flipped gives
Net force = Mass x Acceleration
Changes in Force and Mass
If you increase the force on a cart without
changing its mass, the acceleration will
increase
If something falls out (decrease mass), it will
accelerate faster if you keep pushing just as
hard (same force)
If you want to increase the acceleration of a
bicycle, what must you increase?
If you want to increase the acceleration of a
bicycle, what must you increase?
The force you apply to the pedals
Why are racing bicycles made of lightweight
metals like titanium?
Why are racing bicycles made of lightweight
metals like titanium?
The less mass a bicycle has, the greater the
acceleration the rider can produce
Suppose 4 dogs pull a sled carrying 2 people.
Change the dog/person arrangement to change
the sled’s acceleration in the 2 questions.
Suppose 4 dogs pull a sled carrying 2 people.
Change the dog/person arrangement to change
the sled’s acceleration in the 2 questions.
Which part of the formula would you change if
you added dogs to the team?
Which part of the formula would you change if
you added dogs to the team?
Net force
How does changing the top number of a
fraction affect the value of the answer?
How does changing the top number of a
fraction affect the value of the answer?
It increases the value
Write an “if-then” sentence to show the effect of
adding more dogs
Write an “if-then” sentence to show the effect of
adding more dogs
If you add dogs to the team, then you increase
the acceleration
What else could be done to increase
acceleration?
What else could be done to increase
acceleration?
By removing some people from the sled or a
combination of adding dogs and removing
people
Remember:
Mass units are kg
Acceleration units are m/s2
Force units are N (kg · m/s2)
Force = Mass x Acceleration
2 kg x 2 m/s2 = 4N
It would be cut in half to become 1 m/s2
I get it! Now I know that Newton’s second law
of motion describes the relationship ________
____________________________________
I get it! Now I know that Newton’s second law
of motion describes the relationship among
force, mass and acceleration