An introduction
to Newton’s 2nd
Law of Motion
This work by Kristin Lee is licensed under a Creative Commons Attribution 3.0 Unported License.
What do we remember about
Newton’s Laws of Motion?
• List as many items on a ½ sheet of paper as
you can related to Newton’s Laws of Motion.
• Ideas to discuss:
State laws, examples, real life applications, how
terms are related
You will need a white board for today’s class!
Newton’s First Law
“An object at rest tends to stay at rest
and an object in motion tends to stay
in motion unless acted upon by an
unbalanced force.”
Essential Questions
• What is Newton’s Second Law
& how does it apply to
motion?
An introduction
to Newton’s 2nd
Law of Motion
This work by Kristin Lee is licensed under a Creative Commons Attribution 3.0 Unported License.
What is Acceleration?
Acceleration can be a
very tricky term in
science.
I am sure that you
have heard of a car
accelerating, but
what does that really
mean?
What is Acceleration?
• Acceleration is the rate at which velocity (speed of motion)
changes.
• In Science, acceleration can mean:
Changing
Directions
True or False?
• According to the scientific definition, which of these
statements is true?
– When you step on the gas, the car is accelerating.
• TRUE!
– When you step on the brake, the car is accelerating.
• TRUE!
– When you change lanes but don’t change speed, the car
is accelerating.
• TRUE!
• Acceleration happens whenever something
changes SPEED or DIRECTION! {record on notes sheet}
Acceleration: Changing Direction
• An object can be
accelerating even if
its speed is constant.
• This is possible if it
changes direction
while moving at a
constant speed.
• Changing lanes is an
example of
acceleration?
• Really?
• Yes, let me show you
using my accelerometer.
More Laws of Motion
• Newton’s First Law of Motion says that if a force
acts on an object, the object will move.
• Newton’s Second Law of Motion says that when
a force acts on an object, the object accelerates
{speeds up, slows down, or changes
direction}…meaning the movement can be faster, slower, or
in a different direction!
What Factors Affect
Acceleration?
• Now, let’s see this firsthand.
• Take the blue cube and turn it on its side. Gently
give it a push.
• Now move the cube back to its starting location.
Give it a harder push.
• What factor affects the cube’s acceleration?
FORCE
NEWTON’S SECOND LAW
The more force that is applied,
the more the object will
accelerate.
What Factors Affect
Acceleration?
• Take the blue cube and turn it on its side.
• Take the black cube and place it next to the blue
cube.
• Push both cubes with the same force. Which cube
moved further?
• What factor affects the cube’s acceleration?
MASS
Newton’s Second Law of Motion
• A long row of shopping carts takes more force
to move than one empty cart requires.
Takes more force to move
Let’s Explore This Idea Further:
What factors are involved in determining the
acceleration of an object?
Force and Mass
Let’s Review:
Now think carefully about force and mass. How
do you get good acceleration?
or
mass gives you good acceleration?
or
force gives you good acceleration?
Let’s Explore This Idea Further:
Now think carefully about force and mass. How
do you get good acceleration?
mass gives you good acceleration
force gives you good acceleration?
Newton’s 2nd Law of Motion
• Please record the following on the bottom of
your notes sheet:
“The acceleration of an object depends on the
force acting on it and the mass of the object.”
So what exactly does that mean?
• Applying a larger net force to an object will
result in __________ acceleration.
So what exactly does that mean?
• If you apply equal net forces to two objects with
different masses, the one with the smaller mass
will accelerate _________.
So what exactly does that mean?
• If you want 2 objects with different masses to
have the same acceleration, the object with
the larger mass will require ________ force.
Newton’s Second Law
“The acceleration of an object depends on the force acting on it and the mass of
the object.”
Acceleration = force/mass
Force equals mass times acceleration
Force = mass x acceleration
Can we write a formula?
• If we have a ball that is twice as big (double the
mass} but the acceleration is the same, how
much force do you need to make the ball go?
• You would need double the force to move
the larger ball.
Force, mass, and acceleration are proportional…
Force = mass x acceleration
8 = 2 x 4 smaller mass
16 = 4 x 4 larger mass
What does F = ma say?
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a baseball, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
Let’s Review
• What is the effect of a car’s
acceleration if six people ride
in the car instead of one?
Acceleration…
• Remember our question from the beginning
of class?
If someone told you that a car was accelerating,
what would that mean to you?
• What would you say now?