EQ: What is the
relationship
between force and
acceleration?
Pg. 19
Newton’s First Law
(law of inertia)
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.
http://www.sciencechannel.com/games-and-interactives/newtons-laws-of-motion-interactive.htm
Newton’s First Law
(law of inertia)
• INERTIA is a property of an
object that describes how
much
______________________
it will resist change to the
motion of the object
• more _____
____
mass means more inertia
Balanced Force
Equal forces in opposite
directions cancel out.
Object in motion will stay in motion.
Object at rest will stay at rest.
Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing change in motion
(acceleration- change in velocity)
If objects in motion tend to stay in motion,
why don’t moving objects keep moving
forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.
1st Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.
1st Law
• Once airborne,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction) it
would never
Inertia
Closing Task:
Clearing up those misconceptions with a
shoulder partner
1. A force is needed to continue the objects
motion
2. There are no forces in outer space
3. Acceleration is velocity or acceleration is
speeding up
Closing Task:
Clearing up those misconceptions with a
shoulder partner
1. Misconception - A force is needed to continue the objects motion
An object can fly through space with a constant velocity as long as no
external forces act on it (unbalanced force).
2. Misconception – There are no forces in spaces
A spaceship can be accelerated if acted upon an external force
(example: a meteor hits the ship, fuel gives ship more power to
accelerate)
3. Misconception – Acceleration is velocity or acceleration is speeding up
Acceleration is a change in velocity (speeding up, slowing down,
stopping, changing direction)
Newton’s Second Law
The acceleration of an object as produced by a net
force is directly proportional to the magnitude of
the net force, in the same direction as the net
force, and inversely proportional to the mass of the
object.
Fnet = m • a
http://www.sciencechannel.com/games-and-interactives/newtons-laws-of-motion-interactive.htm
Newton’s Second Law
Fnet = m • a
Quantity
Variable
Unit
Force
F
Newtons (N)
Mass
m
Kilograms (kg)
Acceleration
a
meters/second2 (m/s2)
Newton’s Second Law
gravity on the mass of an object
• WEIGHT is a measure of the force of ________
Newtons
measured in __________
Calculate your weight
Mass = pounds/2.2
Ex:
Remember, acceleration of
gravity, a=g= 9.80 m/s2
130 lbs/2.2 kg = 59 kg
F = ma
F = (59 kg)(9.80 m/s2)
F = 578 ~ 580 N
Newton’s Second Law
A house is lifted from its foundations
onto a truck for relocation. The house
is pulled upward by a net force of 2850
N. This force causes the house to
move from rest to an upward speed of
0.15 m/s in 5.0 s. What is the mass of
the house?
Newton’s Third Law
For every action there is an equal and
opposite reaction.
Newton’s Third Law
• A bug with a mass of 5 grams
flies into the windshield of a
moving 1000kg bus.
• Which will have the most
force?
• The bug on the bus
• The bus on the bug
Newton’s Third Law
• The force would be the same.
• Force (bug)= m x A
• Force (bus)= M x a
Think I look bad?
You should see
the other guy!
If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.
3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.
Consider hitting a baseball with a bat. If
we call the force applied to the ball by the
bat the action force, identify the reaction
force.
(a) the force applied to the bat by the hands
(b) the force applied to the bat by the ball
(c) the force the ball carries with it in flight
(d) the centrifugal force in the swing
Pause it:
talk about these misconceptions with a
shoulder partner
1. Objects with more mass “push harder”
than objects with less mass.
2. Objects with more mass experience
greater gravitational acceleration than
objects with less mass.
Newton’s 3rd Law
• Suppose you are taking a space
walk near the space shuttle, and
your safety line breaks. How
would you get back to the shuttle?
Newton’s 3rd Law
• The thing to do would be to take one of the tools
from your tool belt and throw it is hard as you
can directly away from the shuttle. Then, with
the help of Newton's second and third laws, you
will accelerate back towards the shuttle. As you
throw the tool, you push against it, causing it to
accelerate. At the same time, by Newton's third
law, the tool is pushing back against you in the
opposite direction, which causes you to
accelerate back towards the shuttle, as desired.
What Laws are represented?
Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.
Group Work
Write up:
• Is it possible for you to continue sliding
across a surface if all forces are
balanced? Explain
• What could you do to minimize the
acceleration of an object if something was
to bump into you? Hint, think of F = ma
• You walk into a pole while texting on your
phone, which object experienced the
bigger force? Which object experienced
the greater acceleration?