Newton's first law of motion

1. Let’s recall!
•Net force
•Mass
•Friction

2. Relate you experiences.
 Suppose the jeepney that you are
riding suddenly stops, what
happens to you?
 Or suppose the jeepney which is
initially at rest suddenly moves
forward, what will be your
reaction?

3. Let’s investigate!

4. Inertia
• Objectives
1. Understand inertia
2. Illustrate inertia
3. Understand Newton's laws of motion.

5. Inertia
• Materials
Balls of various sizes, glass, coins,
sheet of paper, straw

6. Inertia
• Activity 1 - Place the glass on the
table. Cover the glass with a sheet
of paper. Place a coin on the
cover. Snatch the paper cover off
the glass. Observe what happens
to the coin.

7. Inertia

8. Inertia
• Activity 2 - Put a ball in motion
and try to blow it off of its path
with a straw.
• Repeat this activity using balls of
various masses. Which ball was
the hardest to move off of the
straight path?

9. Analysis:
1) How does glass and coin activity
help prove the first law of motion?
2) Which ball was the hardest to
move off of the straight path?
3) Does inertia increase with mass?
4) Conclusion.

11. Newton’s Contributions
• Calculus
• Light is composed of
rainbow colors
• Reflecting Telescope
• Laws of Motion
• Theory of Gravitation

12. 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.

13. Balanced Force
Equal forces in opposite
directions produce no motion

15. Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion

16. It means that…
An object in motion
wants to stay in
motion, and an object
at rest wants to stay at
rest.
This is also called
INERTIA

17. 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.

18. Newton’s First Law
(law of inertia)
• MASS is the measure of the
amount of matter in an object.
• It is measured in Kilograms

19. Newton’s First Law
(law of inertia)
• INERTIA is a property of an
object that describes how
______________________ the
motion of the object
• more _____ means more ____
much it will resist change to
mass inertia

20. 1st
Law
• Unless acted
upon by an
unbalanced
force, this golf
ball would sit
on the tee
forever.

21. • There are four main types of friction:
– Sliding friction: ice skating
– Rolling friction: bowling
– Fluid friction (air or liquid): air or water
resistance
– Static friction: initial friction when moving an
object
What is this unbalanced force that acts on anWhat is this unbalanced force that acts on an
object in motion?object in motion?

22. 1st
Law
• Once airborne,
unless acted on
by an
unbalanced force
(gravity and air –
fluid friction) it
would never
stop!

23. The car is at rest, and it is going to stay at rest
until a force gives it a push.

24. Now that is moving, it will continue to move
until another force stops it.
Friction

25. The best place to test inertia is in Space, why?
In outer space there
is no air, or
anything else to
create friction. So
moving objects will
keep moving forever,
and objects that
aren't moving will
stand still forever.

26. Inertia

27. Always wear your seatbelt!
•The crash dummy is not wearing a seat belt
and is moving along with the car.
•Both the car and the dummy are moving at,
say 60 mph.
•When the car hits the cement road divider,
it is stopped (an outside force stops it
from moving).
•The crash dummy, however is not so lucky.
Since he is not wearing a seat belt and is
not connected to the car, he will continue
to move at 60 mph. This means he will go
flying out through the front windshield.
•The dummy will fly through the air until
he hits the ground.
•This is because the earth's gravity
stopped him from moving any further.
•If this collision had happened in zero-
gravity, in a vacuum, the dummy would
theoretically keep on hurtling away from
the car at 60 mph.

28. Another Example
• The person is standing
on the floor.
• The only forces acting
on the person are the
force due to gravity
pulling down & the
normal force pushing
up. The net force is
zero and the person
remains still.

29. Some Problems
An astronaut in
outer space
away from
gravitational or
frictional
forces throws a
rock. The rock
will…..

30. And now the answer
…continue to move in a
straight line at a
constant speed.
The rock’s tendency to do
this is called INERTIA.

31. A rock is being
whirled at the
end of a string in
a clockwise
direction. If the
string breaks,
the path of the
rock is

32. It will follow an “inertial
path” so it will follow path
“C”. Once the rock leaves
the string, there are no
unbalanced forces to affect
its motion.

33. A Weighty Problem
I spend most
Sunday afternoons
at rest on the sofa,
watching football
and consuming large
quantities of food.
What effect (if any)
does this practice
have upon my
inertia? Explain.
My inertia will most
definitely increase.
My mass will
increase because of
this practice and if
mass increases, then
inertia increases.

34. Is a force required?
If you were in a
weightless
environment in
space, would it
require a force
to set an object
in motion?
Yes, because even in
outer space, an object
has mass. If an object
has mass then the
object is going to resist
changes in its motion.
A force must be applied
to set the object in
motion. Newton’s Laws
rule—Everywhere!

35. A Final Problem
Why isn't the
girl hurt when
the nail is
driven into
the block of
wood?

36. And the answer is….
• Due to the large mass of the
books, the force of the hammer
is sufficiently resisted (inertia).
• This is demonstrated by the fact
that the blow of the hammer is
not felt by the girl.

37. Synthesis:
• State the law of inertia.
• What is inertia? How is
inertia related to mass?

38. Assignment:
A. In your ½ crosswise, explain in not less than
3 sentences, how each one is considered a
manifestation of Newton’s first law of motion.
1. when riding on a descending elevator that
suddenly stopped, the blood rushes from your
head to your feet
2. to tighten the head of a hammer onto the
wooden handle, you ram the bottom of the
handle against a hard surface

39. Assignment:
B. by group: Bring the following
materials on June 30, Thursday.
•toy cart/car
•1 pc. manila paper
•2 m-long string/yarn
•pentel pen
•stone inside the small plastic bag

40. Newton’s Second Law
Force equals
mass times
acceleration.
F = ma

41. Newton’s Second Law
• Force = Mass x Acceleration
• Force is measured in Newtons
ACCELERATION of GRAVITY(Earth) = 9.8 m/s2
• Weight (force) = mass x gravity (Earth)
Moon’s gravity is 1/6 of the
Earth’sIf you weigh 420 Newtons on earth,
what will you weigh on the Moon?
70 Newtons
If your mass is 41.5Kg on Earth
what is your mass on the Moon?

42. Newton’s Second Law
• WEIGHT is a measure of the
force of ________ on the
mass of an object
• measured in __________
gravity
Newtons

43. Newton’s Second Law
One rock weighs 5 Newtons.
The other rock weighs 0.5
Newtons. How much more
force will be required to
accelerate the first rock
at the same rate as the
second rock?
Ten times as much

44. Newton’s Third Law
For every action there is an equal and
opposite reaction.

45. Newton’s 3rd
Law
• For every action there is an equal and
opposite reaction.
Book to
earth
Table to
book

46. Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on
your toe (and the more your toe hurts).

47. 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

48. 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!

49. Action: earth pulls on you
Reaction: you pull on earth
Action and Reaction on Different Masses
Consider you and the earth

50. Action: tire pushes on road
Reaction: road pushes on tire

51. Action: rocket pushes on gases
Reaction: gases push on rocket

52. 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
(b) the force applied to the bat by the ball

53. 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?

54. 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.

55. What Laws are represented?

57. 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.

58. 1st
law: Homer is large and
has much mass, therefore he
has much inertia. Friction
and gravity oppose his
motion.
2nd
law: Homer’s mass x
9.8 m/s/s equals his
weight, which is a force.
3rd
law: Homer pushes
against the ground and it
pushes back.