newtons_laws_of_motion_science grade 8 _.ppt

lecture note for science 8

1. Newton’s Newton’s
Laws of Laws of
MotionMotion
I.I. Law of Inertia Law of Inertia
II.II. F=ma F=ma
III.III. Action-Reaction Action-Reaction

2. While most people know While most people know
what Newton's laws say, what Newton's laws say,
many people do not know many people do not know
what they mean (or simply do what they mean (or simply do
not believe what they mean). not believe what they mean).

3. Newton’s Laws of MotionNewton’s Laws of Motion

11
stst
Law Law – An object at rest will stay at rest, – An object at rest will stay at rest,
and an object in motion will stay in and an object in motion will stay in
motion at constant velocity, unless acted motion at constant velocity, unless acted
upon by an unbalanced force.upon by an unbalanced force.

22
ndnd
Law Law – – Force equals mass times Force equals mass times
acceleration.acceleration.

33
rdrd
Law Law – – For every action there is an For every action there is an
equal and opposite reaction.equal and opposite reaction.

4. 11
stst
Law of Motion Law of Motion
(Law of Inertia) (Law of Inertia)
An object at rest will stay at An object at rest will stay at
rest, and an object in motion rest, and an object in motion
will stay in motion at will stay in motion at
constant velocity, unless acted constant velocity, unless acted
upon by an unbalanced force.upon by an unbalanced force.

5. 11
stst
Law Law

Inertia is the Inertia is the
tendency of an tendency of an
object to resist object to resist
changes in its changes in its
velocity: velocity:
whether in whether in
motion or motion or
motionless.motionless.
These pumpkins will not move unless acted on
by an unbalanced force.

6. 11
stst
Law Law

Once airborne, Once airborne,
unless acted on unless acted on
by an by an
unbalanced force unbalanced force
(gravity and air (gravity and air
– fluid friction), – fluid friction),
it would never it would never
stop! stop!

7. 11
stst
Law Law

Unless acted Unless acted
upon by an upon by an
unbalanced unbalanced
force, this golf force, this golf
ball would sit on ball would sit on
the tee forever. the tee forever.

8. Why then, do we observe Why then, do we observe
every day objects in motion every day objects in motion
slowing down and becoming slowing down and becoming
motionless seemingly without an motionless seemingly without an
outside force?outside force?
It’s a force we sometimes cannot see – It’s a force we sometimes cannot see –
friction.friction.

9. Objects on earth, unlike the Objects on earth, unlike the
frictionless space the moon frictionless space the moon
travels through, are under the travels through, are under the
influence of friction.influence of friction.

10. 
There are four main types of friction:There are four main types of friction:

Sliding friction: Sliding friction: ice skating

Rolling friction: Rolling friction: bowling

Fluid friction (air or liquid): Fluid friction (air or liquid): air or water resistance

Static friction: Static friction: initial friction when moving an object
What is this unbalanced force that acts on an object in motion?What is this unbalanced force that acts on an object in motion?

11. Slide a book Slide a book
across a table and across a table and
watch it slide to a rest watch it slide to a rest
position. The book position. The book
comes to a rest comes to a rest
because of the because of the
presencepresence of a force - of a force -
that force being the that force being the
force of friction - force of friction -
which brings the book which brings the book
to a rest position.to a rest position.

12. 
In the absence of a force of friction, the book In the absence of a force of friction, the book
would continue in motion with the same speed would continue in motion with the same speed
and direction - forever! (Or at least to the end and direction - forever! (Or at least to the end
of the table top.) of the table top.)

13. Newtons’s 1Newtons’s 1
stst
Law and You Law and You
Don’t let this be you. Wear seat belts.Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes Because of inertia, objects (including you) resist changes
in their motion. When the car going 80 km/hour is stopped in their motion. When the car going 80 km/hour is stopped
by the brick wall, your body keeps moving at 80 m/hour.by the brick wall, your body keeps moving at 80 m/hour.

14. 22
ndnd
Law Law

15. 22
ndnd
Law Law
The net force of an object is The net force of an object is
equal to the product of its mass equal to the product of its mass
and acceleration, or F=ma.and acceleration, or F=ma.

16. 22
ndnd
Law Law

When mass is in kilograms and acceleration is When mass is in kilograms and acceleration is
in m/s/s, the unit of force is in newtons (N).in m/s/s, the unit of force is in newtons (N).

One newton is equal to the force required to One newton is equal to the force required to
accelerate one kilogram of mass at one accelerate one kilogram of mass at one
meter/second/second.meter/second/second.

17. 22
ndnd
Law (F = m x a) Law (F = m x a)
How much force is needed to accelerate a 1400
kilogram car 2 meters per second/per second?

Write the formulaWrite the formula
F = m x a

Fill in given numbers and unitsFill in given numbers and units
F = 1400 kg x 2 meters per second/second

Solve for the unknownSolve for the unknown

2800 kg-meters/second/second or 2800 N

18. If mass remains constant, doubling the acceleration, doubles the force. If force remains
constant, doubling the mass, halves the acceleration.

19. Newton’s 2
nd
Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
•We know that objects
with different masses
accelerate to the
ground at the same
rate.
•However, because of
the 2
nd
Law we know
that they don’t hit the
ground with the same
force.
F = maF = ma
98 N = 10 kg x 9.8 m/s/s98 N = 10 kg x 9.8 m/s/s
F = maF = ma
9.8 N = 1 kg x 9.8 9.8 N = 1 kg x 9.8
m/s/sm/s/s

21. Check Your UnderstandingCheck Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg 1. What acceleration will result when a 12 N net force applied to a 3 kg
object? A 6 kg object?object? A 6 kg object?
  

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s
22
. .
Determine the mass.Determine the mass.

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 4. What is the force on a 1000 kg elevator that is falling freely at 9.8
m/sec/sec?m/sec/sec?

22. Check Your UnderstandingCheck Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object? 1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s12 N = 3 kg x 4 m/s/s

  

2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s
22
. Determine the . Determine the
mass.mass.
16 N = 3.2 kg x 5 m/s/s16 N = 3.2 kg x 5 m/s/s
  

3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N66 kg-m/sec/sec or 66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

 9800 kg-m/sec/sec or 9800 N9800 kg-m/sec/sec or 9800 N

24. 33
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Law Law

For every action, there is an For every action, there is an
equal and opposite reaction.equal and opposite reaction.

25. 33
rdrd
Law Law
According to Newton, According to Newton,
whenever objects A and whenever objects A and
B interact with each B interact with each
other, they exert forces other, they exert forces
upon each other. When upon each other. When
you sit in your chair, you sit in your chair,
your body exerts a your body exerts a
downward force on the downward force on the
chair and the chair chair and the chair
exerts an upward force exerts an upward force
on your body. on your body.

26. 33
rdrd
Law Law
There are two forces There are two forces
resulting from this resulting from this
interaction - a force on interaction - a force on
the chair and a force on the chair and a force on
your body. These two your body. These two
forces are called forces are called actionaction
and and reactionreaction forces. forces.

27. Newton’s 3rd Law in NatureNewton’s 3rd Law in Nature

Consider the propulsion of a Consider the propulsion of a
fish through the water. A fish fish through the water. A fish
uses its fins to push water uses its fins to push water
backwards. In turn, the water backwards. In turn, the water
reactsreacts by pushing the fish by pushing the fish
forwards, propelling the fish forwards, propelling the fish
through the water.through the water.

The size of the force on the The size of the force on the
water equals the size of the water equals the size of the
force on the fish; the direction force on the fish; the direction
of the force on the water of the force on the water
(backwards) is opposite the (backwards) is opposite the
direction of the force on the direction of the force on the
fish (forwards).fish (forwards).

28. 33
rdrd
Law Law
Flying gracefully Flying gracefully
through the air, birds through the air, birds
depend on Newton’s depend on Newton’s
third law of motion. As third law of motion. As
the birds push down on the birds push down on
the air with their wings, the air with their wings,
the air pushes their the air pushes their
wings up and gives wings up and gives
them lift.them lift.

29. 
Consider the flying motion of birds. A bird flies by Consider the flying motion of birds. A bird flies by
use of its wings. The wings of a bird push air use of its wings. The wings of a bird push air
downwards. In turn, the air reacts by pushing the bird downwards. In turn, the air reacts by pushing the bird
upwards. upwards.

The size of the force on the air equals the size of the The size of the force on the air equals the size of the
force on the bird; the direction of the force on the air force on the bird; the direction of the force on the air
(downwards) is opposite the direction of the force on (downwards) is opposite the direction of the force on
the bird (upwards).the bird (upwards).

Action-reaction force pairs make it possible for birds Action-reaction force pairs make it possible for birds
to fly.to fly.

31. Other examples of Newton’s Other examples of Newton’s
Third LawThird Law

The baseball forces the The baseball forces the
bat to the left (an bat to the left (an
action); the bat forces action); the bat forces
the ball to the right (the the ball to the right (the
reaction). reaction).

32. 33
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Law Law

Consider the motion of Consider the motion of
a car on the way to a car on the way to
school. A car is school. A car is
equipped with wheels equipped with wheels
which spin backwards. which spin backwards.
As the wheels spin As the wheels spin
backwards, they grip the backwards, they grip the
road and push the road road and push the road
backwards.backwards.

33. 33
rdrd
Law Law
The reaction of a rocket is The reaction of a rocket is
an application of the third an application of the third
law of motion. Various law of motion. Various
fuels are burned in the fuels are burned in the
engine, producing hot engine, producing hot
gases. gases.
The hot gases push against The hot gases push against
the inside tube of the rocket the inside tube of the rocket
and escape out the bottom and escape out the bottom
of the tube. As the gases of the tube. As the gases
move downward, the rocket move downward, the rocket
moves in the opposite moves in the opposite
direction.direction.

34. Choose the best answer. Write letter only.
 
1. According to Newton's First Law of Motion,
A. an object in motion eventually comes to a stop.
B. an object at rest eventually begins to move.
C. an object at rest always remains at rest.
D. an object at rest remains at rest unless acted upon by a net
force.
 
2. The greater the mass of an object,
A. the easier the object starts moving.
B. the more space it takes up.
C. the greater its inertia.
D. the more balanced it is.
 
 

35.  
3. The tendency of an object to resist any change in its motion is known
as
A. balance.
B. force
C. inertia.
D. mass.
 
4. Mass of the object is quantitative measure of its inertia stated law is newton's
A. first law
B. second law
C. third law
D. fourth law
 
5. Which of these best describes the concept of inertia?
A. A force that attracts objects with mass
B. The tendency of an object to float in water
C. A force created when surfaces are in contact
D. The tendency of an object to resist a change in motion