Newton's Laws

1. Newton’s Laws of
Motion

2. Newton’s Laws of Motion
•Sir Isaac Newton (1643-1727) an English
scientist and mathematician famous for his
discovery of the law of gravity also
discovered the three laws of motion.
•Today these laws are known as Newton’s
Laws of Motion and describe the motion of
all objects on the scale we experience in
our everyday lives.
•Remember: Technology is limited by laws
of nature such as these!!!

3. Newton’s Laws of Motion
Law 1: An object in motion tends to stay in
motion and an object at rest tends to stay
at rest unless acted upon by an
unbalanced force.
Law 2: Force equals mass times
acceleration (F = ma).
Law 3: For every action there is an equal
and opposite reaction.

4. 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.

5. What does this mean?
•Basically, an object will “keep doing what it
was doing” unless acted on by an
unbalanced force.
•If the object was sitting still, it will remain
stationary. If it was moving at a constant
velocity, it will keep moving.
•It takes force to change the motion of an
object

6. Some Examples of the First Law
Two teams are playing tug of war. They are both exerting
equal force on the rope in opposite directions. This
balanced force results in no change of motion.
A soccer ball is sitting at rest. It takes an
unbalanced force of a kick to change its
motion.

7. •A force is viewed as a push or a pull,
something that changes the motion of an
object.
•Whenever there is an interaction between
two objects, there is a force upon each of
the objects.
•Forces can result from two kinds of
interactions.
1.Contact Interaction
2.Non Contact Interaction
What is a force?

8. Applied Force
•An applied force is a contact force which is
applied to an object by a person or another
object
•If a person is pushing a box across the
room, then there is an applied force acting
upon the object.
•The applied force is the force exerted on the
box by the person.

9. Normal Force
•The normal force is the support force
exerted upon an object which is in contact
with another stable object.
•For example, if a physics book is resting
upon a table, then the table is exerting an
upward force upon the book in order to
support the weight of the book.

10. Friction
•Friction is a contact force exerted by a
surface as an object moves across it or
makes an effort to move across it.
•Friction often opposes the motion of an
object.
•For example, if a book slides across the
surface of a desk, then the desk exerts a
friction force in the opposite direction of its
motion.
•Air resistance is a type of friction.

11. Non Contact Forces
•Non contact forces are those types of
forces which result even when the two
interacting objects are not in physical
contact with each other, yet are able to
exert a push or pull despite their physical
separation.
•Non-Contact forces generate a force field –
the area influenced by the push or pull of
the non contact force.

12. Gravitational Force
•Gravity is a non contact force
•ALL objects attract each other with a force of
gravitational attraction.
•The standard formula for gravity is:
Gravitational force = (G * m1 * m2) / (d2)
•where G is the gravitational constant, m1 and
m2 are the masses of the two objects for which
you are calculating the force, and d is the
distance between the centers of gravity of the
two masses.

13. Gravitational Force
•More massive objects will attract each
other with a greater gravitational force.
– as the mass of either object increases, the
force of gravitational attraction between them
also increases.
•more separation distance will result in
weaker gravitational forces.
–as two objects are separated from each other,
the force of gravitational attraction between
them also decreases.

14. Gravitational Force
•The sun and planets exert a gravitational
pull on each other despite their large
spatial separation.
•Even when your feet leave the earth and
you are no longer in physical contact with
the earth, there is a gravitational pull
between you and the Earth.
•All objects upon earth experience a force
of gravity which is directed "downward"
towards the center of the earth.

15. Electric and Magentic Forces
•Electric and magnetic forces are non contact
forces.
•The protons in the nucleus of an atom and the
electrons outside the nucleus experience an
electrical pull towards each other despite their
small spatial separation.
•Two magnets can exert a magnetic pull on each
other even when separated by a distance of a
few centimeters.
•Electric and magnetic forces are stronger than
gravity but only work over very small distances

16. Net Force
•If multiple forces act on an object, they could
potentially add or cancel, depending on direction
•The net force is the sum of all forces acting on an
object.
Force #1Force #2
Total Force
Force #1
Force #2
Total Force = 0
UNBLANCED
FORCE
BALANCED
FORCE

17. What is meant by
unbalanced force?
•If the forces on an
object are equal and
opposite, they are said
to be balanced, and
the object experiences
no change in motion.
•If they are not equal
and opposite, then the
forces are unbalanced
and the motion of the
object changes.

18. When two parallel forces are acting
on the cart in the same direction,
the net force is the two forces
added together.

19. When two forces are opposite
and of equal magnitude, the net
force is zero.

20. When two parallel forces are not of
equal magnitude, the net force is
the difference in the direction of the
larger force.

21. Newton’s First Law is also called
the Law of Inertia
•Inertia: the tendency of an object to resist
changes in its state of motion.
•Mass is a measure of the amount of inertia
an object has.
•The First Law states that all objects have
inertia.
•The more mass an object has, the more
inertia it has (and the harder it is to
change its motion).

22. Another Example of the First Law
There is the
parlor trick of
quickly pulling
a tablecloth
from under a
setting of heavy
dishes, leaving
them on the
table.

23. Example of the First Law
•This trick works because the inertia of the
heavy objects tends to keep them in place.
•By quickly pulling the tablecloth, the force of
friction is easily overcome.
•If the tablecloth was pulled slowly, the friction
would be greater than the inertia, and the
dishes would follow along.
–(Note: The tablecloth must be pulled down at the
edge, otherwise the dishes may fly upward.)

24. 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 it.
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.
In outer space, away from gravity and any
sources of friction, a rocket ship launched
with a certain speed and direction would
keep going in that same direction and at that
same speed forever.

25. Applications of the First law
•Blood rushes from your head to your feet while quickly
stopping when riding on a descending elevator.
•The head of a hammer can be tightened onto the wooden
handle by banging the bottom of the handle against a hard
surface.
•To dislodge ketchup from the bottom of a ketchup bottle, it
is often turned upside down and thrusted downward at high
speeds and then abruptly halted.
•Headrests are placed in cars to prevent whiplash injuries
during rear-end collisions.
•While riding a skateboard (or wagon or bicycle), you fly
forward off the board when hitting a curb or rock or other
object which abruptly halts the motion of the skateboard.

26. Newton’s Second Law
Force equals mass times acceleration.
F = ma
Acceleration: a measurement of how quickly an
object is changing speed.

27. According to Newton's Second law...
•Acceleration is produced when a force
acts on a mass.
•The greater the mass (of the object being
accelerated) the greater the amount of
force needed (to accelerate the object).
•Everyone unconsciously knows the
Second Law -- heavier objects require
more force to move the same distance as
lighter objects.

28. What is Acceleration?
•Speed = distance/time (m/s, km/hr, mph)
•Velocity: Same as speed but includes
direction of travel
•Acceleration = (change in velocity)/time
(m/s
2
)
t
vv
a
if-
=

29. What is Acceleration?
•In physics, all of these are considered
examples of acceleration
–Speeding up
–Slowing down
–Changing direction!
•Examples of acceleration:
–A car speeding up or slowing down while
traveling in a straight line
–A car rounding a curve at a constant speed
–A car speeding up or slowing down while
rounding a curve

30. Four different ways (A-D) to
accelerate a car.

31. Acceleration due to Gravity
•Gravity pulls objects
toward Earth
•Causes objects to move
faster and faster, thus
accelerate
•Velocity increases by
about 10 m/s, for every
second of fall
•thus, Acceleration of
Gravity (g) ~10 m/s
2

32. Mass vs. Weight
Weight is the effect of gravity on an object.
•Weight is a Force (F = ma)
•Weight = Mass ´ Acceleration of Gravity (g)
–measured in Newtons (N) in the metric system or
pounds (lb) in the British system
Mass is the quantity of matter in an object or the
measurement of the inertia
–measured in kilograms (kg) in the metric system or
slugs in the British system

33. Mass vs. Weight
•The weight of an object
changes with location, the
mass of an object does not.
•For example, your mass is
the same on earth and on
the moon but you weigh less
on the moon because, due
to its smaller size, the moon
has a weaker pull of gravity.

34. So, what does F = ma mean?
•Imagine a ball of a certain mass moving at a
certain acceleration. This ball has a certain force
(double the mass) but keep the acceleration
constant. F = ma says that this new ball has
twice the force of the old ball.
•Now imagine the original ball moving at twice the
original acceleration. F = ma says that the ball
will again have twice the force of the ball at the
original acceleration.
•Now imagine we make the ball twice as big

35. Examples of the Second Law
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still have a
great force. Something very small changing
speed very slowly will have a very weak
force.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still have
great force.

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

37. Newton's Third Law of Motion
•Whenever two objects interact, the force
exerted on one object is equal in size and
opposite in direction to the force exerted
on the other object.
F
A
due to B = F
B
due to A
•In other words…….For every force acting
on an object, there is an equal force acting
in the opposite direction.

38. What does this mean?
Right now, gravity is pulling
you down in your seat, but
Newton’s Third Law says
your seat is pushing up
against you with equal
force. This is why you are
not moving. There is a
balanced force acting on
you– gravity pulling down,
your seat pushing up.

39. 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).

40. Consider the flying motion of birds
•A bird flies by use of its wings. The wings of a
bird push air downwards. Since forces result
from mutual interactions, the air must also be
pushing the bird upwards.
•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 (downwards) is opposite the direction of
the force on the bird (upwards).
•For every action, there is an equal (in size) and
opposite (in direction) reaction. Action-reaction
force pairs make it possible for birds to fly.