2. THREE (3) LAWS OF MOTION
• LAW OF INERTIA
• LAW OF
ACCELERATION
• LAW OF ACTION-
REACTION
3. LAW OF INERTIA
An object at rest will
remain at rest unless
acted on by an
unbalanced force. An
object in motion
continues in motion
with the same speed
and in the same
direction unless acted
upon by an unbalanced
force.
4. What does this mean?
This means that there is a natural
tendency of objects to keep on doing
what they're doing. All objects resist
changes in their state of motion. In the
absence of an unbalanced force, an
object in motion will maintain this state
of motion.
5. What is the motion in this picture?
What is the unbalanced force in this
picture?
What happened to the skater in this
picture?
6. This law is the same reason why you
should always wear your seatbelt.
Seat belts are used to provide safety for passengers
whose motion is governed by Newton's laws. The seat
belt provides the unbalanced force which brings you
from a state of motion to a state of rest. Perhaps you
could speculate what would occur when no seat belt is
used.
7. 1. Blood rushes from your head to your feet while quickly
stopping when riding on a descending elevator.
2. The head of a hammer can be tightened onto the wooden handle
by banging the bottom of the handle against a hard surface.
3. A brick is painlessly broken over the hand of a physics teacher
by slamming it with a hammer. (CAUTION: do not attempt
this at home!)
4. 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.
5. Headrests are placed in cars to prevent whiplash injuries during
rear-end collisions.
Applications of Newton's first law
8. Check Your Understanding
1. The group of physics teachers
are taking some time off for a
little putt-putt golf. The 15th
hole at the Hole-In-One Putt-
Putt Golf Course has a large
metal rim which putters must
use to guide their ball towards
the hole. Mr. S guides a golf
ball around the metal rim When
the ball leaves the rim, which
path (1, 2, or 3) will the golf
ball follow?
9. The answer is 2. Once leaving
the rim, the ball will follow an
"inertial path" (i.e., a straight
line). At the instant shown in the
diagram, the ball is moving to
the right; once leaving the rim,
there is no more unbalanced
forces to change its state of
motion. Paths 1 and 3 both show
the ball continually changing its
direction once leaving the rim.
10. a. 0 N b. 0.5 N c. 2.0 N d. 8.0 N
e. depends on the speed.
ANSWER: If an object is in motion, then it will
stay in motion with those very same motion
characteristics. It doesn't take any force to maintain
that same state of motion. In fact, the presence of a
force would "ruin" such a state of motion and cause
an acceleration.
2. A 4.0-kg object is moving across a friction-free
surface with a constant velocity of 2 m/s. Which one
of the following horizontal forces is necessary to
maintain this state of motion?
11. INERTIA
• " Objects tend to "keep on doing what they're
doing."
• In fact, it is the natural tendency of objects to
resist changes in their state of motion.
This tendency to resist changes in their
state of motion is described as inertia.
• Inertia: the resistance an object has to a
change in its state of motion.
12. • Newton's conception of inertia stood in direct
opposition to more popular conceptions about
motion.
• The dominant thought prior to Newton's day
was that it was the natural tendency of objects
to come to a rest position. Moving objects, so
it was believed, would eventually stop
moving; a force was necessary to keep an
object moving. But if left to itself, a moving
object would eventually come to rest and an
object at rest would stay at rest;
13. •Thus, the idea which dominated
people's thinking for nearly 2000 years
prior to Newton was that it was the
natural tendency of all objects to
assume a rest position.
17. Mass as a Measure of the Amount of Inertia
All objects resist changes in their state of motion.
All objects have this tendency - they have inertia.
But do some objects have more of a tendency to
resist changes than others? Absolutely yes! The
tendency of an object to resist changes in its state
of motion varies with mass.
Mass is that quantity which
is solely dependent upon the
inertia of an object. The
more inertia which an object
has, the more mass it has.
18. ANSWER:According to Newton's first law, the rock will
continue in motion in the same direction at constant speed.
Check Your Understanding
1. Imagine a place in the cosmos
far from all gravitational and
frictional influences. Suppose
that you visit that place (just
suppose) and throw a rock. The
rock will
a. gradually stop.
b. continue in motion in the same
direction at constant speed.
19. 2. A 2-kg object is moving horizontally with a
speed of 4 m/s. How much net force is required
to keep the object moving at this speed and in
this direction?
Answer: 0 N
An object in motion will maintain its state of
motion. The presence of an unbalanced force
changes the velocity of the object.
20. 3. Mac and Tosh are arguing in the cafeteria.
Mac says that if he flings the Jell-O with a
greater speed it will have a greater inertia.
Tosh argues that inertia does not depend
upon speed, but rather upon mass. Who do
you agree with? Explain why.
Tosh is correct. Inertia is that quantity which
depends solely upon mass. The more mass, the more
inertia. Momentum is another quantity in Physics
which depends on both mass and speed. Momentum
will be discussed in a later unit.
21. 4. Supposing you were in space in a
weightless environment, would it require a
force to set an object in motion?
Absolutely yes!
Even in space objects have mass. And if they
have mass, they have inertia. That is, an object
in space resists changes in its state of motion. A
force must be applied to set a stationary object
in motion. Newton's laws rule - everywhere!
22. 5. Ben Tooclose is being chased through the woods by
a bull moose which he was attempting to photograph.
The enormous mass of the bull moose is extremely
intimidating. Yet, if Ben makes a zigzag pattern through
the woods, he will be able to use the large mass of the
moose to his own advantage. Explain this in terms of
inertia and Newton's first law of motion.
The large mass of the bull moose means that the bull
moose has a large inertia. Thus, Ben can more easily
change his own state of motion (make quick changes in
direction) while the moose has extreme difficulty
changing its state of motion. Physics for better living!
27. Drawing Free-Body Diagrams
1. A book is at rest on a
table top. A free-body
diagram for this
situation looks like this:
2. A girl is suspended
motionless from the ceiling
by two ropes. A free-body
diagram for this situation
looks like this:
28. 3. An egg is free-falling from
a nest in a tree. Neglect air
resistance. A free-body
diagram for this situation
looks like this:
4. A flying squirrel is
gliding (no wing flaps) from
a tree to the ground at
constant velocity. Consider
air resistance. A free-body
diagram for this situation
looks like this:
29. 5. A rightward force is
applied to a book in order to
move it across a desk with a
rightward acceleration.
Consider frictional forces.
Neglect air resistance. A free-
body diagram for this
situation looks like this:
30. 6. A rightward force is
applied to a book in order
to move it across a desk at
constant velocity. Consider
frictional forces. Neglect
air resistance. A free-body
diagram for this situation
looks like this:
31. 7. A force is applied
to the right to drag a
sled across loosely-
packed snow with a
rightward
acceleration. A free-
body diagram for
this situation looks
like this:
32. 8. A car is coasting to the
right and slowing down.
A free-body diagram for
this situation looks like
this:
33. LAW OF ACCELERATION
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).
34. What does this mean?
Everyone unconsciously knows the
Second Law. Everyone knows
that heavier objects require
more force to move the same
distance as lighter objects.
35. However, the Second Law gives us an
exact relationship between force, mass,
and acceleration. It can be expressed
as a mathematical equation:
or
FORCE = MASS x ACCELERATION
36. This is an example of how
Newton's Second Law works:
Mike's car, which weighs
1,000 kg, is out of gas.
Mike is trying to push the
car to a gas station, and
he makes the car go 0.05
m/s/s. Using Newton's
Second Law, you can
compute how much force
Mike is applying to the
car. Answer = 50 newtons
37. Practice #1
An applied force of 50 N is
used to accelerate an object to
the right across a frictional
surface. The object
encounters 10 N of friction.
Use the diagram to determine
the normal force, the net
force, the mass, and the
acceleration of the object.
(Neglect air resistance.)
38. Fnorm = 80 N
m = 8.16 kg
Fnet = 40 N, right
a = 4.9 m/s/s, right
39. Practice #2
An applied force of 20 N is used
to accelerate an object to the
right across a frictional surface.
The object encounters 10 N of
friction. Use the diagram to
determine the normal force, the
net force, the coefficient of
friction ("mu") between the
object and the surface, the mass,
and the acceleration of the
object. (Neglect air resistance.)
40. Fnorm = 100 N
m = 10.2 kg
Fnet = 10 N, right
"mu" = 0.1
a =0.980 m/s/s, right
(a = Fnet / m = (10 N) / (10.2 kg) = 0.980 m/s/s.)
41. Practice #3
A 5-kg object is sliding to the
right and encountering a friction
force which slows it down. The
coefficient of friction ("mu")
between the object and the
surface is 0.1. Determine the
force of gravity, the normal
force, the force of friction, the
net force, and the acceleration.
(Neglect air resistance.)
42. Fgrav = 49 N
Fnorm = 49 N
Ffrict = 4.9 N
Fnet = 5 N, left; a = 0.98 m/s/s, left
( Fgrav = 50 N; Fnorm = 50 N; Ffrict = 5
N;
Fnet = 5 N, left; a = 1 m/s/s, left )
a = Fnet / m = (4.9 N) / (5 kg) = 0.98 m/s/s.
43. Check Your Understanding
1. Determine the accelerations which result when a
12-N net force is applied to a 3-kg object and
then to a 6-kg object.
2. A net force of 15 N is exerted on an
encyclopedia to cause it to accelerate at a rate of
5 m/s2. Determine the mass of the encyclopedia.
3. Suppose that a sled is accelerating at a rate of 2
m/s2. If the net force is tripled and the mass is doubled,
then what is the new acceleration of the sled?
44. 4. Edwardo applies a 4.25-N rightward force to a
0.765-kg book to accelerate it across a table top. The
coefficient of friction between the book and the
tabletop is 0.410. Determine the acceleration of the
book.
5. In a physics lab, Kate and Rob use a hanging
mass and pulley system to exert a 2.45 N rightward
force on a 0.500-kg cart to accelerate it across a
low-friction track. If the total resistance force to the
motion of the cart is 0.72 N, then what is the cart's
acceleration?
55. What does this mean?
This means that for every force there is a
reaction force that is equal in size, but
opposite in direction. That is to say that
whenever an object pushes another object it
gets pushed back in the opposite direction
equally hard.
56. Let's study how a rocket works to
understand
Newton's Third Law.
The rocket's action is to push down on the ground with the
force of its powerful engines, and the reaction is that the
ground pushes the rocket upwards with an equal force.
