In
previous chapters, we studied how objects move. In
this chapter, we will study why objects move as they
do. We will study Newton's Laws of Motion, which
explain the relationship between acceleration and
force. We will also use Newton's Laws for problem
solving.

Force can be defined as a push or a pull.
(Technically, force is something that can accelerate
objects.) For example, when you throw a baseball, you
apply a force to the ball. Force is measured by N (Newton). A force that
causes an object with a mass of 1 kg to accelerate at
1 m/s is equivalent to 1 Newton.

You will have to learn a
new terminology here: net
force. Net force is the sum of all forces
acting on an object. For example, in a tag of war,
when one team is pulling the tag with a force of 100
N and the other with 80 N, the net force would be 20
N at the direction of the first team (100 N - 80 N =
20 N).

QUESTION:
If both teams pull the tag with equal
force, what would the net force be? N

When you slide your book on floor it
will stop soon. When you slide it on icy surface, it
will travel further and then stop. Galileo believed
that when you slide a perfectly smooth object on a
frictionless floor the object would travel forever.

Isaac
Newton developed the idea of Galileo further. He
concluded that an object will remain at rest or
move with constant velocity when there is no net
force acting on it. This is called Newton's First
Law of Motion, or Law of Inertia.

Newton's First Law deals with an object with no
net force. Newton's Second Law talks about an object
that hasnet force. It states that when
the net force acting on an object is not zero, the
object will accelerate at the direction of the
exerted force. The acceleration is directly
proportional to the net force and inversely
proportional to the mass. It can be expressed in
formula

F = ma

where:

F is the net force in N,

m is the mass of an object in kg and

a is its acceleration in m/s^{2}.

From this formula, we can say that force is
something that accelerates an object.

QUESTION:
How much net force is required to
accelerate a 1000 kg car at 5.00 m/s^{2}?
N

QUESTION: If
you apply a net force of 1 N on 200 g-book,
what is the acceleration of the book? m/s^{2}

When you kick the wall
in your room, you will probably end up hurting your
foot. Newton's Third Law of Motion can explain why: when
one object applies a force on a second object, the
second object applies a force on the first that has
an equal magnitude but opposite direction. In
other words, when you kick the wall, the wall kicks
you back with equal force. As a result you will get
hurt. These forces are called action-reaction
forces.

Remember when you kick the wall,
you exerts force on the wall. When the wall kicks you
back, it exerts force on you. Therefore, the net
force on the wall is not zero and the net force on
your foot is not zero neither.

QUESTION: What is the net force on
200 g ball when it hits a wall with
acceleration of 10 m/s^{2}? N

Mass and weight are different in
physics. For example, your mass doesn't change when
you go to the Moon, but your weight does. Mass shows
the quantity, and weight shows the size of gravity.

If you know your mass, you can easily find your
weight because

W = mg

where:

W is weight in Newton (N),

m is mass in kg, and

g is the acceleration of gravity in m/s^{2}.

If your mass is 70 kg on Earth, your weight is
W=(70 kg)(9.8 m/s^{2}) = 686 N.

Weight is measured by Newton (N).

QUESTION:
What is the mass of an object that has a
weight of 115 N on the Moon? The gravity of
the Moon is 1/6 of g (which is 9.8 m/s^{2}). kg

You will have to learn another vocabulary before
you proceed: the normal
force. The normal force acts on any object
that touches surface (either directly or indirectly).
The normal force would be applied on a ball on a
table, but not on a ball in the air, for instance. It
always acts perpendicularly to the surface. The
formula to calculate the normal force is

F_{N} = - mg

where:

F_{N} is the normal force in Newton
(N),

m is the mass in kg, and

g is the gravitational force in m/s^{2}.

For example, the normal force acting on a 70
kg-person would be
F_{N} = - (70 kg)(-9.8 m/s^{2}) =
686N

QUESTION:
What is the normal force acting on the same
person on the Moon? N

Now, we will talk about friction.

When
you slide your book on floor, it will come to stop
because of the force of
friction. Friction is the force that acts
between two object in contact because of
action-reaction.

Force of friction can be calculated by the formula

where:

F_{f} is the force of friction in N,

is the coefficient of friction,
and

F_{N} is the normal force in N.

The value of depends on surface you are
dealing with. The following table shows some example
of .

Surface

Value of

rubber on dry asphalt

~1

rubber on wet asphalt

0.95

steel on steel

0.18

steel on ice

0.010

rubber on ice

0.005

For example, if you throw a 500 g book on floor
where = 0.1, the force of friction would be:
F_{f} = = (0.1)(0.5 * 9.8) = 0.49 N

QUESTION:
What is the value of if
the force of friction on a 300 g book was 0.5
N?