Download Newton`s Second Law of Motion

Newton's Second Law of Motion
Force can cause acceleration
The combination of all forces that act on a body is
called the resultant (or net) force.
5N
10 N
5 N (Resultant (net) Force)
The amount of acceleration depends on the
magnitude of the resultant force.
Acceleration is directly proportional to the
Net Force for a given mass.
For example;
a∝F
if a × 3 then F × 3
or a × ¼ then F × ¼
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 1 of 7
Acceleration is inversely proportional to mass for a
given force.
1
a∝
m
For example;
1
if a × 3 then m × 3
or a × ¼ then m × 4
For the same Force;
M⇒a
a
large mass
m⇒
small mass
small acceleration
large acceleration
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 2 of 7
Newton's Second Law of Motion
The acceleration produced by a
net force acting on an object is
directly proportional to the
magnitude of the force and
inversely proportional to the
mass of the object.
acceleration (m ⋅s
-2
net force (N)
)=
mass (kg)
F
a =
m
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 3 of 7
Friction
Friction causes a force that always acts to oppose the
motion.
The force of friction acts on any objects, which are in
contact with each other
Fluids (gases and liquids) also have friction.
For example, air resistance or water resistance.
Fluid friction occurs when an object pushes aside the
fluid as the object is moving through it.
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 4 of 7
Pressure
Pressure is the force per unit area
(units = newtons/m2 = pascals (Pa)).
Force
Area of Application
F
(Pa)
P =
A
Pressure =
For example;
The upright block exerts the same force
but greater pressure on the table, given
that the two blocks have equal masses.
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 5 of 7
Free Fall (in the absence of air resistance)
Galileo showed that falling objects accelerate. The
acceleration is independent of mass.
a=
F
F
or
= g = 9.8 ms - 2
m
m
• all freely falling objects undergo the same
acceleration (g) at the same place on the Earth.
• g is the acceleration due to gravity.
Non-Free Fall
When an object is dropped with air resistance the net
acceleration is less than g;
a=
net force weight - air resistance
=
mass
mass
When air resistance = weight, the net force
equals 0; therefore the acceleration equals 0.
With no acceleration, a constant velocity has been
reached, ie a terminal velocity.
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 6 of 7
The increased area of
the parachute increases
air resistance and
decreases the
terminal velocity.
Terminal Velocity for a Falling Object
• As a result of air resistance objects can reach a
terminal velocity.
• As speed increases the upward force of
air resistance can increase until it equals the
weight of the falling body.
• At that point the Net Force on the body equals
zero (the body falls with zero acceleration
i.e. constant velocity).
Conceptual Physics – 3rd Edition – Paul Hewitt
Chapter 5 – Newton’s Second Law of Motion
Page 7 of 7