4 Newton’s Second Law of
Motion
 Force causes acceleration
 Friction
 Mass and weight
 Mass resists acceleration
 Newton’s second law of motion
 Free Fall and Non-Free Fall
Dr. Jie Zou
PHY 1071
1
Force Causes Acceleration
 Any object that accelerates is
acted on by a push or pull-a
force of some kind.
– Kick the ball and it accelerates.
Force of hand
accelerates the
brick.
F
 Acceleration is caused by force. Twice as much
force produces
 Acceleration produced is
twice as much
directly proportional to the net
acceleration.
force, or
acceleration ~ net force
Dr. Jie Zou
PHY 1071
F
F
2
Friction
 When surfaces slide or tend to
slide over one another, a force of
friction acts.

An object sliding
– No friction exists on a crate that sits down an incline
at rest on a flat floor.
experiences friction
The direction of the friction force directed up the incline.
is always in a direction opposing
motion.
f
v
– When you apply a force to an
object, a force of friction usually
reduces the net force and reduces
the resulting acceleration.
Dr. Jie Zou
PHY 1071
3
An Example-Friction
 Example: A jumbo jet cruises at a constant
velocity of 1000 km/h when the thrusting force of
its engines is a constant 100,000 N. What is the
acceleration of the jet? What is the force of air
resistance.
 Answer:
– Constant velocity
acceleration is zero
net force
is zero
the air resistance is balanced out by the
thrusting force.
– So, force of air resistance on the jet = 100,000 N.
Dr. Jie Zou
PHY 1071
4
Mass and Weight
 The acceleration produced to an object depends on applied
forces, friction forces, and its inertia (the property of things
to resist changes in motion).
 Mass: The quantity of matter in an object; how much
inertia an object possesses depends on the amount of
matter in the object, or its mass.
– Mass is a measure of the inertia of a material object; the greater the
mass of an object, the greater its inertia.
– Think about why it is more difficult to slow down or stop a heavily
loaded truck than a Toyota Corolla.
Dr. Jie Zou
PHY 1071
5
Weight
 Weight: The force upon an object due to gravity
(gravitational attraction to the Earth).
 Mass and weight are directly proportional to one another,
G = mg.
– G: gravity (weight), m: mass, g: the constant of proportionality due
to gravity, also the acceleration of gravity (g = 9.8 m/s2  10 m/s2
on Earth).
– Units of weight: newtons (N), pounds
– Units of mass: kilogram (kg), gram (g)
 Think about how much you will weigh if you are on the
surface of the Moon? And how much your mass will be?
Dr. Jie Zou
PHY 1071
6
Mass Resists Acceleration
 The amount of acceleration
depends not only on the force,
but also on the mass being
pushed.
 For a given force, the
acceleration produced is
inversely proportional to the
mass, or
Acceleration ~ 1/mass
Dr. Jie Zou
PHY 1071
F
Force of hand accelerates
the brick
F
The same force accelerates
2 bricks ½ as much.
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Newton’s Second Law
 Newton’s second law gives the
relationship of acceleration to force
and inertia.
The force is applied in the
opposite direction of the
motion, speed is decreased.
v
F
 Newton’s second law:
– The acceleration of an object is directly
proportional to the net force acting on the
object, is in the direction of the net force,
and is inversely proportional to the mass of
the object.
– a = Fnet/m; a: acceleration produced by the
net force (m/s2), Fnet : the net force (N), m:
the mass of the object (kg).
 The acceleration of an object is
always in the direction of the net
force.
Dr. Jie Zou
PHY 1071
The force is applied in the
direction of the motion,
speed is increased.
v
F
Applied at a right angle, it
will deflect the object.
F
v
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Free Fall
 Galileo could not explain why
objects of various masses fall with
equal accelerations.
 Newton’s second law provides the
explanation:
– A falling object accelerates toward
the Earth because of the gravitational
force of attraction (gravity) between
the object and the Earth.
– The acceleration due to gravity is a
constant g, for the same locality.
Dr. Jie Zou
PHY 1071
m
F/m = g
2m
2F/2m = g
The acceleration of free fall
is independent of an object’s
mass.
Question: In a vacuum, a
coin and a feather fall
equally, side by side,
would it be correct to
say that equal force of
gravity act on both the
coin and the feather
when in a vacuum?
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Non-Free Fall
 In the presence of air resistance, the net force on a
falling object is less than the gravity-it is the
gravity minus air drag, the force arising from air
resistance.
 Air drag is opposing the direction of motion and
decreases the net force. Thus, a < g.
Dr. Jie Zou
PHY 1071
10
Homework
 Ch. 4, p. 71-72, Exercises: #14, 29, 41, 49.
 Ch. 4, p. 73, Problems: #2, 6.
 The above homework problems are
assigned from the 10th edition of the
textbook by Hewitt.
Dr. Jie Zou
PHY 1071
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