6 Newton’s Second Law of
Motion  Force and Acceleration
Hewitt, P.G. 2009. Conceptual Physics
Objectives
• State the relationship between a and F and
between a and m. (6.1 and 6.2)
• State and explain Newton’s second law of motion.
(6.3)
• Describe the effect of friction on stationary and on
moving objects. (6.4)
• Distinguish between force and pressure. (6.5)
• Explain why the acceleration of an object in free
fall does NOT depend up on the mass of an object.
(6.6)
• Describe the effect of air resistance on a falling
object. (6.7)
Introduction
• Recall the definition of acceleration
accelerati on 
change in velocity
time interval
• Most objects in motion do not travel at a
constant velocity, but instead accelerate.
6.1 Force Causes Acceleration
If I kick the ball 2x
harder, its acceleration
is 2x greater. How
about 3x? 0.5x?
• Unbalanced forces
acting on an object
causes the object to
accelerate.
– Another way of saying
net force (F)
• An object’s
acceleration is directly
proportional to the net
force acting on it.
a  F
6.2 Mass Resists Acceleration
• For a constant net
force, an increase in
the mass will result in
a decrease in the
acceleration.
If the rhino was 2x as
massive, acceleration would
be half as much? What
about 3x bigger? 0.5x?
a
1
m
• The acceleration is
inversely proportional
to the mass.
6.3 Newton’s Second Law
• Acceleration is directly proportional to the
net force on the object,
is in the
same
So what happens
to acceleration
if both the and
net force
and
the
direction of the netmass
force,
is
inversely
are doubled? If net force is
doubled
massobject.
is
proportional to the mass
ofand
the
quadrupled?
a
F
m
• In equation form
a
F
m
Questions
1. A car has a mass of 1000 kg. What is the
acceleration produced by a force of 2000 N?
2. If the force is 4000 N, what is the
acceleration?
3. How much force, or thrust, must a 30 000 kg
plane produce in achieve an acceleration of
1.5 m/s2?
4. What kind of motion does a constant force
produce on an object of fixed mass?
6.4 Friction
• Friction is a force that resists motion.
– Acts when materials are in contact with each
other.
– Caused by irregularities on the surfaces of
materials.
– Magnitude of frictional force depends on the
kind of materials in contact and how much they
are pressed together.
• Friction can occur in fluids (liquids or gases).
• Air resistance is the friction that results when
an object moves through the air.
– Happens when the moving object has to push
aside the fluid it is moving through.
– Depends on the shape of the object and on the
speed of the object.
• More surface area in contact with fluid, more air
resistance.
• More speed, more air resistance.
• Free-body diagrams show all the forces acting
on an object.
• Below, the free-body diagrams show that
although an object has a force acting on it, the
object can move at a constant velocity
because of friction.
Questions
1. Two forces act on a book resting on a table:
its weight and the support force from the
table. Does the force of friction act on it as
well?
2. Suppose a jet cruises with a constant velocity
when the trust from its engine is a constant
80 000 N. What is the acceleration of the
jet? What is the force of air resistance acting
on the jet?
6.5 Applying Force—Pressure
• Pressure is the amount of force per unit area.
– For a constant force, an increase in the area of
contact will result in a decrease in pressure.
Pressure 
Force
area of applicatio n
• Units for pressure: Pressure is
measured in newtons per
square-meter, or pascals (Pa).
P 
F
A
Questions
1. In attempting to do the demonstration in
Figure 6.7 on the previous slide, would it be
wise to begin with a few nails and then work
upward to more nails? Why or why not?
2. The massiveness of the cement block plays
an important role in the demonstration.
Which provides more safety, a less massive
block or a more massive one?
6.6 Free Fall Explained
• All freely falling objects
fall with the same
acceleration because the
net force acting on the
object is only its weight,
and the ratio of weight
to mass it the same for
all objects.
Question
1. If you were on the moon and dropped a
hammer and a feather from the same
elevation at the same time, would they strike
the surface of the moon at the same instant?
6.7 Falling and Air Resistance
• Air resistance is directly proportional to both
the speed of the falling object AND the
object’s frontal area.
air resistance
 speed  frontal area
• The force due to air resistance reduces the net
force acting on the falling object.
• Terminal speed is the
speed at which the
acceleration of a falling
object is zero because
friction balances the
weight.
• Terminal velocity
Question
1. If a heavy person and a light person open
their parachutes together at the same
altitude and each wears the same size
parachute, who will reach the ground first?