Download When the Acceleration is g

Newton’s
Second Law of
Motion
Mass
the quantity of matter in an object
the measurement of the inertia
measured in kilograms (kg)
Weight
the force upon an object due to
gravity
 Weight = Mass  Acceleration of gravity
W = mg
measured in Newtons (N) in the
metric system or pounds (lb) in the
British system
The weight of a 10 kg brick is...
•
•
•
•
•
A)
B)
C)
D)
E)
98 N
10 kg
9.8 kg
10 N
98 kg
Mass and Weight should not be
confused with...
Volume
• the quantity of space an object occupies
Density
• the quantity mass per unit volume
Mass and Weight
On the Moon the gravitational force
is only 1/6 as strong as on the Earth.
In space you are “weightless” but
not “massless”.
Your mass does not depend on
where your are.
• (e.g. Earth, Moon, or space).
Location
Mass
Weight
Earth
18.4 kg
180 N
Moon
18.4 kg
30 N
Space
18.4 kg
0N
NEWTON'S 2nd LAW OF MOTION
F
a
m
F
M
a
m
F
F
a
m
M
a
m
m
F
m
1
a
m
F
a
 
aF
or
M
a


F  ma
Newtons’ Second Law
 SF = m a
 The acceleration of an
object is directly
proportional to the
net force acting on
the object…
 …and inversely
proportional to the
mass of the object.
Example Questions
How much acceleration does a 747
jumbo jet of mass 30,000kg
experience in takeoff when the
thrust of all of the engines is
120,000N?
•
•
•
•
•
A)
B)
C)
D)
E)
747 N
4 kg
1/4 kg
4 m/s2
30,000 kg times 9.8 m/s2
Example Questions
How much acceleration does a 747
jumbo jet of mass 30,000kg
experience in takeoff when the
thrust of all of the engines is
120,000N?
•
•
•
•
•
A)
B)
C)
D)
E)
747 N
4 kg
1/4 kg
4 m/s2
30,000 kg times 9.8 m/s2
Example Questions
F
F
The same net force is applied to two
blocks.
If the blue one has a smaller mass
than the yellow one, which one will
have the larger acceleration?
Example Questions
F
F
The same net force is applied to two
blocks.
If the blue one has a smaller mass
than the yellow one, which one will
have the larger acceleration?
• A) Blue
• B) Yellow
If the net force is parallel to the velocity, then the speed of the
object increases.
If the net force is anti-parallel to the velocity, then the speed
of the object decreases.
If the net force is perpendicular to the velocity, the direction
of the velocity changes.
 Force and acceleration are vector
quantities.
 If v is parallel to F, speed increases.
 If v is antiparallel to F, speed decreases.
 If v perpendicular to F, direction of v
changes.
 See Check Yourself questions page 59,
61, 63 & 65.
Freely Falling Objects
Free fall is the motion of an object subject
only to the influence of gravity. The
acceleration due to gravity is a constant, g.
Freely Falling Objects
Free fall from rest:
When the Acceleration is g...
…the object is in Free Fall.
Consider a 1kg rock and a 1gram
feather.
– Which object weighs more?
• A. Rock
• B. Feather
• C. Neither
Freely Falling Objects
An object falling in air is subject to air
resistance (and therefore is not freely falling).
When the Acceleration is g...
…the object is in Free Fall.
Consider a 1kg rock and a 1gram
feather.
– On which is the gravitation force stronger?
• A. Rock
• B. Feather
• C. Neither
When the Acceleration is g...
…the object is in Free Fall.
Consider a 1kg rock and a 1gram
feather.
– Which object has more mass?
• A. Rock
• B. Feather
• C. Neither
When the Acceleration is g...
…the object is in Free Fall.
Consider a 1kg rock and a 1gram
feather.
– Which has a greater acceleration when
dropped from rest?
• A. Rock
• B. Feather
• C. Neither
When the Acceleration Is Less Than g...
…the object is not in Free Fall.
In this case there is a force other than
gravity.
That force is air resistance.
Air resistance depends on size, shape,
air density, and speed.
When Acceleration Is Zero...
 …we say the object is in
Mechanical Equilibrium.
 …the net force is zero.
 For Static Equilibrium the velocity is
zero.
 For Dynamic Equilibrium the velocity is
constant.
Important Equations:
When the Acceleration is g...
 SF=ma.......mg=ma........a=g
When the Acceleration Is Less Than g...
 SF=ma.......mg-R=ma........a=g-R/m
When Acceleration Is Zero...
 SF=ma.......mg-R=0........a=0
After jumping from an airplane a
skydiver will fall until the air
resistance equals her weight. At
that point...
– A) she opens her parachute
– B) she will fall no farther
– C) she will fall faster
– D) she will fall with constant speed
– E) she will hit the ground
Equations
v = d / t
a = Dv / t
W = m g
SF = ma
Note: Bring a calculator to class for
the remainder of the semester.
Real World Forces:
Gravity
Support (normal force)
Tension
Friction (static vs. kinetic)
When Acceleration Is Zero - Equilibrium
Scales pushing up
Static Equilibrium
Velocity is zero
Examples:
Computer setting on a table
Normal up
Weight down
Weighing yourself on a set of scales
Hanging from a tree
Car parked on an incline
Friction
Tree
pulling up
Weight down
Normal
Weight down
Weight down
Dynamic Equilibrium
Velocity is nonzero and constant
Examples:
Driving at constant velocity
Normal up
Air resistance
Force from road
Friction
Weight down
Terminal velocity in parachuting
Weight down