Chapter 4 :
Chapter 4 Newton’s Second Law of Motion Mass :
Mass the quantity of matter in an object
the measurement of the inertia
measured in kilograms (kg) Weight :
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 Slide 4:
The weight of a 10 kg brick is...
A) 98 N
B) 10 kg
C) 9.8 kg
D) 10 N
E) 98 kg Mass and Weight should not be confused with... :
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 :
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). Slide 7:
Weight Location Mass Earth Moon Space 18.4 kg 18.4 kg 18.4 kg 180 N 30 N 0 N http://observe.phy.sfasu.edu/images/KC135-Summer2001/ NEWTON'S 2nd LAW OF MOTION :
NEWTON'S 2nd LAW OF MOTION F a F a F a F a F a F a Newtons’ Second Law :
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 :
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) 747 N
B) 4 kg
C) 1/4 kg
D) 4 m/s2
E) 30,000 kg times 9.8 m/s2 Example Questions :
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) 747 N
B) 4 kg
C) 1/4 kg
D) 4 m/s2
E) 30,000 kg times 9.8 m/s2 Example Questions :
Example Questions 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 :
Example Questions 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 Slide 14:
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. Slide 15:
If the net force is perpendicular to the velocity, the direction of the velocity changes. Slide 16:
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 example questions page 59, 61, 63 & 65. When the Acceleration is g... :
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 When the Acceleration is g... :
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... :
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... :
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... :
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... :
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. Slide 23:
Example: A heavy parachutists will fall faster than a light one.
When the force of air resistance is equal to weight of the falling object, the object will reach a Terminal Velocity. Section Titles :
Section Titles 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 In-Class Exercise :
In-Class Exercise Write down 2 examples of static equilibrium.
Write down 2 examples of dynamic equilibrium.
Write your own exam question that you would like to see on the exam. Slide 26:
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 :
Equations v = d / t
a = Dv / t
W = m g
SF = ma
Note: Bring a calculator to class for the remainder of the semester. End of Chapter :
End of Chapter When Acceleration Is Zero - Equilibrium :
When Acceleration Is Zero - Equilibrium Static Equilibrium
Velocity is zero
Examples: Hanging from a tree Weighing yourself on a set of scales Computer setting on a table Car parked on an incline Normal up Weight down Weight down Weight down Weight down Scales pushing up Tree
pulling up Normal Friction Slide 30:
Dynamic Equilibrium
Velocity is nonzero and constant
Examples:
Driving at constant velocity Force from road Weight down Friction Normal up Weight down Air resistance Terminal velocity in parachuting