chapter 4 newton’s second law of motion newton's 2 nd law of motion f a m fa mm f a m m m fa...
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Chapter 4Chapter 4
Newton’s Newton’s Second Law of Second Law of
MotionMotion
NEWTON'S 2nd LAW OF MOTION
Fa
or amF
F am
F am
m
F a
m
m
m
F a
F a
F aM
m
a1
M
M
Newtons’ Second LawNewtons’ Second LawF = 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 QuestionsExample 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?
Example QuestionsExample 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?
F F
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 example questions page 60, 62 & 64.
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.
When Acceleration Is Zero - Equilibrium
Static EquilibriumVelocity 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 downWeight down
Scales pushing up
Tree pulling up
NormalFriction
Dynamic EquilibriumVelocity is nonzero and constantExamples:Driving at constant velocity
Force from road
Weight down
Friction
Normal up
Weight downAir resistance
Terminal velocity in parachuting
When the Acceleration is g...…the object is in Free Fall.Consider a 1kg rock and a 1gram
feather.– Which object weighs more?
• Answer: The rock
– On which is the gravitation force stronger?• Answer: The rock
– Which has a greater acceleration when dropped from rest?
• Answer: Both have the same acceleration, 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 and speed.
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.
After jumping from an airplane a skydiver will fall until the air resistance equals her weight. At that point...– A) she will fall with constant speed– B) she will fall no farther– C) she will fall faster– D) she opens her parachute – E) she will hit the ground
*
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