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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