ForcesForces1

DynamiDynamicscs

FORCEMAN

What causes things to move?

•Forces•What is a force?

–A push or a pull that one body exerts on another.

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•Balanced3

No change in motion

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Unbalanced• If the forces acting on a body are

"unbalanced" this means that there is a "net" or "resultant" force.

• The body will do one of the following:

1. Speed Up

2. Slow Down

3. Change the Direction of its Motion

• In other words

• The Body will Accelerate

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NewtonsNewtons•SI unit of force is the Newton

(N).•Defined as the amount of

force that when acting on a 1 kg object produces an acceleration of 1m/s2.

•Therefore, 1N = 1 kg • m s2

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Newton’s First Newton’s First LawLaw

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9 Newton’s First Newton’s First LawLaw

Newton’s First LawNewton’s First Law• An object at rest stays at rest,

and an object in motion stays in motion unless acted upon by a net external force.

•What do you mean by “Net Force”?

•Net means: Final Sum, so net external force is the sum of all the forces acting on the object.

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Newton’s First Newton’s First LawLaw

• Inertia is the tendency of an object to resist any change in its motion.

•Newton’s First Law is also called the Law of

Inertia.

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Why does the ladder continue to move?

Inertia!

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Why does the person continue to move?

Inertia!Clip

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Newton’s Second Law• The acceleration of an object is

directly proportional to the net external force acting on the object and inversely proportional to the object’s mass.

•F=ma• Clip

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•Force = mass x accelerationNewton's 2nd Law of

Motion:

Mass is in kg Acceleration is in m/s2

Force is in kg x m/ s2

(or N)

F = (ma)

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Force Problems:What force is needed to give an

object with a mass of 25.0 kg an acceleration of 15.0 m/s2 ?

Given: Unknown:m = 25.0 kg

a = 15.0 m/sec²

F = ?

Formula

F = ma

F = (25 kg)(15 m/sec²)

F = 375 kg•m/sec² = 375 N

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Newton’s Third Law

"Every action has an equal

and opposite reaction."

Newton’s Third Law of Motion

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……to every action to every action force there is an force there is an

equal and opposite equal and opposite reaction forcereaction force

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Reaction forces act

on different objects

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Review Newton’s Laws Clip

Forces that

oppose motion

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-Friction-Friction is a force is a force that opposes that opposes motion between motion between two surfaces that two surfaces that are touching each are touching each other.other.

Forces that oppose motion

Friction Song

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Friction Song

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-Air resistance is the force air exerts on a moving object

Forces that oppose motion

Friction

• What Causes Friction?• Microscopic, electrostatic

interactions between contacting surfaces

• Three types of friction:1. Static Friction2. Sliding Friction3. Rolling Friction

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Types of Friction Types of friction: 1. Static – force required to

overcome inertia of a stationary object

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Static Friction (FS)22

To decrease friction, surfaces can be lubricated with a liquid such as oil.

3. Rolling – force required to keep an object rolling at a constant speed

2. Sliding – force required to keep an object sliding at a constant speed

Types of Friction (cont’.)23

FRICTION LAB

CLIPCLIP

The amount of force required to overcome static friction is always greater than the force required to

overcome sliding or rolling friction.

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GRAVITY!! • The force of gravity is an attractive force The force of gravity is an attractive force

between objects.between objects. • The amount of The amount of GRAVITATIONAL FORCEGRAVITATIONAL FORCE

between objects depends upon their:between objects depends upon their:

• 1- Masses 1- Masses (greater mass means more (greater mass means more gravity)gravity)

• 2- Distance apart 2- Distance apart (greater distance (greater distance means means

less gravity) less gravity) *All objects fall toward the earth at the same rate of acceleration, regardless of

their masses!

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Calculating the Gravitational Force

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27 Gravitation

Weightlessness Weightlessness

Gravity in Gravity in SpaceSpace

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Moon

Moon Hammer & Feather

Free fall and Terminal Free fall and Terminal VelocityVelocity• The opposing force of air resistance increases with speed.

• Objects accelerate towards the Earth until the force of gravity is cancelled or balanced by this air resistance.

• This is the highest speed an object can reach.

• Different objects have different terminal velocities.

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Weight and Gravity30

•Weight is the

force of gravity

between the

earth and a body

on its surface.

•Weight ≠ mass!

•Weight is

measured in

Newtons (N). BECAUSE IT’S A FORCE!

W =m x g

Could your weight

Could your weight

change by simply change by simply

going to the top of

going to the top of

a mountain?a mountain?

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Weight•When the mass of an When the mass of an

object and the object and the acceleration due to acceleration due to gravity are known, the gravity are known, the weight of an object can weight of an object can be calculated.be calculated.

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Ex. Mass = 100kg Acc.= 9.81 m/s2Weight = 981 N

W = (g)(m)

Example Problem• What is your weight if your mass

4.52 kilograms?Given: Unknown:m = 4.52 kg W = ?g = a = 9.8 m/sec²

Formula: W = mg

W = (4.52kg)(9.8 m/sec²)W = 44.296 kg•m/sec² = 44.296 N

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Projectile MotionProjectiles follow a curved path

because of the Earth’s gravitational pull. They have 2 types of motion that are

independent of each other:

1. Horizontal motion

2. Vertical motion

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Projectile MotionWhich will hit the ground first?

A ball that is dropped straight down or a ball that is “shot” out horizontally?

* An object launched horizontally will land on the ground at the same time as an object simply dropped from the same height!!!!

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Another Example of Projectile Motion39

Circular Motion• An object moving in a

circular motion at the same speed is accelerating toward the center because its direction is constantly changing. Centripetal acceleration

is acceleration toward the center of a curved path.

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'Centripetal' comes from the Latin word for 'center seeking'

only centripetal is the real

force

Centrifugal 'force' is really a function of the inertia of the object being pushed into a circle. It is not really a force at all, it is simply the tendency of an object to go in a straight line.

Roller Coaster Clip

Circular Motion cont.’• Centripetal

force is the force that causes a

moving object to move in a curved path

Without a centripetal

force, an object in motion

continues along a straight-line

path.

With a centripetal

force, an object in motion will be accelerated and change its

direction.

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How do How do rollercoastersrollercoasters work!?!work!?!

Amusement Park Forces

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