Law of Inertia:

*If an object is motionless, it will stay motionless unless acted upon by some force.

*If an object is moving, it will move in a straight line unless acted upon at an angle by some force.

*If an object is moving at a constant speed or velocity, it will continue at the speed unless acted upon by some force.

The Truck and Ladder

• an object in motion continues in motion with the same speed and in the same direction - unless acted upon by an unbalanced force.

Changing motion*A force can cause the motion of an object to change

Balanced and Unbalanced Forces

What is a Force ?Force is a Push or Pull on an

object.• Unit of Force is always

measured in Newton.

Balanced forces• do not cause a change in motion. • The force exerted by each person is

equal• but they are pushing in opposite

directions

• Balanced forces are equal and opposite in direction.

• Ex: 10 10 . newtons newtons

Balanced forces in action is in a tug of war.

• They are equal in size and opposite in direction.

• the resulting force is zero and there is no change in motion.

unbalanced forces • always cause a change in motion.• They are not equal and opposite.

• Unbalanced forces are unequal and opposite to each other.

• Example: . 22 2 Newton 8Newton

A net force is found by

subtracting two opposite forces. 9 N 5 N . .

9 N - 5 N = 4 N

What is Friction?

• appears whenever two things rub against each other

• No matter which direction something moves in, friction pulls it the other way

*Move left - pulls right. *Move up - pulls down.

Sticking together• Stronger the force pushing the

two surfaces together. • The micro welds will come in

contact.• To break these micro welds or

move one surface over other ,a force must be applied.

Four types of friction1. Static Friction 2.Sliding Friction3.Rolling Friction4.Fluid friction

Static Friction . Friction between two surfaces

that are not moving past each other.

Act in the direction opposite to that of the applied force

Sliding friction• when two solid surfaces slide

against each other.• Is less than static friction• Factors affecting sliding friction- include weight (normal force)- the stickiness of the two surfaces

Rolling Friction Friction between a rolling object

and the surface it rolls on. is much less than static or sliding

friction Ex:easier to pull a load in a

wagon rather than dragging it.

Fluid Friction Opposes the motion of an object

through the fluid .Ex: Stirring a cake batter Higher the speeds ,greater the frictionEx: Bicyclists and speed skaters wearing

a racing suits to reduce the air resistance

Free Fall and the Acceleration of Gravity

• A free-falling object is falling under the influence of gravity

• any object which is moving and being acted upon only be the force of gravity is said to be "in a state of free fall."

Elephant and Feather - Free Fall

• if the elephant weighs more and experiences a greater downwards pull of gravity compared to the feather, why then does it hit the ground at the same time as the feather?

Elephant and Feather - Free Fall

*In the absence of air resistance, the elephant and

the feather strike the ground at the same time.

Newton's second law • The force/mass ratio is the same for each. • The greater mass of the elephant requires

the greater force just to maintain the same acceleration as the feather.

Law of Gravitation Anything that has mass is attracted

by the force of gravity. The attractive force depends on the

mass of the objects. Gravitational acceleration is always

going to be 9.8 m/s2

•Horizontal . , .Motion a .. ...................Vertical . Motion

•When an object moves in a circle, two forces affect its

motion.•Inertia pulls it outward, trying to resist changes in

motion.

• attempts to pull the moving object toward the center of the circle that it is moving in.

Centripetal force

•If inertia &

centripetal force are equal (balanced), the object will move in a

circular path.

• Inertia tries to pull off road.

Friction tries to keep on road.

If inertia equals friction, car stays on road.

Newton's Second Law

• The net force acting on an object causes the object to accelerate in the direction of the net force

F is the net force in N, m is the mass of an object in kg and

a is its acceleration in m/s2.

Mass Vs. Weight

Fw = m. ag

Fw or w = force of weight (newton, N)

M = mass (kilogram, kg) Ag or g = gravitational

acceleration(9.8m/s2 or m/s/s)

Mass Vs. WeightMass Weight

Fw =mag Fw or w = force of weight

ag or g =gravitational

acceleration(9.8m/s2 )

1 Is always a constant at any time

Depends on gravity at the place

2 Is measured in kilograms in SI unit

Is measured in Newtons (Not in Kilograms as one might think

3 Is measured using balance Is measured using Scales

4 Can never be Zero Can also be Zero

5 Is an intrinsic (the essential nature) property of a body and is independent of external factors

Depends on

1.Mass of object which is attracting it

2.Force with which it is being attracted (which in turn depends on the distance between the two

1.What is the weight of an infant whose mass is 1.76kg?

2.What is the weight of a locomotive with a mass of 18,000kg?

3.An F-14’s mass is 29,545 kg. What is its weight?

4.What is the mass of a runner whose weight is 648 N?

5.A baseball has a mass of 0.5 kg. What is its weight?

6.A sled has a mass of 28 kg. What is the weight of the sled?

The Third Law of The Third Law of MotionMotion

Newton’s Third Law of MotionNewton’s Third Law of Motion

• States that for every action, States that for every action, there is an equal and opposite there is an equal and opposite reaction. reaction.

Action-Reaction Pairs

• All forces come in pairs • One object exerts a force on a One object exerts a force on a

second object and the second second object and the second object exerts a force on the first object exerts a force on the first that is equal in size and opposite in that is equal in size and opposite in directiondirection

Newton's Third Law of Motion

• For every action, there is an equal and opposite reaction.

Action-Reaction Pairs ExamplesAction-Reaction Pairs Examples

Propulsion of Fish Propulsion of Fish • A - Fish use fins to push water A - Fish use fins to push water

backwardsbackwardsR- Water pushes the fish R- Water pushes the fish forwardsforwards

Action-Reaction Pairs ExamplesAction-Reaction Pairs Examples

Flying motion of birdsFlying motion of birds• A - Wings of birds pushes air A - Wings of birds pushes air

downwardsdownwardsR - The air pushes the bird R - The air pushes the bird upwardsupwards

Action-Reaction Pairs ExamplesAction-Reaction Pairs Examples

Rocket PropulsionRocket Propulsion• A - Rocket engine pushes the A - Rocket engine pushes the

gases out gases out the back of the the back of the rocket rocket R - The gases push the car R - The gases push the car forwardforward

RememberRemember

• This law states that there are TWO objects BUT…

Only ONE Force

You Try Some

A swimmer is accelerating forward Action - The swimmer pushes

against the water Reaction - the water pushes back

on the swimmer and pushes her forward.

A ball is thrown against a wall

• Action - The ball puts a force on the wall

• Reaction - the wall puts a force on the ball so the ball bounces off.

A person diving off a raft

• Action - The person puts a force on the raft pushing it

• Reaction - raft puts a force on the diver pushing her in the opposite direction.

Last one!!!!

• Action - A person pushes against a wall

• Reaction - the wall exerts an equal and opposite force against the person

The flying motion of birds

The wings of a bird push air downwards.

the air must also be pushing the bird upward

The size of the force on the air equals the size of the force on the bird

Driving down the road, a firefly strikes the windshield of a bus

• Each force is the same size

Momentum is the property a

moving object has due to its mass and velocity.

Momentum =(mass)(velocity) .

p = m x v

A large object has a large

momentum because of its large mass.(example- a ship)

A fast moving object has a large momentum because of its large velocity.(ex-a bullet)

Law of conservation of momentum

*If no net force acts on a system , then the total momentum does not change*The loss of momentum of one = the gain in momentum of another object-momentum is conserved.

Gravity and Mass

• Gravitational force of two objects depends on the mass of the objects and the distance between the objects.

• Larger the masses exert great gravitational force

• Gravitational force decreases as distance increases.

Compare the forces of gravity acting between each pair of spheres and tell

how you reached your conclusion1a

1b

2a

2b

3a

3b

Reaction # 3

Jane is tired of pushing her VW van to the gas station. She gets an idea, “Hey I can go much faster when I am rollerblading than when I am walking. Why don’t I wear my roller blades?”

KEY TERMSKEY TERMS1. Friction: Resistance to motion that is

opposite the direction of travel. A force that opposed the motion of an object.

2. Air resistance: The force that opposes the movement of an object in air

3. Terminal velocity: Speed at which air resistance and gravity acting on a falling object are equal

4. Vacuum: Empty space

Falling ObjectsFalling Objects1. All falling objects in the Earth’s

atmosphere are subject to two forces:~ GRAVITY / AIR RESISTANCE

2. air resistance~ the greater the surface area the greater the air resistance

FORCEFORCEFORCEFORCE

Gravity Gravity

Falling ObjectsFalling Objects

3. When the air resistance equals the pull of gravity the object stops accelerating and stay at a constant velocity. Gravity

Air Resistance

Terminal Terminal VelocityVelocity

VacuumVacuum1. Vacuum:

~ empty space = no atmosphere~ only subject to one force:

GRAVITY

EARTH’S ATMOSPHERE

Gravity

Air Resistance

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VACUUM

Gravity

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