The Nature of ForceThe Nature of Force

Chapter 3 section 4

What is a forceWhat is a force

Force is a push or pull.

Forces are either balanced or unbalanced.

Unbalanced ForcesUnbalanced Forces Can cause an object to start moving, stop moving,

or change direction. Unbalanced forces acting on an object will change

the object’s motion. When 2 forces act in the same direction they add

together. When 2 forces act in opposite directions they add

together however one number is positive and one is negative.

When 2 equal forces act in opposite directions the object stays put.

What is the net force?What is the net force?

The net force is the overall force after all the forces are added together.

When there is a net force acting on an object the forces are unbalanced.

Balanced ForcesBalanced Forces

Equal forces acting on one object in opposite directions are called balanced forces.

Balanced forces acting on an object will not change the objects motion.

Newton’s First Law of MotionNewton’s First Law of Motion

An object at rest will remain at rest and an object moving at a constant velocity will continue moving at a constant velocity unless acted upon

by an unbalanced force.

InertiaInertia

INERTIA is the tendency of an object to resist change in motion.

Example: when the car stops suddenly and you continue to move forward.

MassMass

Mass is the amount of matter in an object.SI unit is the kilogram.The amount of inertia an object has depends

on its mass. The greater the mass the greater the inertia.

Friction and GravityFriction and Gravity

3.5

FrictionFriction

Friction is the force that one surface exerts on another when the two rub against each other.

Friction opposes motion.

Friction acts in a direction opposite to the object’s direction of motion.

Without friction an object Without friction an object would continue to move at a would continue to move at a

constant speed forever.constant speed forever. The strength of the

force of friction depends upon two factors: the types of surfaces involved and how hard the surfaces push together.

Is friction useful?Is friction useful?

Depends on the situation – helps us walk, ie. Tread on shoes and tires.

Different types of frictionDifferent types of friction

Sliding friction – solid surface sliding over one another.

Rolling friction – object rolls over a surface.

Fluid friction – objects move through a fluid.

Gravity – force that pulls the Gravity – force that pulls the object toward the earth.object toward the earth.

Free fall

Projectile motion

Air resistance

weight.

Weight vs. MassWeight vs. Mass

Weight is the measure of the force of gravity on an object.

Mass is the amount of matter in an object.

Weight = mass x acceleration due to gravity.

WeightWeight

Mass is measured in gramsWeight is a force measured in Newton’s.A kilogram of mass on Earth weighs 9.8N

Universal GravitationUniversal Gravitation

The law of universal gravitation states that the force of gravity acts between all objects in the universe.

Gravitational force depends on mass and distance between the objects.

Force, Mass, and AccelerationForce, Mass, and Acceleration

4.1

Newton’s 2Newton’s 2ndnd Law of Motion Law of Motion

The net force on an object is equal to the product of its acceleration and its

mass.

Force = Mass x AccelerationForce = Mass x Acceleration

Sometimes people refer to the equation as Newton’s 2nd Law.

Units of measurementUnits of measurement

One Newton = 1 kg x m/s/s

Force = ma

Acceleration = f/m

Air ResistanceAir Resistance

Objects falling through the air experience some type of fluid friction called air resistance.

Air resistance is not the same for all objects. Depends on surface area – the greater the surface area the greater the air resistance.

Air resistance increases with velocity. So as a falling object speeds up, the air resistance against it increases. Eventually the air resistance equals the force of gravity. When forces are balanced there is no acceleration.

Terminal VelocityTerminal Velocity

The greatest velocity an object reaches is called terminal velocity.

Free FallFree Fall

When the only force acting upon the falling object is gravity the object is said to free fall.

As an object falls it accelerates at a rate of 9.8 m/s/s

Changes in force and massChanges in force and mass

How can you increase the acceleration of the wheel barrel?

Increase Force Decrease Mass

Acceleration due to the force Acceleration due to the force of gravity.of gravity.

9.8 meters per second squared.After 1 sec. = 9.8 m/s/s2 sec. =19.6 m/s/s3 sec = 29.4 m/s/s

Objects in free fall accelerate at the same rate regardless of mass.

4:2 Projectile and Circular 4:2 Projectile and Circular MotionsMotions

Projectile MotionProjectile Motion

Projectile – an object thrown horizontally.

Will a projectile land on the ground at the same time as an object dropped straight down? Yes

Horizontal MotionHorizontal Motion

Motion that is parallel to the Earth’s surface.

Once the ball is thrown forward its horizontal velocity is constant if you ignore air resistance.

Vertical MotionVertical Motion

When you let go of the ball the ball has horizontal motion but since gravity starts to pull it downward it has vertical motion.

Not the ball has constant horizontal velocity with increasing downward vertical velocity.

Demo: Ball dropping straight down vs. ball being thrown as a projectile.

Motion along curvesMotion along curves

A change in acceleration is a change of speed or direction.

Centripetal AccelerationCentripetal Acceleration

Acceleration towards the center of a curve.

When riding a bike the rider experiences centripetal acceleration and they lean toward the inside of the curve.

Free FallFree Fall

When an object is influenced only by gravity it is said to be in free fall.

Space shuttles, satellites are in free fall – they fall around the Earth

Centrifugal ForceCentrifugal Force

This the force acting away from the center of a curved or circular path.

Action and ReactionAction and Reaction

10-4

Newton’s third Law of MotionNewton’s third Law of Motion

States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.

Equal but OppositeEqual but Opposite

Action and reaction forces will be equal and opposite, the same force acting on a greater mass results in a smaller acceleration.

Action – Reaction In ActionAction – Reaction In Action

Walking – you push on ground and the ground pushes on you.

Squid – Water jet

Do Action-Reaction Forces Do Action-Reaction Forces Cancel?Cancel?

Newton’s third law refers to forces on two different objects.

Example: Soccerball If one player hits the ball –

force is upward. The ball exerts an equal but opposite downward force on the player. The action and reaction forces are acting on different objects and therefore cannot be added together.

Momentum = mass x velocityMomentum = mass x velocity

A small mass can produce a large momentum if it is moving at a high velocity ie. Bullet.

Conservation of momentumConservation of momentum

The total momentum of any group of objects remains the same unless outside forces act on the objects.

Conservation means the conditions before and after some event.

Two Moving ObjectsTwo Moving Objects

Two snowboarders traveling the same direction bump into each other.

Momentum is conserved - The back person slows down while the front person speeds up.

Orbiting SatellitesOrbiting Satellites

10 - 5

Rocket LaunchRocket Launch

Rockets can rise into the air because the gases it expels with a downward force exert an equal but opposite force on the rocket.

What is a satellite?What is a satellite?

Any object that travels around another object in space.

Centripetal force is any force that causes an object to move in a circle.

Centripetal force is the gravitational force that pulls the satellite toward the center of the Earth.

Satellite MotionSatellite Motion

Satellites in orbit around Earth continually fall toward the Earth, but because Earth is curved they travel around it. A satellite is a projectile that falls around the Earth rather than into it.