PHSC 1011PHSC 1011

Chapter 1Patterns of Motion

and Equilibrium

1.1 Aristotle on Motion Two Kinds of Motion – Natural and Violent

Natural Motion depends on the Nature of the Object.

An object has a “proper place” and, if not there, will strive to get there.

Natural Motion is straight up or straight down.

All motion on the Earth is linear .

All motion in the heavens (outer space) is curved .

1.1 Aristotle on Motion

Violent Motion is imposed motion caused by pushing or pulling.

Motion continues only so long as there is an applied force to an object. Removing the force stops the object.

The speed at which an object falls is directly related to the mass of an object .

It would take a man named Galileo to start the wheels of change in this field of knowledge called physics.

1.2 Galileo’s Concept of 1.2 Galileo’s Concept of InertiaInertia

According to Galileo, the motion of a According to Galileo, the motion of a falling object is independent of mass - two falling object is independent of mass - two objects of unequal masses will fall to the objects of unequal masses will fall to the ground when dropped from a set height in ground when dropped from a set height in equal timesequal times

The reason why objects did not always The reason why objects did not always fall in the same time was because of a fall in the same time was because of a retarding force called friction.retarding force called friction.

Once an object is set in motion, it will Once an object is set in motion, it will remain in motion without the application remain in motion without the application of a force except as necessary to overcome of a force except as necessary to overcome friction.friction.

1.2 Galileo’s Concept of 1.2 Galileo’s Concept of InertiaInertia

Galileo did an experiment by rolling balls in a tray with sloped sides.

He observed that the balls increased in speed going downhill, maintained constant speed on the level, and decreased in speed going uphill.

He concluded that if friction were not present, the ball moving on the level would continue at the same speed indefinitely.

He also noted that objects at rest tend to remain at rest.

This resistance an object has to a change in its state of motion is called INERTIA.

1.2 Galileo’s Concept of 1.2 Galileo’s Concept of InertiaInertia

1.3 Mass – a Measure of 1.3 Mass – a Measure of InertiaInertia

The amount of inertia an object The amount of inertia an object possesses depends upon the amount possesses depends upon the amount of matter in the object – the more of matter in the object – the more matter, the greater the inertia.matter, the greater the inertia.

The term, The term, MASSMASS, is used to describe , is used to describe the amount of matter in an object. the amount of matter in an object. The greater and object’s mass, the The greater and object’s mass, the greater the magnitude of its inertia.greater the magnitude of its inertia.

Therefore, mass can be used as a Therefore, mass can be used as a measure of an object’s inertia.measure of an object’s inertia.

1.3 Mass – a Measure of 1.3 Mass – a Measure of InertiaInertia

Mass Versus WeightMass Versus Weight

Mass is a measure of the amount Mass is a measure of the amount of material in an object and also of material in an object and also it’s inertia.it’s inertia.

Weight is the force upon an Weight is the force upon an object due to gravity.object due to gravity.

Mass and Weight are Mass and Weight are proportional to each otherproportional to each other

1.3 Mass – a Measure of 1.3 Mass – a Measure of InertiaInertia

Mass Versus WeightMass Versus Weight

Mass is measured in kilograms Mass is measured in kilograms (kg).(kg).

One kilogram weighs 9.8 newtons One kilogram weighs 9.8 newtons (N).(N).

One kilogram of mass weighs 2.2 One kilogram of mass weighs 2.2 lb.lb.

One pound is equivalent to 4.45 N.One pound is equivalent to 4.45 N.

1.3 Mass – a Measure of 1.3 Mass – a Measure of InertiaInertia

Mass Versus WeightMass Versus Weight

1.4 Net Force1.4 Net Force

A force is a push or a pull.A force is a push or a pull. An object does not speed up, slow down, An object does not speed up, slow down,

or change direction unless a force acts or change direction unless a force acts upon it.upon it.

An object moving through the An object moving through the atmosphere and/or in contact with the atmosphere and/or in contact with the Earth have a variety of forces acting Earth have a variety of forces acting upon it, for example: aerodynamic drag, upon it, for example: aerodynamic drag, gravity, and mechanical friction.gravity, and mechanical friction.

The The NET FORCENET FORCE on the object is a on the object is a combination of all forces acting upon it.combination of all forces acting upon it.

1.4 Net Force1.4 Net Force

1.5 The Equilibrium Rule1.5 The Equilibrium Rule

When the combination of all forces When the combination of all forces acting upon an object add up to a acting upon an object add up to a zero net force (that is – they cancel zero net force (that is – they cancel each other out) the object is said to each other out) the object is said to be in Mechanical Equilibrium.be in Mechanical Equilibrium.

F = 0F = 0

1.5 The Equilibrium Rule1.5 The Equilibrium Rule

1.6 Support Force1.6 Support Force

When an object sits on a surface, When an object sits on a surface, gravity pulls the object downward.gravity pulls the object downward.

Since the object remain motionless, Since the object remain motionless, the net force must be zero.the net force must be zero.

Therefore the surface is exerting a Therefore the surface is exerting a Support ForceSupport Force upward upon the upward upon the book.book.

1.6 Support Force1.6 Support Force

1.7 Equilbrium of Moving 1.7 Equilbrium of Moving ThingsThings

Equilibrium is a state of no Equilibrium is a state of no change.change.

An object at rest is at equilibrium.An object at rest is at equilibrium. An object moving at a constant An object moving at a constant

speed in a straight line is in speed in a straight line is in equilibrium.equilibrium.

Question: Can an object being Question: Can an object being subjected to a single force be in subjected to a single force be in equilibrium?equilibrium?

1.8 The Force of Friction1.8 The Force of Friction

Friction always acts in a direction to Friction always acts in a direction to oppose motion.oppose motion.

The amount of friction between two The amount of friction between two surfaces depends on the kinds of surfaces depends on the kinds of material and how tightly they are material and how tightly they are pressed together.pressed together.

Friction is caused by tiny surface Friction is caused by tiny surface bumps and the “stickiness” of the bumps and the “stickiness” of the atoms on the surfaces of the two atoms on the surfaces of the two materials.materials.

1.8 The Force of Friction1.8 The Force of Friction

1.8 The Force of Friction1.8 The Force of Friction

1.9 Speed and Velocity1.9 Speed and Velocity Speed is the distance traveled in a given time.Speed is the distance traveled in a given time.

Speed =Distance TraveledTravel Time

Instantaneous Speed is the speed at a given instant.Instantaneous Speed is the speed at a given instant. Average speed is the total distance covered divided by the total time for the trip.Average speed is the total distance covered divided by the total time for the trip.

1.9 Speed and Velocity1.9 Speed and Velocity

Velocity is combination of Speed and Velocity is combination of Speed and Direction of Travel.Direction of Travel.

Constant Velocity is constant speed Constant Velocity is constant speed and constant direction – motion in a and constant direction – motion in a straight line at constant speed.straight line at constant speed.

All things in the universe are moving.All things in the universe are moving. When we discuss speed or velocity, we When we discuss speed or velocity, we

are talking about speed or velocity are talking about speed or velocity relative to something else (usually the relative to something else (usually the surface of the Earth).surface of the Earth).

1.10 Acceleration1.10 Acceleration

When the rate of motion of an object When the rate of motion of an object changes, we say that the object is changes, we say that the object is undergoing undergoing AccelerationAcceleration..

AccelerationAcceleration is defined as the is defined as the rate of change rate of change of velocityof velocity..

Acceleration =Change in VelocityTime Interval

1.10 Acceleration1.10 Acceleration

1.10 Acceleration1.10 Acceleration

1.10 Acceleration1.10 Acceleration