Chapter 1: The Science of Physics

1.1 The Science of Physics

1.2 Scientific Inquiry and Natural Laws

1.3 The Nature of Scientific Knowledge

1.1 The Science of Physics

Physics provides the foundation upon which rests the human understanding of other major areas in science.

Biology: everything in biology follows the laws of physics

Chemistry: is the science of matter and energy.

Physics: provides the ground rules for how matter and energy behave.

1.1 A grand tour of physics

What do nerves, cell phones, and lightning have in common?

1.1 Matter and Energy Our universe is made of matter and energy.

Matter is “stuff” that has mass and takes up space.

Energy is the ability to make things change.

1.1 Electricity and energy Energy is active.

Things change by exchanging and transforming energy.

Electricity is useful because electrical energy can easily be transformed into many other kinds of useful energy.

1.1 Energy

Energy is the fundamental constituent of the universe.

Yet, in its pure form, energy cannot be tasted, touched, seen, smelled, or heard.

1.1 Energy and joules

All types of energy are measured in tiny units called joules.

One joule is the amount of energy it takes to raise a pint of ice cream about 21 centimeters.

1.1 Energy and Electricity Electrical energy is carried by an electric

current.

Electric current is measured in amperes, or amps.

1.1 Energy and Electricity

Electric current flows in response to differences in electrical energy.

Electrical energy is measured in volts.

1.1 Energy and Electricity A regular alkaline battery has

a voltage of 1.5 volts.

That means each amp of current flowing through the battery carries 1.5 joules of energy each second.

The electrical outlet on the wall has an energy rating of 120 volts.

That means each amp of current carries 120 joules of energy each second, which is one reason outlets are more dangerous than batteries.

1.1 Mechanics

In physics, however, mechanics is the science of forces and motion.

Mechanics has two parts: dynamics and statics.

Understanding motion is the dynamic part of mechanics.

Understanding why things stand still or break is the static part of mechanics.

1.1 Force Mechanics starts with the idea of force.

A force is an action such as a push or a pull.

Your weight is a force from the gravity of Earth pulling on the mass of your body.

Forces are the cause of action, even when nothing is moving.

1.1 Newton’s first law Changes in motion such as

starting, stopping, turning, speeding up, and slowing down require forces.

An object at rest stays at rest, and an object in motion continues in motion at the same speed and in the same direction forever.

In theory, a kicked ball on a level sheet of frictionless ice would travel forever in a straight line.

1.1 Equilibrium and statics In any normal situation there are

always forces acting, one of which is usually gravity.

If nothing is moving, then physics tells us at least two forces must be acting, with one canceling the other.

The forces are in equilibrium.

This is true even when you don’t know about a second force.

1.1 Equilibrium and net force If we use the value +2 to represent the

force on the right, then –2 represents the force on the left.

Adding +2 and –2 gives a total of zero or zero net force.

Cutting a string removes one force.

The result is that the total or net force is no longer zero, and the block moves.

1.1 Equilibrium and dynamics At the very instant the string is cut, the

block is not yet moving.

It immediately starts moving but it starts from rest.

The force from the connected string accelerates the block by causing its speed to increase.

Unbalanced forces cause objects to accelerate in the direction of the force.

1.1 Harmonic Motion Compare a ball rolling downhill with a swing

going back and forth.

The rolling ball is one kind of motion, with characteristics of speed and position.

The swing is another kind of motion because it repeats.

1.1 Oscillators A swing is an example

of an oscillator.

An oscillator is an object or system that repeats over and over again in cycles.

Many examples of oscillators occur in nature and also in human technology.

1.1 Frequency and cycle The frequency of a repeating motion is

how often it occurs in a given unit of time.

A frequency of once per second means the motion repeats once per second.

A typical swing has a frequency of about of a cycle per second.

A cycle is a complete back-and-forth movement of the swing.

1.1 Waves

Waves carry information and waves carry energy.

A wave spreads its frequency wherever it travels.

1.1 SoundSound is a wave of air pressure.

When a guitar string vibrates back and forth, the soundboard of the guitar pushes the air that touches it back and forth.

Like the water wave, that air pushes the air next to it, which pushes the air next to it, and so on, all the way to your ear drum.

1.1 Light Light is a very rapid wave of electricity

and magnetism.

If you could move a magnet up and down 1 million times per second, you would have a radio wave.

Now imagine moving the magnet up and down 5 thousand trillion times per second (5 × 1015 Hz).

You would make red light!

1.1 Light The colors between red and blue in the

rainbow represent the range of light energy that our eyes can detect.

However, light has an infinite range of energies.

1.1 Heat and Energy

All matter contains heat energy.

The higher the temperature, the more heat energy there is.

You put energy in to make something warmer.

You take energy out to make something colder.

1.1 Heat and Energy

Whenever anything gets warmer or colder, energy is being exchanged.

If you have 1 gram of water, adding 4.18 joules of energy makes the temperature increase by exactly 1 degree Celsius.

1.1 Heat and Energy

Many of the most useful human inventions ultimately transform heat into other forms of energy.

1.1 Heat and atoms In 1827, Scottish botanist Robert Brown

noticed that a speck of pollen floating in water moved around in a jerky, and continuously agitated way.

Brownian motion comes from the impacts of trillions of atoms, constantly moving and jostling each other.

Brownian motion is related to atomic motion, which is dependent on temperature.

1.1 Heat and atoms Energy of motion is

called kinetic energy.

Heat is actually kinetic energy.

The “feeling of heat” is the energy moving between more-energetic atoms to the less-energetic atoms.