Physics (NSC013)

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Physics

Marie Jessica B. Alumaga

Objectives

• To describe the different branches of classical physics

• To appreciate the contributions of different scientists in the field of physics

• To explain the different ways of describing motion

• To state the three laws of motion and be able to solve problems involving motion

Physics

- study of matter, energy, space and time - divided into two main branches

• Classical physics

• Modern physics

Branches and Sub-branches of Classical Physics

A. Mechanics - the study of forces acting on bodies, whether at rest or in motionStatics – about forces acting on bodies at restKinematics – about motion without

regard to its causeDynamics – about motion and forces that affect it

Branches and Sub-branches of Classical Physics

B. Acoustics - the study of the production and propagation of sound waves

C. Optics – the study of light

• Physical optics – production, nature and properties of light• Physiological optics – part played by light

in vision• Geometrical optics – reflection and

refraction of light as encountered in the study of mirrors and lenses

Branches and Sub-branches of Classical Physics

D. Thermodynamics – study of the relationship between heat and other forms of energy

Branches and Sub-branches of Classical Physics

E. Electromagnetism – study of the properties of electric current and magnetism, and

their relationship• Electrostatics – electric charges at rest• Electrodynamics – moving charges• Magnetostatics – magnetic poles at rest

Branches and Sub-branches of Classical Physics

Marble bust of Aristotle. Roman copy after a Greek bronze original by Lysippus c. 330 BC.

Aristotle ((384 BC – 322 BC)

According to him, motion has two kinds:

a. Natural motion – thought to be either straight up or straight down

b. Violent motion – imposed motion, result of forces which pushed or

pulled

Archimedes (287 BC – 212 BC)

- introduced the screw to raise water efficiently

- used buoyancy determined whether the golden crown was less dense than solid gold

- used mirrors acting collectively as a parabolic reflector to burn ships attacking Syracuse

- introduced the lever

Give me a place to stand on, and I will move the Earth.

Galileo Galilei (1564 – 1642)

- first to use a telescope

- devised and improved the geometrical and military compass

- formulated the basic law of falling bodies

‘all things fall at the same rate’

Galileo's geometrical and military compass

A replica of the earliest surviving telescope

Sir Isaac Newton (1643 – 1727)

- lectured on optics

- built the first practical reflecting telescope

- introduced the universal law of gravitation

- stated the three laws of motion

A replica of Newton's second Reflecting telescope that he presented to the Royal Society in 1672[

Newton's law of universal gravitation

states that every massive particle in the universe attracts every other massive particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them

F magnitude of the gravitational force between the two point masses, NG gravitational constant, 6.674×10−11 N m2 kg−2 m1 mass of the first point mass, kgm2 mass of the second point mass, kgR distance between the two point masses, m

Newton's law of universal gravitation

How can the mass of the earth be determined using an apple?

Newton's law of universal gravitation

This illustrates the way scientists can use indirect methods to perform seemingly “impossible tasks”

2

2 2 6 224

11 2 2

Gravitational force on apple F

(9.8 / )(6.4 10 )6 10

6.67 10 /

GmM

R

gR m s mM kg

G N m kg

Speed the rate at which something

moves a given distance

s = distance/time

Velocity • a vector quantity that includes both speed and direction v = distance/time

Acceleration of an object • the rate of change of its velocity• for straight-line motion, average acceleration is the rate of change of speed a = velocityf – velocityi/t

MOTION

Newton’s

Laws of Motion

Newton’s first law of motion(Law of Inertia)

Inertia is a term used to measure the ability of an object to resist a change in its state of motion.

An object with a lot of inertia takes a lot of force to start or stop; an object with a small amount of inertia requires a small amount of force to start or stop.

The word “inertia” comes from the Latin word inertus, which can be translated to mean “lazy.”

Newton’s Second Law of Motion (Law of Acceleration)

The acceleration of an object is equal to the force applied divided by the mass of the object.

a = F m

Newton’s Second Law of Motion (Law of Acceleration)

If more force is applied to an object, the object accelerates at a higher rate.

Newton’s Second Law of Motion (Law of Acceleration)

If an object has more mass it accelerates at a lower rate because mass has inertia.

Newton’s Second Law of Motion (Law of Acceleration)

A cart rolls down a ramp. The cart has a mass of 500

grams (0.5 kg). Using a spring scale, you

measure a net force of 2 newtons pulling the car down.

Calculate the acceleration of the cart.

Sample problem 1

Solution: a = F/m

= 2 N/0.5 kg = 4 m/s2

Answer: 4 m/s2

An airplane needs to accelerate at 5 m/sec2 to reach take-off speed before reaching the end of the runway.

The mass of the airplane is 5,000 kilograms.

How much force is needed from the engine?

Newton’s Second Law of Motion (Law of Acceleration)

Sample problem 2

Answer: 25,000NSolution:F = ma = 5,000kg(5m/s2) = 25,000N

“For every action there is an equal and opposite reaction.”

Newton’s third law discusses pairs of objects and the interactions between them.

Newton’s third Law of Motion (Action-reaction)

Newton’s third Law of Motion (Action-reaction)

More examples

Three people are each applying 250 N of force to try to move a heavy cart.

The people are standing on a rug.

Someone nearby notices that the rug is slipping.

How much force must be applied to the rug to keep it from slipping?

Sketch the action and reaction forces acting between the people and the cart and between the people and the rug.

Sample problem

Newton’s third Law of Motion (Action-reaction)

Answer: 750N

Homework: (Use half cross-wise.)

Give three (3) examples of each of the Newton’s Laws of motion