forces - amazon s3aristotle, greek philosopher, ca 335 bc he believed that terrestrial motion is...

Forces

Chapter 3

Physical Science 2018-2019

A force is a push or pull that transfers energy to an

object.

A force can be the cause of change in motion of an object.

Balanced and Unbalanced Forces:

Balance forces are forces

of equal sizes and exerted

in opposite directions.

Results in no change in

motion.

Unbalanced forces are

forces that are not

opposite and equal.

This always causes a

change in motion.

Combining Forces:

A combined force of 2 unbalanced forces exerted in opposite directions is the difference between the forces. eg. Tug-of-war

A combined force of 2 unbalanced forces exerting in the same direction is the sum of the forces.

Aristotle, Greek Philosopher, ca 335 BC

He believed that terrestrial

motion is always linear and

always come to a halt.

i.e. The natural state of objects is to

be at rest; a constant force is needed

for an object to remain in motion.

e.g. Earth moves down, fire moves

up to their place of rest.

Sir Issac Newton, Physicist, ca 1687

Newton stated that objects in motion remain in motion unless acted on by some force.

Aristotle failed to recognize the influence of friction on Earth.

Friction opposes the motion of an object.

Types of Friction:

Sliding friction is between 2 solid surfaces.

The amount of friction depends on the weight of the objects and texture of their surfaces.

Rolling friction is between a round solid object and a solid surface.

There is less opposition to motion than sliding friction.

Fluid friction is friction of liquids and gases.

There is less opposition to motion than rolling friction.

Friction cont.:

An increase in friction may be desirable.

e.g. Studded snow tires, rough surfaces on

boat ramps, treads on tennis shoe soles increase friction.

Newton’s Law of Universal Gravitation

Gravitational force increases as the mass of

either object increases and as the objects move

closer.

Newton applied Johannes Kepler’s laws of

orbital motion (ca. 1594) and postulated

that all objects in the universe attract each

other by the force of gravity.

https://youtu.be/Jk5E-CrE1zg

Gravity Attractive force between any two objects that

depends on the mass of the objects and the

distance between them.





Newton’s Law of Universal

Gravitation:

Factors affecting gravity include:

1. Mass of the objects -

The force of gravity between 2 objects is

directly proportional to their masses.

The force of gravity is only observable if

the mass of an object is very large (large

asteroid, moon-sized, or bigger).

Newton’s Law of Universal

Gravitation:

The force of gravity between 2 objects is inversely proportional to the distance between the objects.

e.g. The moon is the main cause of tides (not the sun) because it is much closer to Earth.

2. Distance between the objects-

Weight and Mass:

Mass is the amount of matter in an object.

Mass is constant regardless of orientation.

Weight is the measure of force of gravity

between 2 masses.

Weight varies as an object moves farther from

the center of gravity.

Weight and Mass:

Mass and weight are not the same, but are

related.

Weight = mass x acceleration due to gravity

or

Weight (N) = mass (kg) x 9.8 m/s2

Weight and Mass:

Conversion of mass in grams to weight

on Earth.

mass in g /1000 x 9.8 m / s2 = N

Conversion

Grams kilograms

Forces and Motion:

Law of Inertia - Newton’s First Law of Motion An object at rest will remain at rest and an object in motion will remain in motion at constant velocity unless acted on by an unbalanced force.

Based on inertia (Latin word iner meaning “idle” or “lazy”)

Inertia is the tendency of matter to remain in motion or stay at rest, inertia resists a change in motion.

An object’s inertia is determined by its mass.

Forces and Motion Cont’d:

Law of Acceleration –

Newton’s Second Law of Motion

An unbalanced force accelerates an object in the direction of that force.

The smaller the mass of an object, the greater the acceleration of that object if the force remains the same.

i.e. The acceleration of an object is directly proportional to the net force acting upon it.


Force = mass x acceleration

or

F = ma

Where:

F = force measured in newtons (N)

m = mass measured in kilograms (kg)

a = acceleration measured in

meters/seconds/seconds (m/s2)


Law of Interaction - Newton’s third law of motion

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

i.e. Forces act in pairs.

e.g. The prop on a boat motor pushes water behind the boat, the boat is pushed in the opposite direction forward.

Putting it all together… Free Fall Motion

Galileo Galilei, Italian physicist ca. 1600, discovered that the gravity of Earth affects all objects equally.

i.e. Any object of any size will accelerate during free fall at a rate of 9.8 m/s2 near the Earth’s surface in the absence of air resistance.

https://youtu.be/E43-CfukEgs




Putting it all together…

Q. How much will an object accelerate in 5

seconds of free fall?

A. 49 m/s

Q. Why doesn’t a parachutist continue to

accelerate?

A. Air resistance (fluid friction)


Terminal velocity

is reached when air resistance equals

gravitational pull and the object falls at a

constant velocity.

i.e. Both forces become balanced.

Putting it all together… Projectile motion is the

path of a projectile

(hurled object) that

begins somewhat

horizontally.

Acceleration due to

gravity of a projectile

is still 9.8 m/s2.

Projectile motion is

always a curved path.


Q. Which of these stars will hit the ground

first?

A is dropped and B is hurled in projectile motion.

A B

https://youtu.be/zMF4CD7i3hg




Orbital motion is similar to projectile

motion; but the inertia of an object and

gravity becomes balanced.

Centripetal Force & Centripetal

Acceleration:

Centripetal force is the force required to change the direction of a moving object.

Fc = mac

Where:

Fc = centripetal force

m = mass (kg)

ac = centripetal acceleration (m/s2)

Since a change in direction is a change in velocity and the rate of change in velocity is acceleration, an object subjected to centripetal force is accelerating.


Acceleration Cont’d:

Centripetal acceleration requires a force

be directed toward the center of a circular

path.

ac = v2/r

Where:

ac = centripetal acceleration

v = velocity (m/s)

r = radius (m)


Acceleration Cont’d: Q. A 4.0 kg ball is attached to a 0.7 m

string and spun at 2.0 m/s. What is the

centripetal acceleration?

ac = v2/r

= (2.0 m/s)2 / 0.7 m

= 5.71 m/s2


Acceleration Cont’d:

Q. What is the centripetal force?

Fc = mac

= 4.0 kg x 5.71 m/s2

= 23 N

forces - amazon s3aristotle, greek philosopher, ca 335 bc he believed that terrestrial motion is...

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