Forces - Amazon S3Aristotle, Greek Philosopher, ca 335 BC He believed that terrestrial motion is...
Transcript of Forces - Amazon S3Aristotle, Greek Philosopher, ca 335 BC He believed that terrestrial motion is...
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.
Forces and Motion Cont’d:
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)
Forces and Motion Cont’d:
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)
Putting it all together…
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.
Putting it all together…
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
Putting it all together…
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.
Centripetal Force & Centripetal
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)
Centripetal Force & Centripetal
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