# AST101H5 Chapter Notes - Chapter 4: Net Force, Orbital Period, Figure Skating

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Chapter 4
Making Sense of the Universe
Speed tells us how fast an object will go in a certain amount of time.
Velocity tells us both the speed and the direction.
Acceleration an object has acceleration if its velocity is changing in anyway,
whether its speed or direction.
In science, accelerating is used when you are slowing down or turning,
not just motion.
Slowing down occurs when acceleration is in a direct opposite to the
motion acceleration = negative, causing velocity to decrease.
Turning changes your velocity because it changes the direction which you
are moving even if your speed is constant.
You don’t feel effects when moving at a constant speed.
Acceleration of Gravity:
- Galileo demonstrated that gravity accelerates all objects by the same amount,
regardless of their mass.
Air resistance causes a difference of acceleration.
o If you dropped an object on the moon, where there is no air, they
would both fall at exactly the same rate.
- Acceleration of a falling object is called the acceleration of gravity (G).
Earth causes the acceleration of gravity to fall faster by 9.8 m/s or 10 m/s
each passing second.
o Suppose you drop a rock from a tall building, the moment you let it
go, its sped is about 10 m/s. After 1 second, the rock will be falling
at about 10 m/s and after 2 seconds, it will fall at 20 m/s.
o Absence of air resistance will cause speed to increase by about 10
m/s each second until it hits the floor.
o Therefore the acceleration of gravity is 10 meters per second, per
second (10m/s^2)
Momentum the product of its mass and its velocity = mass X velocity.
- The only way to change an objects momentum is to apply force to it.
two concepts that show relationship between interaction of objects.
Example: Truck and bug move at 30 km/hr. If bug slams into you at that
speed, bug will squish and cause little damage because it did not exert
that much speed. However, if a truck slams into you at 20 km/hr, it will
cause more damage. This is due to the fact that it imparted enough of its
momentum to cause dramatic and sudden change in your car’s
momentum. You feel this sudden change in momentum as a force.
Mere presence of a force doesn’t always cause a change in momentum.
o Ex. Car is always affected by air resistance and friction. Forces will
slow your car if you take your foot of the gas pedal. You an
maintain a constant a constant velocity, constant momentum if
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Chapter 4
Making Sense of the Universe
you apply enough force to overcome these slow effects of these
forces.
Forces are always present such as force of gravity or electromagnetic
forces acting between atoms.
Net Force (overall force) acting on an object represents the combined
effect of all the individual forces put together.
o There is no net force on your car, because force from engine to
move wheels, offsets forces of air resistance and road friction.
o A change of momentum occurs when the net force is not zero.
A net force that is not zero causes an object to accelerate.
An object must accelerate whenever a net force acts on it.
o Planets accelerating as they orbit the sun beause travel constantly
changes as they go around their orbits… Some force must be acting
this acceleration.
Mass vs. Weight:
- Bathroom scale = measures your weight NOT mass… doesn’t matter when we are
In Astronomy however:
Weight/Apparent Weight is the force that a scale measures when you
stand on it. Depends on your mass and the forces acting on your mass
(including gravity).
o Example: elevator with scale. When moving upward = greater
force. Downward = lower weight. Shows different weight when
elevator is accelerating, not going up/down at a constant speed.
Your mass is the same no matter where you are, but your weight can vary.
o Example: Moon = your mass is the same as on Earth, but your
weight is less, because the forces of gravity is weaker.
Free-fall falling without resistance to slow you down.
Free-fall makes you weightless.
o Jumping off a chair or of a diving board/trampoline. We
experience it many times in our lives.
- Astronauts are in free-fall, reason to why they are weightless.
They are in constant state of free-fall.
Imagine jumping off a tower, the faster you run from it, the farther you go
from falling from it.
If you go fast enough (28,000 km/hr) you would move far enough, that
you’d not be going down at all.. you’d be high above earth as you were
before = you would be orbiting Earth.
The Space Shuttle, Space Station and all other objects orbiting stay in
orbit because they are constantly “falling around” Earth. Their constant
state of free-fall makes these spacecraft and everything in them
weightless.
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Chapter 4
Making Sense of the Universe
Newton’s Law of Motion:
- Born in Lincolnshire, England on Christmas Day 1642
- Had inspiration in 1666 when he saw an apple fall to the ground, realizing that the
gravity that made it fall was the gravity held the moon in orbit around the Earth.
Shattered the remaining vestiges of the Aristotelian view of the world, for
which centuries has been accepted.
o Belief in an Earth-centered cosmo. Believed that heavens were
completely distinct from Earth and Earth did not apply to heavenly
motion.
When he saw the apple fall, the Copernican revolution had displaced
earth from a central position and Galileo’s experiments had shown that
the laws of physics were not what Aristotle believed.
- Showed that the same physical laws that operate on Earth as well as in the heavens.
Were brought together as one universe for the first time in history.
- Heralded the birth of the modern science of astrophysics (applies physical laws
discovered on Earth to phenomena throughout the cosmos).
- He quantified the laws of motion and gravity, conducted crucial experiments
regarding the nature of the light, build first reflecting telescope, and invented the
mathematics of calculus.
- Published the laws of motion and gravity in “Mathematical Principle of Natural
Philosophy” (principia).
Laws:
1) Newton’s First Law: An object moves at constant velocity of there is no net
force acting upon it.
Example: If a car were in space, moving at a constant speed (no friction or
air resistance), then it would keep on moving forever.
o This is why spaceships needs no fuel to keep going after they are
launched into space (or other objects in space).
Explains why you don’t feel sensation of motion on an airplane because it
is moving at a constant velocity (no net force is acting upon it). You can
walk around, sit, sleep as though as you were on the ground.
2) Newton’s Second Law: Force = mass X acceleration (F=ma)
Tells us what happens when a force is present.
The amount of acceleration depends on the objects mass and strength of
the net force.
o Example: When you can throw a baseball farther than you can
throw a shot in the shot put. A baseball accelerates as the pitcher
applies a force from the pitchers arm ceases and the ball’s path
changes only because the forces of gravity and air resistance.
o Explains why large planets such as Jupiter have a greater effect on
asteroids and comets than small planets such as Earth. Jupiter is
much more massive than Earth, it exerts stronger gravitational
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## Document Summary

Speed tells us how fast an object will go in a certain amount of time. Velocity tells us both the speed and the direction. Acceleration an object has acceleration if its velocity is changing in anyway, whether its speed or direction. In science, accelerating is used when you are slowing down or turning, not just motion. Slowing down occurs when acceleration is in a direct opposite to the motion acceleration = negative, causing velocity to decrease. Turning changes your velocity because it changes the direction which you are moving even if your speed is constant. You don"t feel effects when moving at a constant speed. Galileo demonstrated that gravity accelerates all objects by the same amount, regardless of their mass. Air resistance causes a difference of acceleration: if you dropped an object on the moon, where there is no air, they would both fall at exactly the same rate.