AST101H1 Chapter Notes - Chapter 4: Weightlessness, Earth Mass, Net Force
21 Jun 2018
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4.1, 4.2, 4.3 and Intro to 4.4
4.1 Describing Motion: Examples From Daily Life
●Speed: Speed of a car
○ Tells us how far it will go in a certain amount of time
○ E.g. 100 km per hour, car will cover 100 km in distance if driven at that speed for
an hour
●Velocity of a car:
○ Tells us its speed and direction
○ E.g. 100 km/ hour going NORTH is velocity
●Acceleration of a car:
○ Car has acceleration IF velocity changes in any way (speed or direction)
○ You are accelerating when you also slow down (negative acceleration, causing
velocity to decrease)
○ Turning is also acceleration because you change direction
● Acceleration of Gravity
○ Gravity accelerates all objects by the same amount regardless of their mass.
○ Air resistance causes feather to fall slow while rock fall fast (changes their
acceleration)
○ If there was no air resistance, they would fall at the same time
○Acceleration of gravity (g) → Acceleration of falling objects; acceleration of
gravity causes falling objects to fall faster by 9.8m/s (we say is approx 10 m/s)
● Momentum and Force
○ How an object moves = you must know concepts of velocity, speed and
acceleration
○ Interaction between objects
○Momentum: An object’s momentum is product of its mass and velocity (mass x
velocity), velocity is changed as mass remains constant
○ Momentum stays @ 0 if they don’t move direction
○ To change object momentum is to apply force
○ E.g. Take a truck and mosquito hitting your car; truck has bigger mass so its
momentum is stronger, but both transfer some momentum to your car
○ If you maintain constant velocity = constant momentum, you may overcome the
slowing effects of other force (air resistance and friction on road)
○ Forces of some kind are always present as force of gravity or electromagnetic
forces acting between atoms
○Net force: Overall force, represents the combined effect of all individual forces
together.
○ No net force when driving at constant velocity because the force generated by
the engine to turn wheels offsets the forces of air resistance and road friction
● Angular Momentum
○ Figure skater → Some parts of her body have momentum but her overall
momentum is 0
find more resources at oneclass.com
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○Angular momentum; circling momentum, turning momentum: Any object
that is spinning or moving along a curved path has angular momentum
○ Angular momentum can change only when a special type of force is applied to it
○Torque: Twisting force, amount of torque depends on how much force is applied
but also where its applied
● Mass vs. Weight
○Mass: Amount of matter in your body
○Weight: Force that a scale measures when you stand on it, depending on mass
and forces acting on your mass.
○ Mass is dependent on the amount of matter in your body and is the same
anywhere
○ Weight can vary because of the forces acting on you
● Free-Fall and Weightlessness
○ Freefall → Falling without ANY resistance to slow you down which makes you
weightless as you float freely above the floor
○ You are in free fall whenever there’s nothing to prevent you from falling
○ Astronauts are in a constant state of free fall: They remain weightless
4.2 Newton’s Laws of Motion
● How Newton changed view of universe
○ Moment of inspiration was still when apple fell from tree to ground
○ This moment inspired his idea that gravity held the moon in orbit around earth
○ Recall Aristotle
■ Used his ideas to support his belief in an Earth centered cosmos,
heavens were distinct from Earth
■ Copernicus’s view displaced earth from a central position, gaLILEO’S
EXPERIMENTS HAD SHOWN THAT LAW OF PHYSICS WERE NOT
WHAT ARISTOTLE believed
○ Newton eliminated Aristotle’s distinction between two realms and brought
heavens and Earth together as one universe
○ Birthed astrophysics which applies physical laws discovered on Earth to
phenomena through the cosmos
○ Quantified laws of motion and gravity, conducted crucial experiments regarding
the nature of light
● Newton’s Laws of Motion
○ Principia; Three laws that apply to all motion which we call Newton’s laws of
motion which govern our movements on Earth and everything else in the sky
○ FIRST LAW
■ An object moves at constant velocity if there is no net force acting on it
■ Objects at rest (velocity = 0) tend to remain at rest, objects in motion tend
to remain in motion with no change in speed or direction
■ Newton’s first law says that a car should keep going at the same speed
forever UNLESS a force acts to slow it down, car coming to stop when
find more resources at oneclass.com
find more resources at oneclass.com
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AST101H1 Full Course Notes
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Document Summary
Tells us how far it will go in a certain amount of time. 100 km per hour, car will cover 100 km in distance if driven at that speed for an hour. Car has acceleration if velocity changes in any way (speed or direction) You are accelerating when you also slow down (negative acceleration, causing velocity to decrease) Turning is also acceleration because you change direction. Gravity accelerates all objects by the same amount regardless of their mass. Air resistance causes feather to fall slow while rock fall fast (changes their acceleration) If there was no air resistance, they would fall at the same time. Acceleration of gravity (g) acceleration of falling objects; acceleration of gravity causes falling objects to fall faster by 9. 8m/s (we say is approx 10 m/s) How an object moves = you must know concepts of velocity, speed and acceleration.
