Physics Describing Motion

Aristotle 384-322 BC One of the first to study motion Stated there were two kinds of motion Natural Motion Violent Motion

Natural Motion Due to “nature” of object – the combination of the 4 elements: Objects wanted to get to “natural place” Ex: Smoke rises to be with air; Rocks fall to be with Earth Straight up and Down (on Earth) Circular (in space) Believed different rules applied to the heavens, which were made of quintessence (perfect unchanging substance)

Violent Motion Due to pushes and pulls (FORCES) Imposed motion Ex: Person pushing a cart Wind blowing a ship EXTERNAL CAUSE, not due to “nature” of object

Galileo 1564-1642, showed Aristotle was wrong Showed heavy and light objects fall at same rate Forces are not needed to keep objects in motion Inertia: Tendency of objects to resist change in motion Objects at rest tend to stay at rest, Objects in motion tend to stay in motion until another force acts on it. Tested his ideas through experimentation End of philosophizing about ideas Beginning of modern science

Mass – A Measure of Inertia Amount of Inertia (resist in change of motion) depends on amount of mass Mass: Amount of matter in an object; measured in kilograms (kg) DIFFERENT THAN WEIGHT! Weight: Force on an object due to gravity Weight and mass are directly proportional Double the Mass, Double the Weight! Which bucket would be harder to push? WHY?

Newtons Newton (N) = standard unit of force 1kg = 9.8N on Earth 1kg = 2.2 pounds (lb) 9.8N was rounded to 10

Density Measure of “compactness” – how much mass is squeezed into a given space Density = mass / volume What has more mass: 1kg rocks or 1 kg feathers? Which liquid is the most dense? Which solid is the most dense? What has more weight: 1kg rocks or 1 kg feathers? Which liquid is the least dense? What is more dense: 1kg rocks or 1 kg feathers? Which solid is the least dense?

Net Force Net Force: Combination of all forces on an object Vector Quantities: Forces shown by arrows Have both magnitude (how much) and direction (which way)

The Equilibrium Rule When the net force on something is equal to zero Tension = Stretching Force Is this diagram in equilibrium?

If the Normal Force is less than the Weight, what happens? Support Force The force that supports an object against gravity – often called normal force If the Normal Force is equal to the Weight, is the object in equilibrium? If the Normal Force is less than the Weight, what happens?

Equilibrium of Moving Things Equilibrium = state of no change An object moving at a constant speed in a straight line is in equilibrium Friction: Force that occurs opposite of motion when objects are in contact

Friction Occurs for Solids, Liquids, and Gases ALWAYS acts opposite the direction of movement Ex: Air resistance: Force of friction acting on an object as it moves through the air

Causes of Friction Due to surface bumps and “stickiness” of the atoms on the surfaces of materials Ex: Ridges in fingers allow you to grab things When your hands are wet, water fills the ridges, you can’t hold on to things as well Shooting Stars Meteors are heated to the point of burning when entering our atmosphere due to hitting gas molecules Earthquakes Occur when the friction that is holding rock slabs together is overcome and plates slide past each other

Vector Addition Parallelogram Law: Triangle Method (always head to tail)

Vector Practice Free-Body Diagram: A sketch showing only the forces on the selected particle.

Speed and Velocity Speed: Distance covered per amount of time traveled Units: km/hr; mi/hr; cm/min; km/day Preferred unit: m/s Instantaneous Speed: Speed at a given instant Ex: Speedometer Average Speed: averages all of the instantaneous speeds Velocity: Speed AND Direction Ex: 60mi/hr North; 30km/hr West Is it possible to have constant speed but NOT constant velocity?

Motion is Relative When we discuss speed or velocity, we mean RELATIVE to something else

Relative Velocities Define coordinate system Direction of bus = +x +x +y

Relative Velocities Relative to street vbus= +8m/s Relative to bus vyou = +3m/s

Relative Velocities Vector Diagram: vyou=3m/s vbus= 8m/s vyou, relative to street=11m/s

Relative Velocities Relative to street vbus= +8m/s Relative to street vyou = -3m/s

Relative Velocities Vector Diagram: vyou= -3m/s vbus= 8m/s vyou, relative to street= 5m/s

Acceleration Rate at which velocity changes with time Magnitude (speeding up OR slowing down) Direction Units: m/s2 Ex: Gravity = 9.8m/s2