Newton’s 2 nd Law Air Resistance & Drag

Why does the motion of objects change? Aristotle Objects seek their ‘natural’ places OR ‘Unnatural’ forces act on those objects Greek philosopher, ~2400 BCE. For more, start here.here

Why does the motion of objects change? Aristotle Objects seek their ‘natural’ places OR ‘Unnatural’ forces act on those objects Italian philosopher, ~ For more, start here.here Galileo Challenged Aristotle’s ideas with “thought experiments” and investigations.

Galileo’s thought experiment #1 On rough surfaces, a rolling ball will stop quickly. On a smooth surface, a rolling ball will travel much farther. On a PERFECTLY smooth surface, how far will a rolling ball travel?

Galileo’s thought experiment #2 A ball rolling uphill will come to stop. A ball rolling downhill will speed up. In the absence of friction, at what angle will the ball neither speed up NOR slow down?

Galileo’s big idea #1: Any change in direction or speed requires an outside force! –You, me, horse, exploding gasoline, etc. –Friction, air resistance

Inertia Inertia is the resistance of an object to a change in motion, an intrinsic property of matter Latin In – without erg – force Objects in outer space are weightless, but still have inertia; related to “momentum” NOT the same as weight

Galileo’s thought experiment #3 Heavy things fall faster than lighter things. If you have a block that weighs twice as much as another block, does the heavy block fall twice as fast as the lighter block?

Galileo’s thought experiment #3 What happens to the rate at which the heavy block falls if you cut it in half and connect the two pieces with a string? ???

Galileo’s actual experiment #1 Rolled balls down inclined planes –Timed first with pulse –Later timed with water clock Distance travelled discovered to be proportional to the SQUARE of the time elapsed distance  time 2 –NOT proportional to mass

Galileo’s big idea #2: Weight does not affect rate at which something falls; air resistance does. Dave Scott on the moon

What causes the motion of objects to change? Newton We can describe the motion of objects using three principles: 1 st Inertia An object in straight-line motion will stay in straight-line motion until acted upon by an unbalanced force. An object at rest will stay at rest until acted upon by an unbalanced force.

Balanced forces 5 N 0 N 5 N Describe the motion of the block if the forces are balanced.

Unbalanced forces 5 N 10 N

What causes the motion of objects to change? Newton We can describe the motion of objects using three principles: 1 st Inertia 2 nd Acceleration is proportional to force. Acceleration is inversely proportional to mass.  F   a  m   a

Think about it… If you push on a shopping cart, it will accelerate. 1)If you push 5 x harder, what happens to its acceleration? 2)If you push the same, but the cart is loaded so that it has 5 x the mass, what happens to the acceleration? 3)If you push 5 x harder when its mass is 5 x greater, what happens to the acceleration?

What causes the motion of objects to change? Newton We can describe the motion of objects using three principles: 1 st Inertia 2 nd Acceleration is proportional to force and inversely proportional to mass 3 rd If one object acts on a second, the second acts on the first with equal magnitude in the opposite direction. For every action, there is an equal and opposite reaction. alternatively, forces come in pairs.

Newton’s Laws of Motion To summarize: 1 st Inertia 2 nd a = F / m 3 rd Action & reaction

A note about units of force In the metric system, mass is measured in kilograms (kg) acceleration is measured in (meters per second) per second, (m/s/s OR m/s 2 ) Since force is equal to mass (kg) x acceleration (m/s / s), the units of force are kg  m/s/s. Pronounced: Newton Abbreviated: N In the Imperial system, mass is measured in slugs acceleration is measured in (feet per second) per second, (ft/s/s OR ft/s 2 ) In the Imperial system, force is measured in pounds (lbs)

Modeling Forces of Flight Drag Drag slows a plane down –Drag acts opposite to the direction of flight –Drag limits the forward speed of the airplane

Types Parasitic drag Influenced by how air flows around entire aircraft. Induced drag Influenced by the direction air is deflected by aircraft.

Parasitic Drag Caused by surfaces which deflects or interferes with the smooth airflow around the airplane Size & shapeInterferenceSkin

Induced drag Airfoils deflect air downward and rearward.

Induced drag Downward deflection of air pushes plane upward Rearward deflection of air causes induced drag.

Flaps

Flaps change the shape of the wing to deflect more air. Some of that deflected air creates goes down (and pushes the plane up). Some of that deflected air pushes the plane backwards (and slows the plane down).

Think about it… Flaps generate more lift Allow aircraft to fly at slower speeds Flaps generate more drag Allow aircraft to slow down In what situation(s) is it useful to fly at slower speeds?

Think about it… As a token of appreciation, you take your 9 th grade science instructor skydiving on your 18 th birthday. 1)The moment you leap from the plane, air resistance is ~0 N. At what rate does your speed change when air resistance is ~0 N? 2)After a while, you stop accelerating. Why?