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## Motion

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**Newton’s First Law**• Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.**Motion**• Problem: • Is your desk moving? • We need a reference point... • nonmoving point from which motion is measured**Reference point**Motion Motion • Motion • Change in position in relation to a reference point.**Motion**Problem: • You are a passenger in a car stopped at a stop sign. Out of the corner of your eye, you notice a tree on the side of the road begin to move forward. • You have mistakenly set yourself as the reference point.**d**v t Speed & Velocity • Speed • rate of motion • distance traveled per unit time**Speed & Velocity**• Instantaneous Speed • speed at a given instant • Average Speed**Speed & Velocity**• Problem: • A storm is 10 km away and is moving at a speed of 60 km/h. Should you be worried? • It depends on the storm’s direction!**Speed & Velocity**• Velocity • speed in a given direction • can change even when the speed is constant!**Distance vs. Displacement**• Distance refers to "how much ground an object has covered" during its motion. • Displacement "how far out of place an object is"; it is the object's overall change in position; where the object is in relation to its starting point • Example - You run from your house to a friend's house that is 3 miles away. You then walk home. • a. What distance did you travel? ______________ • b. What was the displacement for the entire trip? _______________**vf- vi**t a Acceleration • Acceleration • the rate of change of velocity • change in speed or direction a: acceleration vf: final velocity vi: initial velocity t: time**You are accelerating whenever you:1-speed up 2- slow down -**People commonly call this deceleration.3- change direction – WHY? velocity is your speed and direction, your velocity changes if your direction changes (even if your speed stays the same). Since you are accelerating if your velocity is changing, you are accelerating when you are changing direction - even if your speed stays the same.**Positive acceleration**• “speeding up” • Negative acceleration • “slowing down”**vf- vi**t a • A roller coaster starts down a hill at 10 m/s. Three seconds later, its speed is 32 m/s. What is the roller coaster’s acceleration? GIVEN: vi = 10 m/s t = 3 s vf = 32 m/s a = ? WORK: a = (vf- vi) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a= 7.3 m/s2**Distance-Time Graph**A B speed • slope = • steeper slope = • straight line = • flat line = faster speed constant speed no motion**Distance-Time Graph**A B • Who started out faster? • A (steeper slope) • Who had a constant speed? • A • Describe B from 10-20 min. • B stopped moving • Find their average speeds. • A = (2400m) ÷ (30min) A = 80 m/min • B = (1200m) ÷ (30min) B = 40 m/min**Distance-Time Graph**• Acceleration is indicated by a curve on a Distance-Time graph. • Changing slope = changing velocity**Speed-Time Graph**• slope = • straight line = • flat line = acceleration constant accel. no accel. (constant velocity)**Speed-Time Graph**Specify the time period when the object was... • slowing down • 5 to 10 seconds • speeding up • 0 to 3 seconds • moving at a constant speed • 3 to 5 seconds • not moving • 0 & 10 seconds**Force**• a push or pull that one body exerts on another • What forces are being exerted on the football? Fkick Fgrav**Balanced Forces**• forces acting on an object that are opposite in direction and equal in size • no change in velocity**N**N • Net Force • unbalanced forces that are not opposite and equal • velocity changes (object accelerates) Fnet Ffriction Fpull S**Newton’s First Law**• Newton’s First Law of Motion • An object at rest will remain at rest and an object in motion will continue moving at a constant velocity unless acted upon by a net force.**Newton’s First Law of Motion**• “Law of Inertia” • Inertia • tendency of an object to resist any change in its motion • increases as mass increases**Friction**• force that opposes motion between 2 surfaces • depends on the: • types of surfaces • force between the surfaces**Friction is greater...**• between rough surfaces • when there’s a greater force between the surfaces (e.g. more weight)**Newton’s Second Law**• Newton’s Second Law of Motion • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. F = ma**F**a m F m F = ma F: force (N) m: mass (kg) a: accel (m/s2) 1 N = 1 kg ·m/s2**Gravity**• force of attraction between any two objects in the universe • increases as... • mass increases • distance decreases**more mass**less distance • Who experiences more gravity - the astronaut or the politician? • Which exerts more gravity - the Earth or the moon?**Weight**• the force of gravity on an object W = mg W: weight (N) m: mass (kg) g: acceleration due to gravity (m/s2) MASS always the same (kg) WEIGHT depends on gravity (N)**Would you weigh more on Earth or Jupiter?**• Jupiter because... • greater mass • greater gravity • greater weight**elephant**feather • Falling WITHOUT air resistance • Accel. due to gravity (g) • In the absence of air resistance, all falling objects have the same acceleration! • On Earth: g = 9.8 m/s2 Animation from “Multimedia Physics Studios.”**Air Resistance**• a.k.a. “fluid friction” or “drag” • force that air exerts on a moving object to oppose its motion • depends on: • speed • surface area • shape • density of fluid**Fair**Fgrav • Terminal Velocity • maximum velocity reached by a falling object • reached when…Fgrav = Fair • no net force no acceleration constant velocity**Terminal Velocity**• increasing speed increasing air resistance until… Fair = Fgrav Animation from “Multimedia Physics Studios.”**heavier objects fall faster because they accelerate to**higher speeds before reaching terminal velocity • Falling with air resistance Fgrav = Fair • larger Fgrav need larger Fair need higher speed Animation from “Multimedia Physics Studios.”**True or False?**• Is the following statement true or false? • An astronaut has less mass on the moon since the moon exerts a weaker gravitational force. • False! Mass does not depend on gravity, weight does. The astronaut has less weight on the moon.**Nonlinear Motion**Projectile Motion Circular Motion Free-fall**Projectile**• any object thrown in the air • acted upon only by gravity • follows a parabolic path called a trajectory • has horizontal and vertical velocities PROJECTILE MINI-LAB**Projectile Velocities**• Horizontal and vertical velocities are independent of each other!**Horizontal Velocity**• depends on inertia • remains constant • Vertical Velocity • depends on gravity • accelerates downward at 9.8 m/s2**A moving truck launches a ball vertically (relative to the**truck). If the truck maintains a constant horizontal velocity after the launch, where will the ball land (ignore air resistance)? A) In front of the truck B) Behind the truck C) In the truck • C) In the truck. The horizontal velocity of the ball remains constant and is unaffected by its vertical motion. Animation from “Multimedia Physics Studios.”**Centripetal Acceleration**• acceleration toward the center of a circular path • caused by centripetal force B-BALL DEMO PLATE DEMO**On the ground...**• friction provides centripetal force**In orbit...**• gravity provides centripetal force ROUND LAB**Free-Fall**• when an object is influenced only by the force of gravity • Weightlessness • sensation produced when an object and its surroundings are in free-fall • object is not weightless! CUP DEMO**Weightlessness**• surroundings are falling at the same rate so they don’t exert a force on the object**Free-Fall**Space Shuttle Missions Go to Space Settlement Video Library. NASA’s KC-135 - “The Vomit Comet” Go to CNN.com. Go to NASA.