Ch. 11 Motion & Forces I. Newton’s Laws of Motion “If I have seen far, it is because I have stood on the shoulders of giants.” - Sir Isaac Newton (referring to Galileo)

A. 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.

B. 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

C. Newton’s Third Law Newton’s Third Law of Motion  When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

Ch. 11 Motion & Forces II. Describing Motion  Motion  Speed & Velocity  Acceleration

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 constant velocity net force

A. Motion Problem:  Is your desk moving? We need a reference point...  nonmoving point from which motion is measured

A. Motion Motion  Change in position in relation to a reference point. Reference point Motion

A. Motion: Displacement The distance an object has been moved from one position to another.

A. 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.

B. Speed & Velocity Speed  rate of motion  distance traveled per unit time v d t

B. Speed & Velocity Instantaneous Speed  speed at a given instant Average Speed

B. 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!

B. Speed & Velocity Velocity  speed in a given direction  can change even when the speed is constant!

C. Acceleration Acceleration  the rate of change of velocity  change in speed or direction a: acceleration v f : final velocity v i : initial velocity t: time a v f - v i t

C. Acceleration Positive acceleration  “speeding up” Negative acceleration  “slowing down”

D. Calculations Your neighbor skates at a speed of 4 m/s. You can skate 100 m in 20 s. Who skates faster? GIVEN: d = 100 m t = 20 s v = ? WORK : v = d ÷ t v = (100 m) ÷ (20 s) v = 5 m/s You skate faster! v d t

D. Calculations 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: v i = 10 m/s t = 3 s v f = 32 m/s a = ? WORK : a = ( v f - v i ) ÷ t a = (32m/s - 10m/s) ÷ (3s) a = 22 m/s ÷ 3 s a = 7.3 m/s 2 a v f - v i t

D. Calculations Sound travels 330 m/s. If a lightning bolt strikes the ground 1 km away from you, how long will it take for you to hear it? GIVEN: v = 330 m/s d = 1km = 1000m t = ? WORK : t = d ÷ v t = (1000 m) ÷ (330 m/s) t = 3.03 s v d t

D. Calculations How long will it take a car traveling 30 m/s to come to a stop if its acceleration is -3 m/s 2 ? GIVEN: t = ? v i = 30 m/s v f = 0 m/s a = -3 m/s 2 WORK : t = ( v f - v i ) ÷ a t = (0m/s-30m/s)÷(-3m/s 2 ) t = -30 m/s ÷ -3m/s 2 t = 10 s a v f - v i t

E. Graphing Motion slope = steeper slope = straight line = flat line = Distance-Time Graph A B faster speed constant speed no motion speed

E. Graphing Motion Who started out faster?  A (steeper slope) Who had a constant speed? AA Describe B from 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 A B

E. Graphing Motion Acceleration is indicated by a curve on a Distance-Time graph. Changing slope = changing velocity

E. Graphing Motion Speed-Time Graph slope = straight line = flat line = acceleration  +ve = speeds up  -ve = slows down constant accel. no accel. (constant velocity)

E. Graphing Motion 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

Ch. 3 & 4 Motion & Forces III. Defining Force  Force  Newton’s First Law  Friction

A. Force Force  a push or pull that one body exerts on another  What forces are being exerted on the football? F kick F grav

A. Force Balanced Forces  forces acting on an object that are opposite in direction and equal in size  no change in velocity

A. Force Net Force  unbalanced forces that are not opposite and equal  velocity changes (object accelerates) F friction W F pull F net NN

B. 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.

B. Newton’s First Law 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

Concept Test 1 TRUE or FALSE? The object shown in the diagram must be at rest since there is no net force acting on it. FALSE! A net force does not cause motion. A net force causes a change in motion, or acceleration. Taken from “The Physics Classroom” © Tom Henderson, The Physics Classroom

Concept Test 2 You are a passenger in a car and not wearing your seat belt. Without increasing or decreasing its speed, the car makes a sharp left turn, and you find yourself colliding with the right-hand door. Which is the correct analysis of the situation?...

Concept Test 2 1. Before and after the collision, there is a rightward force pushing you into the door. 2. Starting at the time of collision, the door exerts a leftward force on you. 3. both of the above 4. neither of the above 2. Starting at the time of collision, the door exerts a leftward force on you.

C. Friction Friction  force that opposes motion between 2 surfaces  depends on the: types of surfaces force between the surfaces

C. Friction Friction is greater...  between rough surfaces  when there’s a greater force between the surfaces (e.g. more weight) Pros and Cons?

Ch. 3 & 4 Motion & Forces IV. Force & Acceleration  Newton’s Second Law  Gravity  Air Resistance  Calculations

A. 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

A. Newton’s Second Law F = ma F:force (N) m:mass (kg) a:accel (m/s 2 ) 1 N = 1 kg ·m/s 2 m F a F m

B. Gravity Gravity  force of attraction between any two objects in the universe  increases as... mass increases distance decreases

B. Gravity Who experiences more gravity - the astronaut or the politician? less distance more mass Which exerts more gravity - the Earth or the moon?

B. Gravity Weight  the force of gravity on an object MASS always the same (kg) WEIGHT depends on gravity (N) W = mg W:weight (N) m:mass (kg) g:acceleration due to gravity (m/s 2 )

B. Gravity Would you weigh more on Earth or Jupiter? greater gravity greater weight greater mass  Jupiter because...

B. Gravity Accel. due to gravity (g)  In the absence of air resistance, all falling objects have the same acceleration!  On Earth: g = 9.8 m/s 2 elephant feather Animation from “Multimedia Physics Studios.”Multimedia Physics Studios

C. Air Resistance 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

C. Air Resistance Terminal Velocity  maximum velocity reached by a falling object  reached when… F grav = F air F air F grav  no net force  no acceleration  constant velocity

C. Air Resistance Terminal Velocity  increasing speed  increasing air resistance until… F air = F grav Animation from “Multimedia Physics Studios.”Multimedia Physics Studios

C. Air Resistance Falling with air resistance F grav = F air Animation from “Multimedia Physics Studios.”Multimedia Physics Studios  heavier objects fall faster because they accelerate to higher speeds before reaching terminal velocity  larger F grav  need larger F air  need higher speed

D. Calculations What force would be required to accelerate a 40 kg mass by 4 m/s 2 ? GIVEN: F = ? m = 40 kg a = 4 m/s 2 WORK : F = ma F = (40 kg)(4 m/s 2 ) F = 160 N m F a

D. Calculations A 4.0 kg shotput is thrown with 30 N of force. What is its acceleration? GIVEN: m = 4.0 kg F = 30 N a = ? WORK : a = F ÷ m a = (30 N) ÷ (4.0 kg) a = 7.5 m/s 2 m F a

D. Calculations Mrs. J. weighs 557 N. What is her mass? GIVEN: F(W) = 557 N m = ? a( g ) = 9.8 m/s 2 WORK : m = F ÷ a m = (557 N) ÷ (9.8 m/s 2 ) m = 56.8 kg m F a

Concept Test 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.

Motion & Forces V. Free-Fall Motion  Free-fall  Circular Motion  Free-fall

A. Free-Fall 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

A. Free-Fall Weightlessness  surroundings are falling at the same rate so they don’t exert a force on the object

Motion & Forces VI. Action and Reaction  Newton’s Third Law  Momentum  Conservation of Momentum

A. Newton’s Third Law Newton’s Third Law of Motion  When one object exerts a force on a second object, the second object exerts an equal but opposite force on the first.

A. Newton’s Third Law Problem:  How can a horse pull a cart if the cart is pulling back on the horse with an equal but opposite force? NO!!!  Aren’t these “balanced forces” resulting in no acceleration?

A. Newton’s Third Law  forces are equal and opposite but act on different objects  they are not “balanced forces”  the movement of the horse depends on the forces acting on the horse Explanation:

A. Newton’s Third Law Action-Reaction Pairs The hammer exerts a force on the nail to the right. The nail exerts an equal but opposite force on the hammer to the left.

A. Newton’s Third Law Action-Reaction Pairs The rocket exerts a downward force on the exhaust gases. The gases exert an equal but opposite upward force on the rocket. FGFG FRFR

A. Newton’s Third Law Action-Reaction Pairs Both objects accelerate. The amount of acceleration depends on the mass of the object. F m Small mass  more acceleration Large mass  less acceleration

JET CAR CHALLENGE CHALLENGE: Construct a car that will travel as far as possible (at least 3 meters) using only the following materials. scissors tape 4 plastic lids 2 skewers 2 straws 1 balloon 1 tray How do each of Newton’s Laws apply?