Long 50slideschapter 5 motion notes [autosaved]

Ch. 5 Motion & Forces II. Describing Motion Using Newton’s Laws of Motion

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 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.

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 N N

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? Which exerts more gravity - the Earth or the moon? less distance more mass

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 Animation from “ Multimedia Physics Studios .” elephant feather

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

B. Newton’s First Law Newton’s First Law of Motion “ Law of Inertia” Inertia (Greek word meaning Lazyness) tendency of an object to resist any change in its motion increases as mass increases

ConcepTest 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? ...

ConcepTest 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.

What is a Force? Push or pull Measured in (N) Newton

Net forces Net force: all forces exerted on an object

Unbalanced and Balanced Forces Unbalanced forces produce a change in motion(acceleration) Balanced Forces produce no change in motion

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?

Types of Friction Sliding Friction Rolling Friction Fluid Friction Static Friction

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

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

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

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 Ms. Wills 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

ConcepTest 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.

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? Aren’t these “balanced forces” resulting in no acceleration? NO!!!

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. F G F R

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

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

E. Graphing Motion 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 A B

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

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

E. Graphing Motion 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 Speed-Time Graph

B. Momentum Momentum quantity of motion p = mv p : momentum (kg ·m/s) m : mass (kg) v : velocity (m/s) m p v

B. Momentum Find the momentum of a bumper car if it has a total mass of 280 kg and a velocity of 3.2 m/s. GIVEN: p = ? m = 280 kg v = 3.2 m/s WORK : p = mv p = (280 kg)(3.2 m/s) p = 896 kg·m/s m p v

B. Momentum The momentum of a second bumper car is 675 kg·m/s. What is its velocity if its total mass is 300 kg? GIVEN: p = 675 kg·m/s m = 300 kg v = ? WORK : v = p ÷ m v = (675 kg·m/s)÷(300 kg) v = 2.25 m/s m p v

C. Conservation of Momentum Law of Conservation of Momentum The total momentum in a group of objects doesn’t change unless outside forces act on the objects. p before = p after

C. Conservation of Momentum A 5-kg cart traveling at 4.2 m/s strikes a stationary 2-kg cart and they connect. Find their speed after the collision. BEFORE Cart 1 : m = 5 kg v = 4.2 m/s Cart 2 : m = 2 kg v = 0 m/s AFTER Cart 1 + 2 : m = 7 kg v = ? p = 21 kg·m/s p = 0 p before = 21 kg·m/s p after = 21 kg·m/s v = p ÷ m v = (21 kg·m/s) ÷ (7 kg) v = 3 m/s m p v

C. Conservation of Momentum A 50-kg clown is shot out of a 250-kg cannon at a speed of 20 m/s. What is the recoil speed of the cannon? BEFORE Clown : m = 50 kg v = 0 m/s Cannon : m = 250 kg v = 0 m/s AFTER Clown : m = 50 kg v = 20 m/s Cannon : m = 250 kg v = ? m/s p = 0 p = 0 p before = 0 p = 1000 kg·m/s p after = 0 p = -1000 kg·m/s

C. Conservation of Momentum So…now we can solve for velocity. GIVEN: p = -1000 kg·m/s m = 250 kg v = ? WORK : v = p ÷ m v = (-1000 kg·m/s)÷(250 kg) v = - 4 m/s (4 m/s backwards) m p v

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