Can You Snap a Card Out From Under a Coin?

Transcription

1 Can You Snap a Card Out From Under a Coin? 1. Balance half of a 3 x 5 index card on the tip of an index finger. 2. Place a penny on the card, just above your fingertip. 3. Give the card a quick horizontal snap with the fingernail of your index finger. 4. Repeat steps 1 through 3 using a quarter.

2 Analyze and Conclude What happened to the penny when the card was quickly removed? Did changing the coin affect the results? Do you think this would work with a card made of sandpaper? Why were you able to snap the card without moving the coin?

3 Newton s Laws: Part I F L T : I C A N D E S C R I B E W H A T A N O B J E C T W I L L D O B A S E D I N T H E D E F I N I T I O N O F I N E R T I A A N D D E T E R M I N E A N O B J E C T S W E I G H T W H E N G I V E N I T S M A S S.

4 THE BIG IDEA FORCES cause changes in MOTION!

5 A Little Bit of History Aristotle: There is natural motion and violent motion. It is natural for heavy things the fall and very light things to rise. Copernicus: The Earth moves around the sun not the other way around! Uh-Oh the Earth is not the center of the universe anymore! Galileo: Only when friction is present (as it usually is) is a force needed to keep an object moving. Without friction, a ball rolling forward would roll forward forever.

6 Newton s Law of Inertia Newton built on these ideas, and came up with his first (of three) law of motion. Newton s 1 st Law: An object at rest will remain at rest. An object in motion will keep moving in a straight line, at constant speed, unless acted on by a force. AKA: The Law of Inertia Inertia = the property of a body to resist change to its state of motion.

7 Checking For Understanding Miss Stein rolls a bowling ball in an open field. Assuming no friction, what happens to the ball? It keeps rolling, in a straight line, at constant speed, until it is acted upon by a force An object at rest wants to. An moving object wants to in a at a speed.

8 Mass vs. Weight Mass = the amount of stuff in an object It is related to the type and number of atoms in an object More Mass = More Inertia Units: [kg] = kilograms Weight = the force of gravity (F g ) on an object = the planet s pull on an object Weight depends on an object s location (Earth, moon, Jupiter) Weight is a FORCE. Units: [N] = Newtons

9 Units & g Quantity Units Mass kg Force N Acceleration m/s 2 g 10 m/s 2 (on earth) **g is the contribution of a planet in creating the weight of an object

10 Weight Formula F g = mg [ N = kg *10 m ] s 2 F g = Force of gravity = Weight [N] m = mass [kg] g = planet s influence in creating an object s weight = 10 m/s 2 on earth

11 F g = mg Example 1 Ken, who weighs 700 N on earth, flies to Jupiter where g = 26.4 m/s 2. Note, mass does not change as we visit other planets, since mass is related to the number of atoms in our body. Find his mass Find his weight on Jupiter

12 Checking For Understanding m = 5 kg F g =? m = 10 kg F g =? m = 7.5 kg F g =? F g = 100 N m =? F g = 50 N m =? F g = 1800 N m =?

13 Newton s Laws: Part II F L T : I C A N D E S C R I B E N E W T O N S 2 ND L A W A N D U S E T H E F O R M U L A F = M A T O S O L V E F O R A N Y U N K N O W N Q U A N T I T Y.

14 THE BIG IDEA An object ACCELERATES when a NET FORCE acts on it.

15 Forces Cause Acceleration Net forces acting on an object cause the object to accelerate.

16 Net Force The combination of forces acting on an object is the net force. Acceleration depends on the net force. For a constant force, an increase in the mass will result in a decrease in the acceleration. more mass + same force = less acceleration

17 Checking for Understanding What is the net force on this object? 4N 8N 3N Note: Another word for net force is unbalanced force.

18 Checking for Understanding 2 Is this object accelerating? Why or why not? 20 N 20 N

19 Newton s 2 nd Law Newton s 2 nd Law: Net forces cause objects to accelerate. F net = ma F net = add up all the forces acting on the object (with the appropriate + or signs) [N] m = mass of object [kg] a = acceleration [m/s 2 ]

20 Example A truck driver is zooming down the freeway in his 3000 kg truck. Aaaah! A ahead! is in the road The truck driver slams on the brakes, providing a friction force of 1000 N. What is the acceleration of the truck?

21 Weight Formula The weight formula is just a special application of Newton s 2nd Law. Fnet = ma

22 Newton s Laws: Part III F L T : I C A N D I S T I N G U I S H A M O N G D I F F E R E N T F O R C E S A N D D R A W P I C T U R E S O F T H E S E F O R C E S.

23 Gravity (F g ) Types of Forces Acts between any two objects and always between an object the earth Friction (F f ) Opposes motion Air drag is a type of friction Tension (F T ) Force of a string Always PULLS Spring (F S ) Can push or pull Normal (F N ) Force between any two objects that are touching

24 Free Body (Force) Diagrams Recipe: 1. Put a dot in the center of the object. 2. Put your brain on the dot. 3. Think: What forces does this object feel? 4. Draw arrows (representing the forces) coming OUT of the dot.

25 Examples (1) Draw the free body diagram for the following systems. 1. Hamburger hanging from a string on the ceiling. 2. Box being pulled along the floor (w/friction) by a rope.

26 Examples (2) 3. A spring is stretched with a mass on one end 4. Miss Stein parachutes down from an airplane

27 Example (3) Miss Stein pulls a 2 kg block along a horizontal surface with a force of 40 N. A friction force acts with 5 N. What is the acceleration of the block?

28 Newton s Laws: Part IV F L T : I C A N S O L V E N E W T O N S L A W E Q U A T I O N S I N 2 D A N D U S E T H E F O R M U L A F O R F R I C T I O N

29 Newton s 2 nd Law: 2D F x = ma x Block pulled With friction F y = ma y

30 Warm-Up A 500 kg car is driving on the 405 at 30 m/s. Traffic! The driver slams on the brakes, bringing the car to rest in 5 seconds. 1. Draw a force diagram for the vehicle. 2. Write Newton s 2 nd Law Equations in x & y 3. Find the magnitude of the frictional force produced by the brakes.

31 Types of Friction 1. Sliding Friction occurs when two solid objects slide over each other. Static Friction just before motion Kinetic Friction just after motion Factors that affect Sliding Friction: The normal Force acting on an object (F N ) The coefficient of Friction (µ) The texture of the surfaces. The stickiness between surfaces. 2. Rolling Friction wheels and ball bearings 3. Fluid Friction solid object moves through gas or liquid

32 Formula for Friction F f = µf N F f = friction force [N] F N = normal force [N] µ = mu = coefficient of friction [no units]

33 Newton s 2 nd Law: 2D w Friction F x = ma x F y = ma y F f = µf N F g = mg

34 Stephen Hawking is moving a 100 kg block from one side of his laboratory to the other. To accomplish this he ties a rope to the block and then ties it to his wheelchair such that the rope is pulled horizontally to the floor. If the coefficient of sliding friction is 0.35, what force must Mr. Hawking apply to the rope to move at constant speed? Ans: 350 N

35 A crate having a mass of 60 kg falls horizontally off the back of a truck which is traveling at 30 m/s. Compute the coefficient of kinetic friction between the road and the crate if the crate slides 90 m on the ground with no tumbling along the road before coming to rest. Assume that the initial speed of the crate along the road is 30 m/s. Ans. 0.5

36 Newton s Laws: Part V F L T : I C A N I D E N T I F Y A N A C T I O N A N D R E A C T I O N U S I N G N E W T O N S 3 RD L A W.

37 Newton s 3 rd Law A force is always part of a mutual action that involves another force. Newton s third law states that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. If Body A exerts a force on Body B, Body B exerts an equal and opposite force on Body A.

38 Check for Understanding Miss Stein gets really mad and punches the wall. Ouch! If she exerts a force of 200 N on the wall, how much force does the wall exert on Miss Stein s fist? **You cannot hit the wall any harder than the wall can hit you back.