VIEWS: 272 PAGES: 53 POSTED ON: 5/4/2010
Forces & Newton’s Laws • Identifying and defining “forces” • Free Body Diagrams • Gravitational Forces • Mass and weight • The Law of Inertia • The Law of Acceleration • The Law of Force-Counterforce • Normal force, Tension, and friction • The vector nature of forces SHS Physics 1 So, what is a “force”, anyway? SHS Physics 2 So, what is a “force” Forces cause cars to move! SHS Physics 3 So, what is a “force”? Forces cause planes to fly! SHS Physics 4 So, what is a “force”? Forces cause planets to orbit the sun! SHS Physics 5 So, what is a “force”? Forces hold atoms together! SHS Physics 6 So, what is a “force” Defn: a force is a push or a pull that causes a change in motion, shape, or direction. Iftwo forces are equal and opposite they are called balanced forces. More than one force can combine to make a net force (total force). SHS Physics 7 Think About It... Think of all the different kinds of forces in the universe... List as many of them as you can. How many different forces are there? SHS Physics 8 The Forces of Nature I. Gravitational: •attraction bet. masses •tides, gravity, weight II. Weak Nuclear: •helps to explain II. Electromagnetic: atomic collisions •friction •tension •adhesion •lift III. Strong Nuclear: •electrostatic •drag •binds atomic nuclei •buoyant •magnetic SHS Physics 9 Representing Forces... Forces are vectors – Forces are drawn as arrows (vectors) – forces add like vectors. – the sum of all the forces is called the net force. A picture of a body with arrows drawn representing all the forces acting upon it is called a FREE BODY DIAGRAM. SHS Physics 10 Try it... Draw a picture of your book sitting on the desk. Identify all the forces acting on it. SHS Physics 11 Free Body Diagrams... T (table) Book W (weight) SHS Physics 12 Free Body Diagrams... What forces are acting on a skier as she races down a hill? SHS Physics 13 The Answer... FN d&f W SHS Physics 14 The Answer... FN f and d W SHS Physics 15 Gravity and Weight... • The force pulling on your mass (toward the ground) is called your weight. W =mxg 1 Newton What are the (N) = 1 units of weight 2 (force)? kg·m/s SHS Physics 16 Guess Some Common Masses/Weights Weight Mass SI (N) Eng (lb) SI (kg) Eng (slug) you a car a bowling ball a quarter a bus SHS Physics 17 How’d you do? Weight Mass SI (N) Eng (lb) SI (kg) Eng (slug) you 667 150 68 4.66 a car 8,896 2,000 907 62.17 a bowling ball 71.2 16.0 7.3 0.50 a quarter 0.139 0.031 0.0 0.00 a bus 177,920 40,000 18,141 1,243 SHS Physics 18 As the skydiver falls what are the magnitudes of the forces acting on him? QuickTime™ and a GIF decompressor are neede d to see this picture. SHS Physics 19 Apparent Weight •Is your weight always the same on the Earth? •How about in an elevator? •What is this QuickTime™ and a sky diver’s Cinepak decompressor are needed to see this picture. apparent weight? SHS Physics 20 Apparent Weight •When accelerating up… Your rate of WA = mg + ma acceleratio n! •When accelerating down… WA = mg - ma SHS Physics 21 Apparent Weight Example On the Mantis (at Cedar Point) at the bottom of the first hill you experience an acceleration of 27 m/s/s (about 3 g‟s). What is your apparent weight at that point? What is your apparent weight at the top of the 2nd hill when you experience an acceleration of 2.2 m/s/s? SHS Physics 22 Free Fall •In the absence of air resistance ALL objects accelerate at 9.81 QuickTime™ and a GIF dec ompres sor are needed to see this picture. m/s/s! SHS Physics 23 True or False 1. The elephant and the feather each experience the same force of gravity. 2. The elephant experiences a greater force of gravity, yet both the elephant and the feather have the same mass. 3. On earth, all objects (whether an elephant or a feather) have the same acceleration. 4. The elephant clearly has more mass than the feather, yet they each weigh the same. 5. The elephant has the greatest acceleration, yet the amount of gravity is the same for each. SHS Physics 24 Think about it... What does “gravity” depend on? If you‟re standing on a scale and the pressure in the room rises what happens? If the Earth stopped spinning how would your weight change? So, what is gravity caused by? SHS Physics 25 Law of Universal Gravitation • Any two bodies with mass will attract eachother with a gravitational force! • More mass ==> more force • farther away ==> less force SHS Physics 26 m1m2 Fg G 2 r 2 Nm G 6.67 10 11 2 kg SHS Physics 27 Examples Find the force of attraction between you and your neighbor (estimate distances and masses). Find the force of gravitational attraction between the Earth and you. SHS Physics 28 Think about it... When a baseball is hit, what causes it to slow down? What prevents it from slowing down? When your car stops fast, why do you lunge forward? Under low friction circumstances which is harder to start moving, a large object or a small object? SHS Physics 29 Inertia... Def: An object’s tendency to resist a change in motion is called its inertia.intrinsic property that depends only on inertia is an mass – more mass --> more inertia! QuickTime™ an d a GIF decompressor are need ed to see this p icture . SHS Physics 30 The Law of Inertia... An object will stay in a state of rest or straight line motion unless acted on by an unbalanced force. an inertial reference frame is a f.o.r. in which the law of inertia holds true. SHS Physics 31 IRF’s or not??? a car moving w/ constant velocity the Earth a bus that is slowing down a skier going down a hill a person on a merry-go-round a skateboard rolling on a level sidewalk? SHS Physics 32 Think about it... A little girl sits on a skate board and her friend pulls her with a constant force. Describe the motion of the girl. SHS Physics 33 The Law of Acceleration When an unbalanced force is applied to an object it will accelerate in the direction of the net force with an acceleration proportional to the force applied and inversely prop. to the mass. ∑F = m x a If there is only one force then the equation is simply: F=m x a SHS Physics 34 Examples What force is needed to decelerate a 2,000 lb car from 35 mph to rest in 5 seconds? ~2900N SHS Physics 35 Examples A jet thruster applies a force of 200,000N at maximum burn. If the jet has a mass of 15,500kg what is the acceleration of the jet? How long will it take to increase speed from 100 m/s to 150 m/s? a ~ 13 m/s2 and t ~ 4sec!! SHS Physics 36 Think About it... A textbook rests on a table. What forces act on the book? On the table? True or False. When you jump the Earth accelerates. An 8 ton bus crashes into a 1200 lb VW Rabbit. If the bus applies a force of 20,000 lb on the car, what is the force of the car on the bus? SHS Physics 37 The Law of Force-Counterforce When one body exerts a force on another body the second one exerts an equal and opposite force on the first. unaccompanied forces do not exist in nature. „action-reaction‟ forces are not the same as „balanced‟ forces. SHS Physics 38 Questions to consider... Identify the “counterforce”… – The push of a rifle on your shoulder when you shoot a gun. – The hit of the lineman on the QB. – The backwards force of the tires on the road. – The gravity of the Earth on the Moon. Why can you push harder on the pedals when pulling up on the handle bars? SHS Physics 39 Think About It... A feather rests on a strong, solid oak table. Does the table exert any forces on the feather? I lean on a solid cement wall. Does it exert forces on me? What force causes a ball to bounce? SHS Physics 40 Normal Forces... when you stand on the ground your weight pushes down on it. It pushes up on you with the same force! That‟s called a normal force. a normal force is always perpendicular to the surface in contact. Take a minute and identify all the normal forces in the room. SHS Physics 41 Think about it... What is the reading on the scales in the two diagrams below? fig. 1 fig. 2 (weights are 200 N each) SHS Physics 42 Tension... Tension is a force that pulls things apart. Tension is an electromagnetic force. Tension is the force that pulls in BOTH directions SHS Physics 43 Tension…A Deeper Analysis Massless ropes and things... SHS Physics 44 Think about it... an ice skater coasts to a stop... a marble rolls across the table and comes to rest… A boy in the back of a truck stays still while the truck accelerates. a skydiver reaches terminal velocity... What force is responsible for the above? If you said FRICTION, you are correct! SHS Physics 45 Brainstorm... So what exactly is friction and what does it depend on? Friction is a resistive force: – surface locking – electrostatic attraction – air drag – fluid viscosity SHS Physics 46 Friction... In mechanics, f=µxN µ (Greek “mu”) is a constant which is different for each situation. N is the normal force – What is “N” for a book sliding across the floor? SHS Physics 47 Types of friction... Sliding Friction (kinetic) fk = µk x N Static Friction (static) fs = µs x N In each case the only difference is the coefficient of friction, µ. µk ≤ µs SHS Physics 48 Determining µ... To determine the coefficient of friction between two surfaces just pull a block at a constant speed across the table top with a spring scale. What force is necessary to do this? Draw a Free Body Diagram. How can you find µ (this is for kinetic friction)? How is it possible to find the coefficient of static friction? SHS Physics 49 Examples What is the force of friction when a 150 lb boy (bike weight included) applies the brakes w/o skidding while riding down the street on his bike (µs=0.9)? How would it change if he locked up his brakes (µk=0.7)? How many meters will the boy above take to come to rest in each situation if his initial speed is 26 mph? SHS Physics 50 Forces are vectors Forces can be resolved into vertical and horizontal components. Ex. A boy pushes on a lawn mower with a force of 400 N. If the mower weighs 65 lbs and the handle makes an angle of 45° with the ground what is the horizontal acceleration (ignore friction)? How would the problem change if you don‟t ignore friction? SHS Physics 51 Lawnmower Problem Fx Fcos(45) Fx 400cos(45) 283N Now, W mg and 65lb 289N So, 289N m(9.81) m 29.5kg Finally, Fx 283N ax 9.59 m / s2 m 29.5kg SHS Physics 52 Wrap-Up Questions What are the four basic forces of the universe? Which is the strongest? What is inertia? How does it relate to Newton‟s first Law? What is equilibrium? What kind of motion do forces cause? How does the force of your car‟s windshield on a mosquito compare to the force of the mosquito on the wind shield? What is a tension? Normal force? Friction? SHS Physics 53