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# 1 newton's laws notes

## on Sep 24, 2012

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• Show the free body diagram of the block.
• Draw picture of a boulder & earth on the board. jump off lab table. have students watch the floor.

## 1 newton's laws notesPresentation Transcript

• Force & Motion Dynamics  how things move
• Aristotle (4-300 BC) Believed everything moved to return to its own. ie. rock to earth, smoke to air Objects want to be in a state of rest. Galileo (15-1600 AD) Dropped objects from the Leaning Tower of Pisa. Too quick, Too Fast  after 3s, v = 29 m/s (64 mi/hr), y = 44 m (1/2 football field) per second
• Slowed things down by rolling balls down variousinclines. ? It always came to about the same height. What would happen on a smooth horizontal surface? Inertia  tendency of an object to maintain a state of rest or motion
• Newton (16-1700 AD) Mass is a measure of inertia The more mass you have, the less it wants to change its state of motion.1st Law (Law of Inertia)  An object in motion tends to stay in motion with a constant velocity and an object at rest tends to stay at rest, unless acted on by an unbalanced force.
• Force  a push or pull on an object (measured in Newtons)Net Force (Fnet)  is the sum of all the forces acting on an object Balanced UnbalancedF1 = 2 N F2 = -2 N F1 = 2 N F2 = -5 N Fnet = 2 + -2 = 0 N Fnet = 2 + -5 = -3 N
• Force Diagrams (Free Body Diagram)  a vector diagram of all the forces acting on1. Draw the object as a single dot. an object2. Draw the force vectors as coming out of the dot. Label each of the vectors. F1  Force of object 1 on object 23. Check to make sure that all forces are accounted for.2 Types of forcesContact – Look at the objects actually touchingNoncontact – look at gravity, electricity, and magnetism
• Normal Force  force perpendicular to the ground pushing up Samples Draw a force diagram of you sitting in your desk.Accelerating out of the parking lot, you put your foot onthe gas and the engine applies 2500 N of force forwardon your 10,000 N car. If the ground puts 1000 N offriction back on the car:a. draw a free body diagramb. what is the net force?
• Newton’s 2nd LawIf all objects resting want to stay resting, how do they move?If all objects moving want to stay moving, how do they stop?Force!!!F↑a↑ m↓a↑Fαa 1/m α aForce is directly mass is inverselyproportional to accel. proportional to accel.
• 2nd Law (Law of Acceleration) Force makes an object want to accelerate. Mass makes an object not want to accelerate. a α Fnet/m a = k Fnet / m Const = 1 a = F/m F = ma 1 Newton = amount of force necessary to accelerate a 1 kg mass at 1 m/s2 1 N = 1 kg m/s2 This would be like holding 100 g in your hand. 1 N = (.1 kg) (9.8m/s2) 1lb is about 4 N
• Net force in theNo Angle y direction Angle Fnety = 0 usually cancels Fnety = 0FN – Fg = 0 out to zero: FN + Fy = Fg FN = F g FN FN Fyou FyouY FF Fyou FF FyouX Fg Fg Net force in x direction causes acceleration: Constant VelocityAccelerating Fnetx = 0 Fnetx = ma Fx – Ff = 0Fx – Ff = ma F =F
• To find the mass, F1 = 2 N 196 N F2 = -5 N take the weight, divide by 9.8. Fnet = 2 + -5 = -3 NWeight  force of gravity F = ma Fg = m g F = ma196 N= m (9.8 m/s2)m = 20 kg -3 = 20 a a = -0.15 m/s2
• Use vf, x, or t to find a, then a to find Fnet, or use Fnet to find a then a to find vf, x,or t vf x t x = vit + ½ at2 a Fnet = ma Fnet vf = vi + at vi vf2 = vi2 + 2ax
• You pull a 50 N block with a force of 15 N at 26 °.a. draw a force diagramb. find the mass of the block.c. find the components of your force.d. find the normal forcee. find the acceleration of the block if friction is 4 N?f. find the block’s final velocity after 3 s? Givens
• You pull a 50 N block with a force of 15 N at Y direction26°. Fnet = m aa. draw a force diagramb. find the mass of the block. Fnet = 0c. find the components of your force. FN + Fy – Fg = 0d. find the normal force FN + 6.58 – 50 = 0e. find the acceleration of the block if friction FN = 43.4 Nis 4 N?f. find the block’s final velocity after 3 s? FN Givens X direction Fx = 15 cos 26 = 13.5 N 15 N vi = 0 Fnet = m a Fy = 15 sin 26 =a = 1.86 m/s2 6.58 N Fx = 5.10 a t=3s 13.5 -4 = 5.10 a vf = ? a = 1.86 m/s2 Fg = m g g a vf = vi + at 50 = m(9.8) vf = 0 + (1.86)(3) m = 5.10 kg vf = 5.59 m/s
• Newton’s Third Law
• Newton’s 3 Law rdEvery Action has an equal and opposite reaction.∴Every interaction happens in 2’s. Action Force and Reaction ForceHow do we know what action & reaction are? Action: Object A exerts a force on object B Reaction: Object B exerts a force on object A Notice that these forces are acting on two different objects.
• example: Earth exerts a force on Boulder Boulder exerts a force on Earth If the boulder exerts a force on the earth that is Force of equal, why doesn’t the Earth on earth accelerate to the Boulder boulder? Force of Boulder on Earth
• Mass! 2nd Law!!! F=F m AM = aSo the earth moves, but it is soooo small that we can’t tell!Another example… horse and cart.Are the equal and opposite forces acting on the same object?