Draw picture of a boulder & earth on the board. jump off lab table. have students watch the floor.
1 newton's laws notes
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 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?