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Unit 2 Forces & Motion

Unit 2 Forces & Motion. Forces. Force - an action applied to an object to change its motion(push or pull) Units of lb, N (equal to kg . m /sec 2 ) If forces are balanced then the object won’t accelerate and it is said to be in equilibrium Unbalanced forces =acceleration

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Unit 2 Forces & Motion

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  1. Unit 2 Forces & Motion

  2. Forces • Force- an action applied to an object to change its motion(push or pull) • Units of lb, N (equal to kg.m/sec2) • If forces are balanced then the object won’t accelerate and it is said to be in equilibrium • Unbalanced forces=acceleration • Forces always occur in pairs!!!!!

  3. Newton’s Laws of Motion • Newton’s First Law: • “AKA” the Law of Inertia • Inertia- property of an object to resist change in state of motion • an object will stay at rest until acted on by an unbalanced force • In other words, things tend to keep on doing what they were doing in the first place unless you apply a force.

  4. Newton’s Laws of Motion • Newton’s Second Law: • Unbalanced force causes an object to accelerate while the object’s mass resists acceleration • Fnet=ma • a=acceleration (m/sec2), F= force (N), m=mass (kg) • Fnet= Right – Left • Fnet= Top - Bottom • EX. Lighter cars go faster than heavier ones pushed with equal force. Lighter cars resist the force of acceleration less allowing them to move faster than heavier ones.

  5. 3.3 Newton’s Third Law • Newton’s 3rd Law: • For every action force there is a reaction force equal in strength & opposite in direction • Can be positive or negative • Examples: rockets, stepping into a boat, throw a ball when on a skate board

  6. Net Forces • Net Force - Total sum of all the forces that act on an object • In the same direction – add forces together • In opposite directions –subtract forces • largest vector wins when forces act in opposite directions • At right angles (a2 + b2 = c2) • When net Force = 0, there is no acceleration. This means the object is: • Not moving OR • Moving at a constant velocity

  7. Gravity, Weight & Mass • Fw=mg • Fw= weight force (N) • m=mass (kg) • g=acceleration due to gravity (-9.8 m/sec2) • Gravity- • Force pulling toward mass of planet • Depends on mass • Mass- amount of matter in an object compared to a standard • Changes on other planets • On Earth= -9.8m/s2 • Weight- force created by gravity, depends on mass • It is a force acting on object in units of lb or N Fw m g

  8. Law of Universal Gravitation • force of attraction directly related to masses & indirectly related to distance • F=G m1m2 r2 • F= Force of attraction • G= Gravitational constant • m1 = mass object 1 • m2 = mass object 2 • r = radius btw the objects

  9. Free Fall- • Drop straight down in the absence of air resistance. • Acceleration due to gravity -9.8 m/sec2 • Object’s will experience uniform acceleration (uniformincreases in velocity) when in free fall! • Terminal Velocity- • Highest velocity reached by a falling object • When an object stops accelerating, but continues to fall, terminal velocity is achieved. • Occurs when force of air resistance balances the weight of the object.

  10. Falling without air resistance – feather and elephant

  11. Falling with air resistance

  12. Skydiver

  13. What is friction? • Friction: • Reduces acceleration • Works against motion • Occurs when surfaces move against one another • Causes wear on parts • Examples of friction: • Air friction- (air resistance)- aka – “drag” • Sliding friction (rub hands together) • Viscous friction- (oil in car engines & joint fluid) • Rolling friction- (wheel on road, ball bearings)

  14. The frictional force that must be overcome to get an object moving (Ffs) μs = Ffs FN μ=coefficient of friction (ratio of frictional forces to the normal force) Fw: Weight Force = mass X gravity FN: Normal Force Often = to the Weight Force (Fw) The frictional force that exists once an object is in motion (Ffk) AKA kinetic friction μk = Ffk FN Static Friction (Ffs) Sliding Friction (Ffk)

  15. Free-body diagrams • It is a diagram that visually represents the various forces being applied on an object: • Fstatic = friction force (Ff) • Fpush = applied force (FA) • N = the normal or support force (FN); perpendicular to surface • W = weight of object (FW) The sum of all of the forces involved is called the FNET

  16. Rules of Friction Fp or Fa Ff • If an object is slowing down, then the frictional force (Ff) is greater than the pulling force • If an object is moving at a constant speed, then the frictional force (Ff) is equal to the pulling force • If an object is speeding up, the frictional force (Ff) is less than the pulling force. • Fp=pulling force = Fa=applied force

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