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Forces and Newton’s Laws of Motion

Forces and Newton’s Laws of Motion. The Nature of Force. Just like Velocity & Acceleration Forces have both magnitude and direction components. By definition, a Force is a push or a pull. Forces. A force causes an object to move, accelerate, change speed, or direction

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Forces and Newton’s Laws of Motion

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  1. Forces and Newton’s Laws of Motion

  2. The Nature of Force Just like Velocity & AccelerationForces have both magnitude and direction components By definition, a Force is a push or a pull.

  3. Forces • A force causes an object to move, accelerate, change speed, or direction • Forces are represented by: • Arrows • Direction • Strength • represented by the length of the arrow

  4. Balanced & Unbalanced Forces • Balanced forces – opposite and equal forces acting on the same object • result in NO motion of the object • Unbalanced forces – two or more forces of unequal strength or direction acting upon on an object • results in motion of the object

  5. Balanced or Unbalanced Forces? Unbalanced but WHY? Balanced but Why?

  6. Force/Free body diagrams (to show forces) • used to show the relative magnitude and direction of all forces acting upon an object in a given situation. • a special example of vector diagram • size of the arrow represents • amount of force

  7. Force / Free-body Diagrams direction of the arrow shows the direction which the force is acting each force arrow is labeled The object is usually represented by a box. Force arrows are drawn from the center of the box outward in the direction which the force is acting. http://www.physicsclassroom.com/Class/newtlaws/u2l2c.cfm#1 (look at types of forces also)

  8. Combining Forces • Net force: All forces are added or subtracted to create one total force • If net force is Zero then there is no change in motion. • If there is a net force there is an acceleration

  9. Newton’s 1st Law of Motion(cont.) Anything slowing this Coke as it slides across the bar? • If you roll a ball across the floor, why does it eventually come to a stop. • Is there a force acting against it? • If so, what is it? • Friction—force that opposes motion. • Always acts in the direction opposite of motion.

  10. Newton’s 2nd Law of Motionpg 94 of textbook called the “Law of Acceleration”. Relates force, mass, and acceleration. Force and acceleration directly related. Mass and acceleration inversely related.

  11. Newton’s 2nd Law of Motion(cont.) Uses this Formula: F = m a • Where: • F = force in Newton’s(N) • m = mass in kilograms(kg) • a = acceleration in m/s2

  12. Ex. What force is required to accelerate a 1500 kg car by a rate of 1.5 m/s/s? Given: m = 1500 kg a = 1.5 m/s/s F = ? F = m a F = 1500kg(1.5 m/s/s) F = 2250 N

  13. Ex. A force of 290 N is applied to a 20 kg rock. At what rate will the rock accelerate? F = m a a = 14.5 m/s2 Given: m = 20 kg a = ? F = 290 N

  14. Ex. A force of 140 N is applied to a mover’s dolly to accelerate a load of Cokes across the grocery store at 3 m/s2. What is the mass of the load of Cokes? Given: F = 140 N a = 3 m/s2 m = ?

  15. A force is applied to accelerate a 50 kg mass at a rate of 2.5 m/s/s. What force is required? 20 N 25 N 120 N 125 N

  16. D • F = 125 N

  17. A 250 N force is applied to a 12 kg mass. At what rate will the object accelerate? 3000 m/s/s 240 m/s/s 20.8 m/s/s .05 m/s/s

  18. C • A = 20.8 m/s/s

  19. Newton’s 3rd Law Forces always occur in pairs!! For every action, there is an equal but opposite reaction.

  20. Which of the following scenarios does not represent Newton’s 3rd Law? A rocket being launched into space. A book resting on a desk. A person paddling a canoe. A large force pushing on a small mass, causing it to accelerate.

  21. Choice D • is Newton’s 2nd law.

  22. Force Problem What is the net force on an object being pulled toward the west with a force of 30N and another force pulling the object toward the east with a force of 75N?

  23. Types of Friction • a force that opposes the motion of objects that touch as they move past each other • acts at the surface where objects are in contact • All moving objects encounter friction • Without it most motion would be impossible • 4 types, static, sliding, rolling, fluid

  24. Static Friction • The force that keeps an object from moving • It is the largest frictional force • Always opposite direction of the applied force • Pushing a Car • Walking

  25. Sliding Friction • Once the object is in motion it experiences sliding friction • Opposite direction from applied force • Less than static friction so less force is needed to keep it in motion

  26. Rolling Friction • As something rolls, the object and floor bend slightly • This bend causes rolling friction • It is a much smaller force than static friction • As much as 1000 times smaller • Allows you to move heavy objects • Ball bearings reduce friction

  27. Fluid Friction • It opposes the motion in the liquid • Like swimming, it is hard to move • If you are in the air, fluid friction is called air resistance • At higher speeds it is very noticeable

  28. Force of Gravity • a force that acts between two objects • an attractive force, pulls objects together • causes objects to accelerate downward • Falling objects, gravity pulls down, What pushes up? • air resistance acts in opposite direction • Terminal Velocity- when Gravity and air resistance cancel each other out (velocity of falling body becomes constant) • Skydiving and falcons • Forces Montage

  29. Terminal Velocity See elephant problem on page 10 and 11 of Netwon’s laws Smart Notebook file!!

  30. Projectile Motion • When an object not only goes up and down but out. • It is a curved path • Air resistance and gravity are the only forces acting on a projectile • What falls faster, an object that falls straight down, or one that has a horizontal velocity? See Videos They are very good…. • Myth busters bullet #2 • This one shows the end?

  31. May the “forces”be with you and not against you!! haha

  32. Class Activity- Balanced and Unbalanced Forces 1.Two tugboats are moving a barge. Tugboat A exerts a force of 3000 newtons on the barge. Tugboat B exerts a force of 5000 newtons in the same direction. What is the combined force on the barge? 2.Draw arrows showing the individual and combined forces of the tugboats in #1.

  33. 3.Now suppose that Tugboat A exerts a force of 2000 newtons on the barge and Tugboat B exerts a force of 4000 newtons in the opposite direction. What is the combined force on the barge? 4.Draw arrows showing the individual and combined forces of the tugboats in #3.

  34. 5.Could there ever be a case when Tugboat A and Tugboat B are both exerting a force on the barge but the barge doesn't move? Draw arrows showing the individual and combined forces in such a situation.

  35. Newton’s Laws of Motion Begins Here!!

  36. Great Scientists • Aristotle- Incorrectly said force was required to keep an object in constant motion • Galileo- with no resistance objects would move indefinitely • Newton- based on Galileo’s findings he came up with his law’s of motion

  37. Newton’s 1st Law of Motion Remember: The greater the mass of an object the greater the inertia • AKA The Law of Inertia • Inertia- an objects tendency to resist changes in motion • an object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity until acted on by another force.

  38. Check your Understanding on Newton’s first law http://www.physicsclassroom.com/Class/newtlaws/u2l1b.cfm http://www.physicsclassroom.com/Class/newtlaws/u2l1d.cfm

  39. You need to remember this about Inertia!!! • Mass is the measure of inertia of an object!!! • Remember: The greater the mass of an object the greater the inertia • So, which would have more inertia? Bowling ball • 0.1 kg baseball traveling at 20 m/s • 5 kg bowling ball traveling at 3 m/s • 10 kg sled traveling at 0 m/s • 0.001 kg bumblebee traveling at 2 m/s • Which would have the least? • 0.001 kg bumblebee traveling at 2 m/s

  40. Newton’s 2ndLaw • The acceleration of an object is equal to the net force acting on it divided by the objects mass • A= F/m so F=ma • Acceleration is always in the same direction of the net force • An object will have greater acceleration if a greater force is applied

  41. Newton’s Second Law of Motion F=ma • Force = mass x acceleration • F=ma ; a= F/m ; m= F/a • What is the basic unit for mass? • Kilogram • What is the basic unit for acceleration? • Meter/sec/sec • Therefore the basic unit for Force is (kilogram)( meter/sec/sec) • An object with a mass of 1 kg accelerating at 1 m/s/s has a force of 1 Newton

  42. Newton’s 2nd Law & Force of Gravity Since objects fall at the same speed, their acceleration is the same. All objects accelerate at the rate. Here on Earth the rate is: Ag=9.8 m/s2 Or Ag=32 ft/s2 With this experiment, Galileo proved Aristotle wrong Air resistance keeps things from falling equally With this experiment, Apollo 15 astronauts proved Galileo right. (link to You Tube) Have you heard of the FORCE of gravity? Gravity: the force that pulls objects towards each other Since gravity is a force it also obeys Newton’s second law F=ma

  43. Weight and Mass • Weight is the force of gravity acting on an object • Weight (N) = mass (g) x acceleration of gravity (m/s2) • Note: just a version of F=ma, F = mass x gravity • FYI: • 1 pound = 4.448 Newtons • so 1/4 lb is slightly more that 1 N • so a 1/4 lb burger is a "Newton" burger

  44. Newton’s 2nd Law & Weight Remember: 1 newton = 0.22 pounds F=ma So, weight is a type of Force The formula for weight: Weight = mass x Ag Since Ag= 9.8 m/s2 then Weight = mass x 9.8 m/s2

  45. Your weight on other planets& 3 different types of stars

  46. Newton’s 2nd Law

  47. F= ma Math Practice 1. A boy pushes forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N? 2. What is the upward acceleration of a helicopter with a mass of 5000 kg if a force of 10,000 N acts on it in an upward direction?

  48. F= m a formula problems… 3. An automobile with a mass of 1200 kg accelerates at a rate of 3.0 m/s2 in the forward direction. What is the net force acting on the automobile? 4. A 25-N force accelerates a boy in a wheelchair at 0.5 m/s2 What is the mass of the boy and the wheelchair?

  49. During a test crash, an air bag inflates to stop a dummy’s forward motion. The dummy’s mass is 75 kg. If the net force on the dummy is 825 N toward the rear of the car, what is the dummy’s deceleration?

  50. A bicycle takes 8.0 seconds to accelerate at a constant rate from rest to a speed of 4.0 m/s. If the mass of the bicycle and rider together is 85 kg, what is the net force acting on the bicycle? (Hint: First calculate the acceleration.) a=(vf-vi)/t = (4.0 m/s) / 8.0 s = 0.50 m/s2 F=ma = 85 kg x 0.50 m/s2 = 43 N

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